Alternative air vehicles for sterile insect technique aerial release


L. T. Tan (corresponding author), Department of Civil, Environmental & Geomatic Engineering, University College London, London WC1E 6BT, UK. E-mail:


The majority of current sterile insect technique (SIT) programmes release chilled adult sterile insects into the wild by means of small fixed-wing aircraft or occasionally helicopters. Besides being the fastest method of release, it has been demonstrated to provide uniform distribution over target areas and to help ensure sterile insect quality and survivability. It is, however, also the most expensive method of release, and on average, current aerial release contracts constitute about 40% of the annual operating budget of sterile fly emergence and release centres. This is mainly due to the fact that the aircraft used are highly over-specified relative to the requirements of small to medium SIT aerial release programmes. Furthermore, this stage of the SIT process has traditionally been neglected, while research and process optimization efforts have focused mainly on the mass rearing and sterilization of insects. This study addresses this gap by presenting an extensive survey of other forms of aerial transport, of which five alternative types of aerial vehicles are identified as having the potential to make significant cost savings compared with the current aircraft of SIT programmes. These different alternatives were further evaluated against the requirements of current SIT programmes and also compared in terms of cost. Of these, two fulfilled all requirements, but given that an unmanned aircraft could bring even more cost savings, we present two further proposals for unmanned vehicles, the first a custom-built fixed-wing unmanned aerial vehicle or long-range radio-controlled aircraft and the second an unmanned dirigible, which is easier to control remotely in view of its natural tendency to stay airborne. Both these alternative aircraft proposals present major cost savings in terms of pilot wages, maintenance and fuel consumption, with the largest cost saving made by eliminating the need for a pilot, as the take-off weight is the largest source of fuel consumption. Considering that current aerial release contracts represent a significant proportion of the operating budget of SIT programmes, the financial savings of implementing either of these options will have a significant impact on the financial attractiveness of the overall SIT process. Cheaper alternatives to currently used aircraft will also allow smaller SIT programmes to use aerial release methods, thereby widening applicability of aerial release methods.


The sterile insect technique (SIT) is a biological pest control method widely considered as the most environment-friendly and sustainable pest control method (Dyck et al. 2005). The principle of the technique involves the systematic release of large amounts of sterile male insects into the target pet control areas to mate with the indigenous female populations. As a result of the sterile sperm transferred, non-viable offspring is produced, which then reduces the number of pest individuals in the next generation. In isolated situations, pest populations can not only be suppressed but even be eradicated after many generations of systematic and area-wide sterile male releases in the target areas. To ensure effectiveness and minimize matings among indigenous males and females, released sterile males need to outnumber the indigenous male population by a ratio of at least 10 : 1 (Hendrichs 2000). In addition, the overall performance and sexual behaviour of the sterile males must be as similar as possible to that of wild males (Hendrichs et al. 2002). Advantages of SIT, in addition to its minimal impact on the environment, are that it is safe and non-intrusive to other organisms and that it works well in conjunction with other pest control methodologies, including biological control and male annihilation (Vreysen et al. 2006).

In the majority of large-scale insect population–management programmes that include an SIT component, aerial release is the route of choice for releasing insects into the wild. While some smaller programmes release sterile insects by ground (Salvato et al. 2003; FAO/IAEA 2007), above a certain operational scale, aerial release has been shown to be more cost effective and facilitates a more uniform distribution of insects because of the higher altitude of release and release paths not being dictated by access routes/roads.

There are two main categories of aerial release: the dropping of the traditional and simple paper bag or cardboard box, and the more sophisticated chilled insect release. In the paper bag or box release, the sterile insects will emerge within the containers, which are then opened upon release from the aircraft. The main advantage, particularly of the paper bags, is the low infrastructural or set-up costs (FAO/IAEA 2007). However, in practice, there are major deficiencies with this method, which include the (i) labour-intensive packing of insects into bags/boxes, (ii) less uniform distribution of the sterile insects because of intermittent intervals (2–8 s) of release from the aircraft, (iii) higher predation rate due to many sterile insects staying in the vicinity of the bags after release, (iv) sterile insects not being properly watered and fed, (v) damage of insects caused by compaction during transport, and (vi) littering by the slow-degrading bags or boxes (FAO/IAEA 2007). Thus, the second category, chilled adult release, is generally the preferred option. In addition to overcoming most of the disadvantages of the paper bag or box release, the chilled adult release method is capable of releasing much larger numbers of sterile insects per trip because of the concentration of sterile insects and the lack of packing (FAO/IAEA 2007). The disadvantages of this method include the need for a more sophisticated holding and collection infrastructure and the comparatively expensive equipment required. Nevertheless, a new generation of the chilled boxes utilizes less-complicated mechanical components (such as phase-change materials or frozen carbon dioxide as a replacement for the conventional cooling facilities) that are more cost effective and reliable (FAO/IAEA 2007; Tween and Rendón 2007). This new chilling technology should be incorporated or utilized in any alternative vehicle for the aerial release of sterile flies. As such, it will not be discussed in this paper, particularly for the aerial release of sterile fruit flies after semiochemical exposure.

Although much work has been carried out in researching the pest insect biology and in developing the industrial-level insect mass rearing and sterilization technology, the final step in any SIT programme, i.e. the release of the insects, is still lacking in cost efficiency. Aerial release is expensive, amounting to more than a quarter (and in extreme cases up to 72% (USDA APHIS 2009)) of the operating budget of fly emergence and release centres, i.e. the costs of transporting sterile pupae, as well as emerging, feeding, chilling and releasing sterile flies. Any cost saving that can be made in this area is therefore likely to have significant impact on the overall cost of SIT programmes.

The aerial release requirements of SIT programmes are far from onerous in terms of the capabilities of commercially available aircraft; thus, one of the main reasons for the high costs of aerial release is that the current aircraft used in these programmes are over-specified for the job. Furthermore, the major component of fuel consumption in the majority of aircraft is the take-off weight. In the smaller SIT programmes, the actual weight of insects to be released in each flight can be as low as 50 kg, whereas the weight of the pilot can be twice this amount, i.e. an 80-kg pilot would be 60% of the total load (130 kg). Even in the larger programmes, for example the medfly programme in Guatemala has an average payload of about 200 kg each flight. An 80-kg pilot is about 30% of the total load, which is still a significant percentage. Significant cost savings can be made by eliminating the need for an on-board pilot. It needs to be noted that unmanned aircraft are not yet suitable to be flown over busy metropolitan areas as a result of aviation regulations. However, with the ever increasing research, development and demand for unmanned vehicles we expect these regulations to change. Hence, if the SIT target area is an agricultural area (assumed to be the majority of the cases), an unmanned vehicle would be suitable, otherwise a manned aircraft is required.

Furthermore, the payload capacity of the aircraft currently being utilized, the Turbo-LET 410 in the Guatemala medfly programme, is 1615 kg, while the payload on each flight together with the pilot is about 280 kg (17% of the capacity). This highlights one of the main issues this study is trying to address, i.e. the aircraft currently in use has such large capacities that the insect payload and the pilot’s weight are insignificant compared with the capacity of the aircraft; hence, vehicles better suited to the job are urgently needed.

The purpose of this study is to investigate alternative aerial release vehicles, for the release of chilled adult insects that more closely match the requirements of small-to-medium scale area-wide SIT programmes and hence, should be more cost efficient than the aircraft currently in use. Following a description of aircraft currently used in SIT programmes, a list of requirements for these programmes is specified. Alternative aerial vehicles are then proposed and evaluated to ascertain whether they meet these requirements, and the alternatives are compared in terms of fixed and operating costs. Given the fuel savings that can potentially be derived from reducing the take-off weight, the focus of the survey is to further increase cost savings by eliminating the need for the on-board pilot, i.e. using unmanned vehicles. The study shows that none of the currently available alternatives are able to meet all requirements with the need to make substantial cost savings; hence, two proposals for adapted/redesigned vehicles are presented.

Current Aircraft Used in SIT Programmes

Aircraft currently used for aerial release of sterile insects in the application of the SIT are either fixed-wing aircraft, or occasionally helicopters for use in narrow mountainous terrain owing to their better manoeuvrability. The types of aircraft include the Cessna 172 (and 206, 207, 208), Beechcraft King Air 90, Piper PA-28 and Helicopter Bell 206 (and 212) (FAO/IAEA 2007). Table 1 lists the main specifications of these different aircraft which can impact aerial release programmes. The large SIT programmes of Guatemala and Mexico MOSCAMED use the Turbo-Let-410 (Guatemala), Cessna 401 and 402-twin engine (Mexico). These have payload capacities of more than 1000 kg and hence, are not suitable for the smaller-scale SIT programmes. The main specifications and costs of the aircraft used in this study have been extracted from the various internet sources of the aircraft manufacturers.

Table 1. Key specifications and cost of selected aircraft currently used for aerial release of sterile insects in the application of the sterile insect technique
Alternative aircraftCessna T206H StationairPiper PA-28 CherokeeBeechcraft Baron G58Helicopter Bell 206L-4Gyrocopter MAGNI M16Cessna 402Turbo-LET L-410
  1. The data have been extracted from various internet sources of aircraft manufacturers (see list of References), the information from the table is derived from the sources Aircraft Specifications 2009; 2009; Barnard Microsystems Ltd 2009; BBC News 2005; Fort Meade Flying Activity (FMFA) 2009; 2009; Hawker Beechcraft 2009.

  2. SIT, sterile insect technique.

SIT locationUSA, MexicoMexico, ChileGuatemala, Mexico, Portugal, USAMexicoSouth AfricaMexicoGuatemala
Maximum range594 NM (1100 km)880 NM (1630 km)1052 NM (1948 km)336 NM (622 km)304 NM (563 km)1273 NM (2360 km)297 NM (550 km)
Fuel consumption75% power: 16.5 GPH at cruise speed of 142 kts
65% power: 14.3 GPH at cruise speed of 134 kts
75% power: 10 GPH75% power: 32 GPH at cruise speed of 202 kts
65% power: 30 GPH at cruise speed of 192 kts
55% power: 26 GPH at cruise speed of 177 kts
38 GPH (average)5.5 GPH (average)40 GPH37 GPH
Maximum speed151 kts (285 km/h)146 kts (271 km/h)202 kts (357 km/h)115 kts (213 km/h)100 kts (185 km/h)235 kts (435 km/h)205 kts (380 km/h)
Cruise speed142 kts (263 km/h)137 kts (254 km/h)190 kts (352 km/h)87 kts (160 km/h)82 kts (151 km/h)205 kts (380 km/h)197 kts (365 km/h)
Fuel type100/100 LL100/100 LL100/100 LLAviation Turbine Fuel95 octane or 101 octane100/100 LL100/100 LL
Usable fuel std.87 US gal (329 l)72 US gal (273 l)194 US gal (734 l)110.7 US gal (419 l)19 US gal (72 l)206 US gal (778 l)443 US gal (1675 l)
Useful load1373 lbs (623 kg)960 lbs (437 kg)1504 lbs (682 kg)1980 lbs (900 kg)550 lbs (250 kg)2781 lbs (1262 kg)4409 lbs (2000 kg)
Service ceiling15 700 ft (4785 m)16 200 ft (4983 m)20 688 ft (6306 m)20 200 ft (6100 m)22 000 ft (6705 m)26 900 ft (8199 m)22 966 ft (7000 m)
Cost of aircraft in US$503 500323 8501 112 1501.6–1.8 million133 000Out of production∼1.5 million

Specifications of an Aerial Release Vehicle for SIT

This section outlines, from the perspective of the chilled aerial release programmes, the main requirements of an aerial release vehicle. These requirements will influence the choice of vehicle and assist in the design of new vehicles geared specifically to the needs of SIT programmes using aerial releases. The characteristics of the target area, polygon shape and size, geographical situation, pest situation, programmed density to release, and swath must also be considered, but as these are programme specific, they will not be considered in this study. It needs to be noted that the vehicle requires a power supply for the insect aerial release machine; this can either be portable or an integrated part of the vehicle.

Flight/weather conditions

With the current navigation technology, most commercial aircraft have the ability to fly in bad weather onditions; however, for an SIT programme, this ability is not usually needed as sterile insects are released only when conditions are best for their survival and performance. Furthermore, releases are usually not carried out in severe weather conditions as this results in the excessive drift of the released insects.

Flight range

Sterile insect technique programmes require area-wide releases not only to ensure suppression of the pest population over cultivated areas but also to include some surrounding areas to prevent the immigration of unmated fertile females. Areas covered are commonly in the range of 100 square miles per flight (USDA FDACS 2009), considering a linear flight path in a polygon of 16 × 16 km at a 500-m swath or lane spacing; the required range is 32 × 16 km = 512 km.

Release airspeed

The main objective of the release operation is to deliver the required sterile insect release density or densities at the desired insect quality. Hence, the airspeed during release, which is dependent on several other factors such as the release machine’s rate and the relation between lane length and lane width, can vary considerably. For instance, in the large Mexico programme with aircraft speeds of 230 km/h and a polygon release area of 256 km (25 600 hectares) and chosen rate of 2000 sterile insects per hectare, a total of 51 200 00 insects can be released in a period of 2 h and 14 min.

Hence, the airspeed for release determines the duration of each flight and impacts on the range, fuel consumption, labour charges and operational fees. Furthermore, too slow an airspeed may result in the insects being held for a longer time period under cold temperatures, and hence, the quality of these chilled adults could potentially be affected (Shelly et al. this volume). Another factor that impacts the chilled holding time of insects is the ferry time or the distance between the release area and the airport. The FAO/IAEA guideline (2007) notes an average speed of 162 km/h for paper bag release rates and 144 km/h for chilled adult release.


The irradiated insects are kept in a release box maintained at low temperature to keep them immobile. When these chilled insects are released, they will warm up as they descend and by the time they are near the ground, they should have regained normal activity. The decision of release altitude is hence a vital issue. If the altitude is too low, the sterile insects will not warm up in time and may succumb to predation. If the altitude is too high, it can affect the uniformity of distribution because of drift. The altitude range currently being utilized varies from 100 to 762 m above ground (FAO/IAEA 2007). The lower end of the range varies depending on the climate; in the USA, sterile insects are released at no lower than 150 m, while in Mexico, where air temperatures are usually warmer, the altitude for release is no lower than 100 m (FAO/IAEA 2007).

Wind varies with in altitude and time, making exact prediction of wind velocities along a particular trajectory impossible. However, various methods of measurement and analysis are available to aid in estimating wind speeds. Sources of error include the very causes of wind: movement of weather systems and associated pressure gradients; orographic influences, both meso- and micro-scale effects; convection, which causes the most rapid time variation; terrain variations, and other diurnal thermal effects. Even direct measurements of the wind by dropsonde are inexact for precision airdrop purposes. Sources of measurement error are associated with time and location. Winds will change between the time of the measurement and the time of the airdrop. The shorter the interval between measurement and release, the more accurate the result. Location errors come from an inability to drop along the exact intended trajectory and tend to be greatest for the low altitude portion of the data and in mountainous terrain.

Release accuracy and adherence to release path

Sterile insects need to be released along predetermined equally spaced release paths to ensure full coverage of the target area and uniform distribution of insects. Hence, the aircraft needs to have the capability of flying accurately along these release paths, and to record with global positioning system (GPS) and appropriate software the adherence to these paths for purposes of effective area-wide coverage, quality control and correct swath distance. The importance of wind data for all types of systems is emphasized. We note that the availability of accurate wind profile data is rapidly improving and will continue to improve in the future. We believe that unmanned aerial vehicles (UAVs) have advantages over manned aircrafts in this matter.


The majority of the small-to-medium USDA fruit fly SIT programmes release between 3–8 million flies per flight (Dowell et al. 2000, USDA APHIS 2009). To release 8 million sterile Ceratitis capitata per flight, the insect payload is about 48 kg (based on a fly weight of 6 mg). However, the weight of the pilot can be more than twice this value, which makes the current aircraft inefficient. Considering the fact that takeoff and landing account for the majority of the total fuel consumption, reducing the takeoff and landing weight is the key to decreasing fuel consumption and minimizing cost. The weight of some fruit fly release machines is estimated to be around 20–40 kg (with newer versions of the release machines being lighter). This gives a minimum payload requirement of about 90 kg for an unmanned aircraft, and of 170 kg for a manned aircraft. It needs to be noted that there are different scales of SIT programmes that also deal with different sterile insect species, for example in the large programmes of Mexico and Guatemala, up to 60 million C. capitata flies are released in one load (equivalent to 300 kg) using Cesna 402s, while for Anastrepha ludens, a Cesna 206 is used and the load consists of 7.2 million flies equivalent to 108 kg.

Ground facility requirements

Ground facilities required for aircraft maintenance, housing and inspection vary according to the aerial release vehicle being used. For fixed-wing aircraft, these are comprised of air traffic control, hanger space and a runway. The less the ground facilities required for these purposes, the lower the operational costs. Aerial vehicles that can take off and land vertically avoid the need for a runway and are also not limited by the distance of the release area from the airport, as they can be loaded with sterile insects anywhere.


A high standard of safety is required for the aerial vehicles to ensure the safety of the pilot, to avoid loss of the expensive release vehicles and release system, and to avoid any economic damage occurring to third parties in case of aircraft failure.

Environmental issues

Ideally, the aerial release vehicle should have low fuel and noise emissions to reduce air and noise pollution. Current SIT aerial release vehicles, however, have relatively high air and noise pollution characteristics.


The current aerial release methods are comparatively expensive. Hence, reducing the cost of aerial release remains a key challenge in the effort to increase their application in SIT programmes. The cost of aerial release can be broken down into:

Fixed costs

The fixed or capital costs to purchase any of the aircraft used in current SIT programmes range between US $320 000 and $1.8 million. Hence, in many cases, aerial release is contracted out to specialized companies the contract normally includes the aircraft, operational costs and pilot fees. In financial year 2002, the Sarasota SIT facility in Florida, which conducted 35 h/week of aerial releases had an aerial release service contract of $846 000 for that year USDA FDACS, 2009).

Operational costs

These costs consist of fuel costs, pilot and ground crew salaries, hanger rental, airport and runway charges, and maintenance costs. Operational costs can be substantial, so that annual operational costs may even exceed the purchase price of the aircraft. However, they are also dependent on the type of aircraft. For instance, fixed-wing aircrafts require a runway for taking off and landing. In addition, there may be a need for airplanes to be housed in a hanger when unused and during bad weather conditions. Therefore, there is a long-term subsequent cost from runway charges and hanger rental. Depending on the air vehicle, maintenance checks are required before and after each flight and they must be in accordance with strict regulations by the country’s aviation authority; hence, maintenance costs can also be very high.

A recent review of eight different fruit fly emergence and release facilities (USDA APHIS 2009) found that in 2007, the total aerial release costs of the eight facilities amounted to US $10.1 million of a total operating budget of US $24.3 million. This is a staggering 42% of the operating budget of these facilities and accounts for an even greater percentage than the personnel costs which were 38%.

Aerial Vehicle Alternatives

In this section, five different aerial vehicle alternatives are proposed and their characteristics discussed. Based on the list of requirements in the previous section, the aerial vehicles are assessed for their suitability as an SIT aerial release vehicle. These findings are summarized in tables 2–9.

Table 2. Compliance of alternative aircraft with flight range requirements for aerial sterile insect release
Alternative aircraftCompliance with range requirements for aerial sterile insect release
  1. UAV, unmanned aerial vehicle.

Currently used aircraftYes. Required range ∼300 km
UAVsYes. 160 km for small size UAVs that weigh 7–45 kg and up to 1600 km for medium UAVs that weigh 45–225 kg
AirshipsYes. >1000 km
Radio-controlled aircraftNo. Required to fly within visual range The range is also severely limited by fuel capacity
Hot-air balloonsNo. Average range is only 16–20 km
MicrolightsYes. Up to 700 km cruise range but dependent on regulations. For example, in the US, the fuel capacity is limited to 19 l hence reducing the range to about 300 km
Table 3. Compliance of alternative aircraft with payload requirements for aerial sterile insect release
Alternative aircraftCompliance with payload requirements for aerial sterile insect release
  1. SIT, sterile insect technique; UAV, unmanned aerial vehicle.

Currently used aircraftYes. 190 kg for piloted versions and 90 kg for proposed unpiloted versions (minimum 48 kg of insects, 40 kg for release system). Current aircraft used in SIT are underutilized (payload capability∼500 kg)
UAVsYes. There are some existing UAVs able to carry this payload
AirshipsYes. It can be designed to carry loads up to 100 tons by altering the envelope size
Radio-controlled aircraftNo. The required unpiloted payload can only be achieved by enlarging current models and increasing engine capacities
Hot-air balloonsYes
Table 4. Compliance of alternative aircraft with airspeed requirements for aerial sterile insect release
Alternative aircraftCompliance with airspeed requirements for aerial sterile insect release
  1. SIT, sterile insect technique.

Currently used aircraftYes. SIT missions are generally flown at around 150 km/h. There is no fixed SIT requirement for airspeed, as this is dependent on the required sterile insect density and the adjustable release rate from the release machine to avoid too long chilling times of sterile insects. There is however a limit of this release speed as speeds which are too high result in progressively higher insect damage (the maximum airspeeds of the current SIT aircraft are 285 km/h (151 kts)
Unmanned arial vehiclesYes. Normally around 150–200 km/h
AirshipsLow. Although the maximum speed is about 200 km/h, the average cruising speed is only 56 km/h to 76 km/h (the majority of which are used for advertising or media coverage). This is however model dependent with some airships having cruise speeds of over 100 km/h
Radio-controlled aircraftLow but depending on the type and model they can fly up to 395.64 km/h
Hot-air balloonsNo. They fly only at the same speed as the wind
MicrolightsNo. The maximum speed is 55 kts (102 km/h) as required by the US regulations
Table 5. Compliance of alternative aircraft with flight/weather condition requirements for aerial sterile insect release
Alternative aircraftCompliance with flight/weather condition requirements for aerial sterile insect release
  1. SIT, sterile insect technique; UAV, unmanned aerial vehicle.

Currently used aircraftYes. Current SIT fixed-wing aircraft are more likely to fly in flat to uneven terrain, while helicopters are used in very mountainous areas. Although both types of vehicles can fly in bad weather conditions flies are not usually released during bad weather because of safety concerns and lower fly survivability. In addition, insects are normally released under low wind conditions to avoid excessive drift
UAVsYes. UAVs can operate in bad weather with some designed especially for bad conditions which are dangerous for humans
AirshipsYes. It can fly at severe conditions, with the restriction being the takeoff and landing wind speed <30 kts (55 km/h), with a mooring mast system this can go up to 100 kts (185 km/h)
Radio-controlled aircraftNo. Cannot usually tolerate bad weather conditions.
Hot-air balloonsLow. The flight of hot-air balloons is severely limited by weather. They require stable wind conditions to ensure a safe flight; hence, early morning and evening are the best period in a day for operation
MicrolightsLow. Because of regulations, micro-lights can only be flown in daylight and good weather. In several countries, they are also not allowed to be flown over populated areas, so they cannot be used when the release programmes are near urban area
Table 6. Compliance of alternative aircraft in terms of flight path accuracy requirements for aerial sterile insect release
Alternative aircraftCompliance with flight path accuracy requirements for aerial sterile insect release
Currently used aircraftYes. They have high standard of accuracy due to the fact that the control systems are based on GPS
Unmanned aerial vehiclesYes. They have relatively high standard of accuracy due to the fact that the control systems are based on GPS
AirshipsYes. The control system of airships is very precise again because of GPS
Radio-controlled aircraftNo. Traditional radio controlled aircraft have very low accuracy; the flight path is determined by the sight of a ground-based pilot
Hot-air balloonsNo. The flight path is influence by the wind, and directional changes can only be achieved by changing the altitude
MicrolightsYes. The control system of microlights is similar as the ones used in a fixed-wing aircraft
Table 7. Comparison of alternative aircraft in terms of ground facility requirements for aerial sterile insect release
Alternative aircraftComparison of ground facility requirements for aerial sterile insect release
Currently used aircraftHigh. There is a need for the aircraft to be housed in a hanger for most of the day and definitely during bad weather conditions. Runway required for takeoff and landing
Unmanned aerial vehiclesHigh. Similar to the traditional fixed wing aeroplane. However depending on the design some do not need a runway
AirshipsLow. There is no need for a permanent hanger except twice per year for maintenance and checking. They are kept outside by majority of operators. They can takeoff and land within a short distance and in many cases vertically hence no need for a conventional runway
Radio-controlled aircraftMedium. Because of their small size, they can be kept in a garage. No need for a conventional runway just a 100–300 m flat strip of ground for takeoff and landing
Hot-air balloonsLow. Vertical takeoff and landing hence only require some open ground
MicrolightsMedium. A hanger is not always necessary as certain models have foldable wings hence a garage will suffice. They do not require a conventional runway for takeoff and landing just 100–150 m of hard surface
Table 8. Compliance of alternative aircraft with safety level requirements for aerial sterile insect release
Alternative aircraftCompliance with safety-level requirements for aerial sterile insect release
  1. UAV, unmanned aerial vehicle.

Currently used aircraftYes. A high standard of safety is required and achieved, because of the risk to the pilot, ground-based third parties, and the aircraft itself
UAVsLow. But no loss of life. UAVs have a very high rate of mishap compared to the commercial and military fixed wing aircraft. This is only partially because of mechanical or software failures, but alsoinfluenced by human factors (both pilot and ground maintenance crews). About 32% of the total UAV accidents are caused by human error (Hobbs and Herwitz 2005). This is mainly because of the time delay in communication with the control systems of the plane which make it difficult for the pilot to react in real time to emergency situations. Pilots are also not able to have the sense of the flying environment because of the lack of tactile feel (Tyabji 2007). However, due to the fact that these are remotely piloted, failure does not lead to the loss of life
AirshipsHigh. The historical causes of accidents have been amended and with new technology, the new generation has not just a high standard of safety, but also fail-safe systems. Furthermore, the lighter than air working principle means that in the case of a control or engine failure, the airship will not crash catastrophically but slowly descend
Radio-controlled aircraftLow. But no loss of life. There is a high chance of failure as a result of interruptions in radio communications, limitations of visual control, limited communications range and the lack of tactile feel. No risk to the pilot’s life if failure occurs
Hot-air balloonsHigh. If it is only flown in good weather the safety levels are relatively high
MicrolightsLow. These vehicles are designated only for leisure purposes hence there has traditionally been very low investment levels in their development and maintenance
Table 9. Comparison of alternative aircraft in terms of environmental issues, such as air and noise pollution, for aerial sterile insect release
Alternative aircraftEnvironmental issues comparison for aerial sterile insect release
Currently used aircraftHigh air pollution, as aeroplanes are one of the largest contributors to carbon dioxide emissions and to noise pollution
Unmanned aerial vehiclesLower air pollution due to lower fuel consumption as a result of less weight
AirshipsLower air pollution because of lower fuel consumption and low noise emissions because of smaller engines
Radio-controlled aircraftVery low air pollution because of low fuel consumption and also low flight altitudes
Hot-air balloonsLow air pollution, although fuel consumption is comparable to fixed wing aircraft. The fuel used is propane which is a low carbon emitting fuel and has a cleaner combustion process than aviation fuel. Furthermore, it is used to heat up air and not for engines hence allows for more complete combustion. Almost silent operation
MicrolightsHigh pollution, similar to conventional aircraft

Hot-air balloons

These devices produce lift by heating up the gas inside the envelope/balloon, thus making the balloon buoyant. The cabin of light-weight material is attached under the envelope. The device can be designed to carry a payload by altering the size of the envelope and the gas temperature inside it. In terms of fixed costs, hot-air balloons are a very low-cost option, with the size required for an SIT release mission costing approximately US $30 000 (<1/10 of the cheapest aircraft utilized in the current release systems) (Cameron Balloons 2009). As for the operation and maintenance fees, the cost of fuel and the allowance for maintenance is about US $30 per hour (Cameron Balloons 2009), including annual insurance costs. However, after 500 h of operation, the envelope needs to be changed at a cost of about US $18 000. For example, in the case of the Sarasota, Florida facility’s aerial release programme, which releases sterile flies for 1820 h/year (USDA FDACS 2009), the envelope would need to be replaced at least 3.5 times per year. This will increase operating costs, but still significantly lower than current SIT aircrafts. In terms of staff cost, the balloon pilot’s salary is lower than that of an aeroplane pilot, while the maintenance operations require less ground crew time than fixed-wing aeroplanes. Hot-air balloons have no problem meeting the altitude requirements for SIT as they can fly at altitudes from above tree canopy to 3000 ft (914 m). The average flight range of 16–19.3 km can be adjusted by increasing the fuel capacity and the envelope size. However, because of navigational problems, hot-air balloons are not capable of flying upwind or crosswind. Hence, hot-air balloons are not capable of releasing insects along pre-determined release paths.

A further limitation is that they can only fly in good weather conditions and at very low wind speeds. Hence, despite the low costs, hot-air balloons are not suitable as aerial release vehicles for SIT programmes.


Dirigibles, airships, zeppelins or blimps are similar to hot-air balloons in that they are lighter than air vehicles, with the majority using helium to provide lift. Helium is an inert gas and unlike hydrogen which was used in the past, it is non-flammable. The main distinction between dirigibles and hot-air balloons is that dirigibles are handled and steered by an engine, which provides precise control and power for on-board systems and hence, do not suffer from the navigation problems of hot-air balloons. Dirigibles may be controlled either by a pilot or by unpiloted systems, with unmanned dirigibles being easier to control remotely compared with fixed-wing aircraft as they have a natural tendency to stay aloft.

There are three different categories of dirigibles depending on their architecture. However, since the 1970s, non-rigid dirigibles, which do not have an internal skeleton, have been the preferred type. These designs have better operation during bad weather conditions as the stresses imposed on the airship are absorbed by the flexible envelope (Airship Management Service [AMS] 2009). During the 20th century, dirigibles were replaced by airplanes as the mainstream method of transportation owing to their faster velocity. In addition, the availability of helium was very limited, and the use of flammable gases for airships was dangerous. Today, the high price of oil coupled with the advancement in helium production and material technology means that dirigibles have once again become cost effective, and for some situations, they represent a financially viable alternative to fixed-wing aircraft. Moreover, computerized modelling programmes have further optimized the design of envelope size relative to payload requirement, especially for smaller load requirements, thereby becoming potentially suitable for SIT requirements. The initial cost of an airship is considerably less than that of any comparable airplane. According to one manufacturer (Lindstrand Technologies Ltd., Oswestry, Shropshire, UK), the cost of the new hybrid model ranges from US $80 000 to US $300 000 depending on the required features.

The operational costs are also substantially lower than current SIT aircraft mainly because of the increase in fuel economy – the cost of aviation fuel is less than half of a comparable length flight on a fixed-wing aircraft. In conventional aircraft, the lift force required to keep the aircraft in the air is a function of the forward velocity, which is provided by the thrust of the engine. However, in lighter than air vehicles, the gas (usually helium) provides the lift to keep the dirigible aloft; hence, engine power is only needed for forward speed and directional capabilities and does not help in the generation of lift. Other operational costs such as maintenance are also noticeably lower, as the dirigible only requires maintenance checks 2–3 times a year and does not need to be sheltered in a hanger. The vertical takeoff and landing eliminate the need for an airport and runway, and hence, the airship can be permanently moored at the fly emergence and release facility, thus eliminating travel times to the airport. Advances in envelope material technology (lower permeability) have also further lowered the operational cost of the airship as this reduces the frequency of helium replacement. This procedure now needs only to be performed every 10 years (in 2009 the cost of helium was approximately US $6/m).

In the past, dirigibles filled with hydrogen did not have high safety levels because of the flammability of this gas. However, the availability of inert helium gas, enhancements in manufacturing skills, quality of materials and knowledge of aeronautics have led to more reliable airships and a safe flying record. In case of engine failure, only used to change direction and to provide forward speed, these lighter than air vehicles will remain afloat and slowly descend with the on-board payload, preventing loss of life and equipment.

Dirigibles are also much more environment-friendly than aeroplanes owing to the lower fuel consumption. For example an airship with a 500-kg payload capacity has 180 hp engine power (2 × 90 hp), while a Cessna 206 aeroplane with the same payload capacity requires 300–350-hp engine power. Therefore, airships also have lower levels of noise pollution, making them more suitable for operations near or over urban/sub-urban areas.

The dirigibles must always be kept inflated because of the degradation of the envelopes, which occurs if they are frequently inflated and deflated (Newbegin 2003). Once inflated the dirigible stays inflated for about 10 years with only minimal top-ups of gas. This brings about a minor logistical problem of delivering the dirigible to its mooring station. They are normally flown to the target area; however, long-distance delivery could be a major issue because of their slow cruising speed (70 km/h). If ground transportation of a deflated dirigible is used instead, the deflated dirigible would fit in a large trailer. Another potential issue is the buoyancy of the aircraft, making it very slow to change altitude (Khoury and Gillet 1999); it hence takes a long time to land. However, the desired release altitudes for an SIT programme ranging from 100 to 800 m and the fixed altitude release do not pose a major issue.


Microlights are in essence light-weight and slow-speed aircrafts. Used mainly for recreational purposes, they evolved from hang gliders – the first models being hang gliders with small engines attached. This category of transport was officially recognized by the United States Aviation Agency in 1982 (Ultralight Aircraft 2006). Since then, the advances made in technology with higher performance at relatively affordable prices have made them an attractive alternative to other fixed-wing aircraft. The payload regulations on microlights varies in different countries; in Europe and India, the total weight is required to be below 300 kg for a one-seat version and 450 kg for a two-seat model (Steiger 2009).

There are several forms of microlights such as the fixed wing microlight, flex-wing trike – a type of powered hang glider and the powered parachute – a parachute on top of the motor trike; in some countries, gyrocopters or gyroplanes are also considered microlights. Of these different forms of microlights, the MAGNI M16 gyrocopter is currently in operation at the ‘False codling moth’ SIT programme in Citrusdal, South Africa; it has a purchase price of about US $130 000 and an operational cost of US $120/hr (Hofmeyr personal communications 2009). Gyrocopters operate on the principle of autorotation, where the rotor is not powered but relies on the wind from the forward motion of the vehicle to turn the rotor and hence create lift. Forward propulsion is generated by a propeller engine.

Structurally the fixed-wing microlight is the most similar form to the conventional fixed-wing aircraft and is in essence a scaled down version of one. Both fixed and operational costs are much lower than for commercial fixed-wing aircrafts. The initial cost of purchase is low, and assembly of the air vehicle does not require highly skilled expertise. A standard specification microlight costs between US $15 000 and $65 000 ( 2009), while more advanced delivery systems such as the LH-10 Ellipse will cost only US$100 000 (LH Aviation – Introducing LH-10 Ellipse). The operational costs are also very competitive as the fuel consumption is particularly low, in the region of 12–15 l of fuel per hour, even though they can fly at speeds of up to 195 km/h. According to the official Indian Micro-light Association, the operational cost of a microlight is about US $37.5 per hour (Microlight Aviation Private Ltd. 2009). Unlike commercial fixed-wing airplanes, there are less stringent regulations on the maintenance of the microlights, and consequently, owners prefer to register their air vehicles as microlights rather than aircrafts when they meet both specifications. Technically, microlights can achieve a 700-km flight range, although regulations in several countries have lowered this range, but even with the strictest regulations, the required range for an SIT programme is achievable. Furthermore, the short distance needed for landing and takeoff means that a runway is not a necessity and 100–150 m of hard surface is sufficient, hence avoiding expensive airport fees. Hangers are also not necessary as the wings can be folded; there is only the need for a relatively small covered place for parking (i.e. a small garage) (Microlight Aviation Private Ltd. 2009).

The main drawback of microlights is in the regulations governing them. In the USA, there are strict regulations on microlights to ensure the safety levels of their flight and to comply with security policies – microlights are only allowed to be used for the purpose of recreation and sport (Federal Aircraft Regulations [FAR], 2009 Part 103). Furthermore, in the majority of countries, they are not allowed to fly either over populated areas or within controlled airspace (160–1600 m above sea level) and hence, cannot be used when the release programmes are near urban areas or need to fly within controlled airspace. In most countries, they are only allowed to be flown under Visual Flight Rules, i.e. in daylight and good weather (high visibility) to ensure safety levels (Steiger 2009). A major disadvantage of microlights is their poor safety records. The safety records of microlights are still much lower than current military or civilian aeroplanes. This is probably due to the relatively low investment in their development because of their limited market for leisure and sport usage, and their primary use for sport/leisure purposes.

Remotely piloted aerial vehicles

The philosophies behind UAV and radio-controlled aircraft (RCA) are very similar. The distinguishing factor between the two is the level of sophistication and the principle of the control system. The pedigree behind UAVs originates from RCAs, but UAVs have had far more investment in terms of research and development, and are, in many cases, completely autonomous. On the other hand, RCAs are used mainly for recreational purposes and although they have a dedicated following with a large community of enthusiasts, the available designs are rather simplistic.

Unmanned aerial vehicle

Unmanned aerial vehicles are defined by the US Department of Defence (DOD) as ‘aerial vehicles that are heavier than air; the lift is created by the engines, and they are capable of being piloted remotely by ground crews or fly autonomously following a pre-determined flight path (Bone and Bolkcom 2003). The US Defense Technical Information Center defines unmanned as being ‘guided without an onboard crew’ (Cupp et al. 1998) and has recognized the cost-effectiveness of UAVs replacing manned aircraft missions, especially when the job is ‘dull, dirty or dangerous’.

Although the main utilization of UAVs is in military fields for reconnaissance and intelligence gathering, in the last 10 years the development of UAV systems has accelerated and they are starting to become sufficiently cost effective to be used in the civilian sector. However, there are still very few models that are commercially available.

The operational costs of UAVs are less than the conventional aircraft used in SIT programmes, as the fuel consumption of a medium-sized UAV is on average only around 20 l/h, due to the fact that there is no on-board crew to transport. The salary of the ground-based pilot is also cheaper than an on-board one.

Despite these merits, the main drawback of current UAVs is their exorbitantly high fixed cost – with the majority of models that would fulfil the SIT requirements costing over US $2 million. This is similar and in some cases even more expensive than the larger fixed-wing aircraft currently being used. This is because of the sophisticated technologies required for military purposes, which would not be needed for an SIT programme. There are, however, cheaper models that are built for civilian purposes, and that are starting to emerge in the marketplace. There are also many projects in universities investigating/building cheaper and reliable UAV systems.

Another disadvantage of UAVs is the relatively high rate of mishap compared with the average manned military aircraft. According to the data from the US Air Force, military UAVs fail an average of 4.28 times for every 1 million departures compared with 1.12 for manned aircraft (Tyabji 2007). The main cause of these incidents is human error, which accounts for 32% of total UAV accidents (Hobbs and Herwitz 2005). Unlike conventional aircrafts, the two main reasons behind pilot errors are: (i) the communication software creating a delay in the pilot’s reactions, which are most crucial during emergencies when fast reactions are required, (ii) pilots no longer have the sense of the flying environment (Tyabji 2007). However, even though the mishap count is higher in UAVs compared with the conventional aircrafts, the outcome of the incidents are not comparable, i.e. the irreplaceable loss of a human life compared with the loss of only machinery. To minimize the mishap rate caused by technical malfunctions, a system called Preventive Maintenance Check, where all aspects of the UAV and its operation are checked, is required before each flight (Hobbs and Herwitz 2005). Because of this high mishap rate and the fact that UAVs are a relatively new technology, aviation restrictions in many countries limit the use of UAVs in certain levels of controlled airspace or over busy metropolitan areas.

Radio controlled aircraft

In complete contrast with UAVs, RCAs are an extremely low-cost solution. However, the only method of control is through visual navigation. Aviation authorities all over the world dictate that the ground-based pilot must at all times, keep visual contact with the aircraft. This requirement is due to the fact that even though the control is done via radio waves, RCAs do not possess on board instrumentation that communicate flight data such as air speed, altitude or exact position down to the pilot. Even though the initial cost of purchasing an RCA is extremely cheap (approximately US $1500 for a model that can carry a 40 kg payload), they cannot fulfil the range requirement because of the visual contact regulations. For small programmes under conditions of favourable topography, one solution to the visual range requirement is to divide up the target area into several ‘release sections’, which are small enough to maintain visual contact with the RCA. The Moscamed programme in Juazeiro, Brazil has developed an in-house RCA with a payload capacity of 1 million flies per flight, ∼5 kg of C. capitata (Moscamed Brasil 2007; Cavalcanti et al. 2009).

While RCAs are very low cost, they have a high accident rate and require additional ground transportation infrastructure not only to transport the RCA, crew and flies to the target area, but also to ferry them to the other release sections within the target area (Malavasi personal communication 2009). This also increases personnel requirements as a truck driver may be also required. Nevertheless, for small areas in flat terrain with good road infrastructure and low labour costs, a fleet of RCAs to achieve the required release capacity can be a cost-effective solution. In terms of fixed maintenance and operational costs, RCAs are by far the cheapest option; the RCA at Moscamed Brazil has an operational cost of US $16/flight hr (Malavasi personal communications 2009). Although RCAs are not able to fulfil all the requirements of an SIT programme, they do have some useful aspects and components that can be integrated into an unmanned solution for special situations.


Having generated the key specifications for aerial release vehicles for SIT programmes, it is evident that currently used aircraft, although technically proficient, often vastly exceed these specifications and are consequently not very economical in performing the tasks required to fulfil programme needs. Other forms of aerial transport were surveyed, and five alternative aerial vehicles were reviewed for their suitability and potential to make significant cost savings compared with currently used aircraft (results summarized in tables 2–9 and compared in terms of costs in tables 10 and 11). These alternatives were analysed for compliance with SIT insect release requirements pertaining to flight/weather conditions, flight range, release airspeed, altitude, adherence to flight path, payload, and costs (both fixed and operational). In addition, safety levels, air and noise pollution and general regulatory restrictions were reviewed. Of these five alternative aircrafts, two are lighter than air, while the other three are heavier than air. Of these, only microlights, UAVs and dirigibles were found to meet, or have the potential to meet, the key specifications. Hot-air balloons and RCAs did not meet these specifications because of navigation and range limitations. Microlights, which are the air vehicles most similar to aircraft currently used in SIT programmes, can bring significant fixed and operational cost savings. While they are technically capable of fulfilling the SIT requirements, restrictive governmental regulations pertaining to flights in or near urban areas and within controlled airspace, their lower safety records, and their flight restrictions in bad weather conditions, may prevent them from becoming viable options in most situations. Nonetheless, a gyrocopter, a type of microlight, is currently being used to release sterile false codling moths over citrus orchards in South Africa (The Packer 2009).

Table 10. Comparison of alternative aircraft in terms of fixed cost requirement for aerial sterile insect release
Alternative aircraftFixed cost comparison for aerial sterile insect release
  1. SIT, sterile insect technique; UAV, unmanned aerial vehicle.

Currently used aircraftCurrently, aircraft used in SIT programs cost from US$280 000 to 1.1 million, while helicopters, which are generally used in mountainous regions, cost around US$1.5 million. However, operations are usually contracted out at US$465/h for at least 35 h/week or US$2325/day
UAVsCurrently available military versions cost between US$4–8 million. However, there are many ongoing research projects, aimed at producing low cost UAVs. Leasing rates of the US Army Altair UAV is US$2000/day
AirshipsBetween US$200 000 and 4 million per unit, depending on specifications
Radio-controlled aircraftStarting from US$1000 per unit. By far the cheapest option
Hot-air balloonsApproximately US$20 000 per unit
MicrolightsAbout US$18 000–50 000 per unit. This option is low cost because of the relatively low technical requirement of the models. However, as the standards vary from country to country they are not internationally recognized and hence difficult to use commercially
Table 11. Comparison of alternative aircraft in terms of operating and maintenance costs for aerial sterile insect release
Alternative aircraftOperating and maintenance costs comparison for aerial sterile insect release
  1. RCA, radio-controlled aircraft; UAV, unmanned aerial vehicle.

Fuel consumption
 Currently used aircraftHigh. 80 l/h
 UAVsLow. 20 l/h
 AirshipsLow. 17 l/h
 RCALow. They only need the batteries recharged or use small amounts of fuel. Current models are not capable of lifting the required payload of 60 kg, hence an upgraded version with larger engines is expected to have similar fuel consumption to a UAV ∼ 20 l/h
 Hot-air balloonsMedium to High. 67 l/h
 MicrolightsLow. 15 l/h
Labour requirements
 Currently usedHigh. One pilot and often one fly disperser are needed on board during operation. A group of ground crew who are responsible for the communication on the ground, inspection, takeoff, landing and maintenance
 UAVsMedium. For military purposes, it requires at least 3 ground crew members – the ground pilot who is in charge of aviation, navigation and sensor operation; the ground searcher, and the incident commander to manage the search effort. For civil purposes, this can be lowered to 2 people
 AirshipsMedium. One pilot and 2–3 ground handlers required during takeoff and landing
 RCAMedium. One ground pilot and 2–3 ground handlers for transport to the target area, assembly, takeoff and landing
 Hot-air balloonsHigh. They should have one pilot to operate the hot-air balloon, who is required to have the pilot certificate of ‘lighter than air-free balloon’ from the Federal Aviation Administration (FAA). Six or more ground crew are required during takeoff and landing for a balloon with 105 000 cu ft capacity
 MicrolightsLow. One pilot and one member of ground crew during takeoff and landing.
Maintenance costs
 Currently used aircraftHigh. because of safety regulations, a large amount of the costs go to maintenance fees and regular safety inspections
 UAVsHigh. In view of the high rate of mishap for UAVs compared with conventional aircraft, a Preventive Maintenance Check is required before each flight. This includes checks on the vehicle, communication system and ground station
 AirshipsLow. Inspection and maintenance carried out twice a year, thus dramatically reducing operational costs. Helium replaced every 10 years at $6/m3. Minimal amount of top-up gas required during maintenance
 RCALow. Technically simple structure/system hence cheap to maintain and even changing complete components can be done at a relatively low cost
 Hot-air balloonsMedium. After 500 h of operation the major maintenance cost is the re-coating of the envelope at US$5/m2 which lasts for a further 100 h. A new envelope costs ∼US$18 000. Additionally, simple routine inspections of the main components, such as the fuel system and burner need to be performed
 MicrolightsMedium. It needs regular inspection of the vehicle for the safety level of the pilot to be maintained. However, there is no legal regulation for this issue; therefore, it depends on users. This is definitely cheaper than current aircraft, as there is a lower level of regulation and mandatory inspections. Additionally, the hardware is cheaper than the aircraft, which makes it less costly to replace when broken

With the exception of purchase price, UAVs meet and in most cases surpass the SIT aerial release specifications. These remotely piloted or autonomously controlled systems could provide substantial cost savings in terms of take-off weight and hence, fuel consumption, and could be a potential option were it not for the high purchase price. Currently, there appears to be little flexibility in modifying UAVs as the majority are built for military purposes. On the other hand, one option could be to utilize the very large, competent and accessible fan base that are constantly modifying and improving the concept of RCAs to custom-build a long-range RCA with autonomous capabilities. Dirigibles meet all the specifications and can be adapted to have unmanned capabilities, hence eliminating the need for an on-board pilot on each and every mission and further reducing operational costs. Additionally, the lighter-than-air working principle makes dirigibles much easier to pilot compared with air vehicles that require engine power for both thrust and lift, and also reduces the catastrophic failure rate. It has also been demonstrated that remotely piloted/unmanned aircraft can provide significant savings over manned vehicles, particularly in terms of lower take-off weights, fuel consumption and other operational costs, e.g. pilot fees, maintenance checks, insurance fees. Hence, this study proceeds to make two recommendations for suitable alternative air vehicles: the first being an unmanned/remotely piloted dirigible and the second a custom-built UAV based on upgrading an RCA to have long-range remote control.

Proposal 1 – custom-built long-range RCA

Owing to the similarities between UAVs and RCAs, combining the advantages of both does not cause major compatibility issues. This can be carried out by either scaling down current UAVs models by introducing smaller engine, simplifying the navigation systems and using lower-grade airframe materials, or by upgrading an RCA with larger engines and installing a sophisticated autonomous remote control system on-board to navigate it.

Owing to the high cost of current UAVs, the custom-built upgraded RCA option appears to be the more cost-effective route. A twin piston engine with a two-blade pusher propulsion system would allow the air vehicle to carry payloads of up to 60 kg, travel at speeds in excess of 110 knots (Aird 2007) and fulfil the 300-km-range requirement. A ballistic parachute system, which can parachute up to 272 kg back to the ground (Cupp et al. 1998), would also need to be included as a safety feature. Alternatively, dividing the release area into small sections would allow, in smaller SIT programmes with favourable topography, the use of a fleet of smaller, shorter-range custom-built UAVs with reduced payload capacity. This would be economically viable in such situations because of the low cost of these vehicles.

Proposal 2 – unmanned dirigible

An unmanned dirigible would further reduce the operational costs of these airships because of the lower fuel-consumption rates as a result of not having to carry a pilot. Furthermore, because of the buoyancy of helium providing the lift forces, and the fact that insect release is carried out at a fixed altitude, the remote control of these vehicles is much simpler compared with aircraft that require additional engine power for both lift and directional capabilities (e.g. RCAs or UAV helicopters). However, there are very few unmanned dirigibles in operation and the majority of which are designed for surveillance and not for their load carrying capacity (one example is Lindstrand Technologies’ GA 22 operated by the Spanish Ministry of Defence), and hence, the capital cost can be rather high, but would still be lower than the average yearly cost of aerial release contracts of many SIT programmes (USDA APHIS 2009). The operating costs, however, would be significantly lower than for fixed-wing aircraft.

The slow average cruising speed of 70 km/h can be a drawback of the use of dirigibles for SIT programmes. This is a disadvantage particularly in situations where the required average sterile insect release densities are low, as is the case for some pest insects, for example in screwworm (2500/km) or even lower in tsetse (>100/km) programmes, compared with high rates in C. capitata and other fruit flies (ca. 200 000/km) (Hendrichs et al. 2005). As a result, release rates have to be low, and often the maximum chilled holding time is the governing factor to avoid affecting sterile insect quality (Hendrichs et al. 2005).

Fortunately, there are several commercially available airships that can reach the required release speeds of ca. 150 km/h or more, while the record speed is 395 km/h. Furthermore, release systems are under development that reduce chilling time only shortly before release of some holding containers, while keeping temperatures only cool for the others during release and transport to the target area (FAO/IAEA 2009). The other possible drawback of dirigibles is the slow process of altering altitude; however, sterile insects are usually released at a fixed altitude. Landing is usually a time-consuming process and requires several ground personnel; however, a ‘suck down’ motor or a landing system consisting of an anchor, cable and electric motor that is used to winch the airship down for landing, would alleviate this problem.


Current SIT aerial release programmes use small fixed-wing aircraft with specifications far beyond those necessary to fulfil the requirements of SIT programmes and hence, are not cost effective. An extensive survey of alternative forms of aerial transport was performed and five alternative aerial vehicles were identified as having the potential to make significant cost savings compared to the current fixed-wing aircraft used in SIT programmes. Of these, two are lighter than air, while the other three are heavier than air, with two being remotely piloted. The different alternatives were evaluated against the requirements of current SIT programmes and also compared in terms of costs. Two fulfilled all requirements, but neither solution achieved its full potential of cost savings. Hence, we present two optimized proposals, the first a custom-built UAV, or long-range RCA, and the second an unmanned dirigible. Both of these optimized proposals present major cost savings in terms of pilot wages, maintenance and fuel consumption, with the largest cost saving made by eliminating the need for a pilot and thereby reducing the take-off weight, the largest source of fuel consumption. Considering the fact that the cost of current aerial release contracts is on average 40% of the yearly operating budget of the centres that receive sterile pupae, emerge, feed and hold sterile flies close to sexual maturity and then release them, the financial savings of implementing either of these options will have a highly significant impact on the overall cost of SIT operations. Furthermore, reducing the cost of aerial release will not only improve the cost efficiency of SIT programmes, it will also increase the wider applicability of aerial release methods.