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Keywords:

  • canegrub;
  • Dermolepida albohirtum;
  • Queensland;
  • sugarcane

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BIOLOGY AND LIFE CYCLE
  5. CONCLUDING REMARKS
  6. ACKNOWLEDGEMENTS
  7. REFERENCES

The greyback canegrub, Dermolepida albohirtum, is a major sugarcane pest that occurs between Mossman to Sarina in Queensland, Australia. Over a period of more than 100 years, BSES Limited has conducted extensive field and laboratory studies on this pest species and information is available in several reports, articles and scientific papers. This document summarises much of the published work on D. albohirtum as well as anecdotal observations on its biology, ecology and management. D. albohirtum has an annual life cycle, and adult beetles usually lay eggs in cane crops around December–January, by which time the crop has become well advanced making pesticide application difficult. Hence, fields that require chemical treatment need to be carefully selected prior to beetle flights and egg laying. To assist in field selection, a prediction system is currently being developed to ensure that chemicals are strategically applied in areas likely to receive grub damage. Knowledge of the life cycle and population dynamics of this pest is essential in developing robust forecast systems. This document is designed to serve as a comprehensive reference for both researchers and cane growers seeking detailed information on the biology, ecology and management of this pest species.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BIOLOGY AND LIFE CYCLE
  5. CONCLUDING REMARKS
  6. ACKNOWLEDGEMENTS
  7. REFERENCES

The greyback canegrub (Dermolepida albohirtum) is the most damaging sugarcane pest in Australia, with estimated annual losses of up to $10 million and with periodic outbreaks where losses may reach $40 million in damage and management expenses (Chandler 2002). Greyback canegrub occurs from Mossman to Sarina in Queensland in a wide range of soil types including loam soils along the Burdekin and Herbert rivers and red volcanic soils between Tully and Mossman (Allsopp et al. 1993). The greyback canegrub has a 1-year life cycle and the larval stage has three instars that feed on the root mass in the soil leading to reduced growth, stool tipping and ultimately plant death (Allsopp et al. 1993). Knowledge of grub behaviour and dynamics is available in the literature; however, information is scattered in many sources. This paper attempts to collate all existing information on greyback population ecology, with the aim of understanding factors governing their dynamics in nature. This knowledge should improve our capability to predict changes in grub dynamics and possible population outbreaks. Ultimately this information will provide growers with decision-support tools and enhanced means of managing the pest effectively.

BIOLOGY AND LIFE CYCLE

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BIOLOGY AND LIFE CYCLE
  5. CONCLUDING REMARKS
  6. ACKNOWLEDGEMENTS
  7. REFERENCES

Adults

Detailed information on greyback biology and life cycle is available from Jarvis (1933). Adult D. albohirtum beetles fly following the first fall of spring rain, which normally starts in October. Beetles will emerge until January and in some cases they keep emerging until March, but this depends on the onset of spring rain; if rain is late (January) then beetles may still be emerging in March, but if rain comes early (October) then flight will cease around December. In areas relying on rain, a fall of 38–50 mL of rain can trigger emergence, and beetles may not emerge at all until it rains (Buzacott 1947). Alternatively, irrigation was found to trigger adult emergence in areas mainly relying on irrigation such as the Burdekin (Robertson & Walker 2001).

The sex ratio of adults in the field is normally around 1:1 when beetles start emerging, but it gradually becomes female-biased (70–80% females) (Bell 1934; Mungomery & Buzacott 1935).

Beetles emerge from different fields and head towards trees where they aggregate and feed for 10–14 days. This is considered crucial to their subsequent fecundity (Illingworth & Dodd 1921). Females fly back to fields to lay eggs, and can fly up to 1.6 km (Mungomery & Buzacott 1935). There are anecdotal observations that some females may have a ‘homing instinct’ and thus head back to where they originally came from hence damage keeps recurring in the same spot. However, another proportion of the population is likely to disperse, and may end up ovipositing in unexpected spots such as under nearby lawn, other crops such as pawpaws and peanuts and even in dead corn stubble or in fallowed fields (N Sallam and D Burgess, unpublished 2006; K Chandler pers. comm. 2006). Adults live for about 8 weeks, and longevity is negatively correlated with temperature. Females tend to live longer than males (Illingworth & Dodd 1921; Jarvis 1933).

There is ample evidence to show that adult greyback prefer to fly to tall crops. Illingworth (1918) and Mungomery (1949) reported that egg-laying females may be preferentially attracted to tall and dense sugarcane crops. While Ward and Cook (1997) and Logan et al. (2000a) showed that early planted and early cut cane is more frequently damaged by greyback than other classes of cane in the Burdekin. This led to the development of the ‘trap crop’ concept. A trap crop is a section of a field that was either planted or harvested earlier than the rest of the crop, and so stands out in comparison to the rest of the crop at the time of beetle flight. Trap crops, in several cases, were shown by Horsfield et al. (2002) to harbour more grubs/stool compared with adjacent blocks in selected areas in the Burdekin and the Herbert regions, and similar observations were also made by Sallam (2008) in Far North Queensland (FNQ). In addition, Ward (2003b) showed a clear negative relationship between date of planting or harvesting and subsequent damage in the Burdekin, and concluded that sugarcane height is the primary determinant of where adult beetles lay their eggs.

Eggs

Females lay 23–36 eggs at depths between 20 and 37.5 cm, and eggs hatch in 10–14 days (Illingworth & Dodd 1921; Jarvis 1933). Females may lay fewer eggs in drier or heavier soils. Ward (2003a) showed that, in a choice experiment, female beetles laid fewer eggs in the heavier silty clay soil than in light loam and silty loam soils, and that hatchlings suffered higher mortality in the silty clay soil. Ward (2003a) also recorded higher egg mortality at field capacity than at lower moisture content in silty clay soil, while moisture content did not affect egg mortality in lighter loam and silty loam soils. The same author also noted that adult females retained more eggs when they were introduced to silty clay soil in a ‘no-choice’ experiment compared with other lighter soils. Ward (1998) and Ward and Rogers (2007) hypothesise that, since high soil moisture adversely affects egg development mainly in the non-endemic heavy soil types (such as silty clay), adult females exhibit clear oviposition preference against high soil moistures in that soil type, while no or minimal preference is demonstrated for the endemic lighter soils where eggs and larvae achieve better survival under a wide range of soil moistures.

Generally females tend to lay the majority of eggs (24 eggs) in one occasion and in a single cell. Although they may come back and lay more, the second batch contains fewer eggs (5–10 eggs) (Jarvis 1933). Illingworth and Dodd (1921) noted that some females may be able to oviposit several lots of 20–25 eggs over their lifespan. However, they made the assumption that gravid adult females found on trees in February (3 months after the first emergence took place) had previously laid eggs. This may not have been the case because these females may have emerged late and were relatively fresh when they were examined by the authors.

Larvae

Larval durations for first, second and third instars are 4 weeks, 4–5 weeks and 7 months, respectively (Jarvis 1933), with grubs showing faster development in lighter soil types (Illingworth & Dodd 1921). First-instar grubs make their way towards the soil surface when they emerge (Illingworth & Dodd 1921). Robertson and Walker (2001) showed that first instars are more laterally dispersed than later instars, with later stages aggregating gradually under the stool area during the first 2–4 months of development. They inferred that larvae feed on organic matter and fine roots as first instars before they move towards the centre of the row as they develop into second and third instars and travel deeper in the soil profile. Illingworth and Dodd (1921) also recorded that second-instar larvae descend in soil and aggregate under the cane stool.

Very saturated, water-logged soils cause high mortality of hatchlings, and grubs are more destructive in well-drained land (Illingworth & Dodd 1921). Ward (2003a) suggests that heavy silty clay soils become more difficult for grubs to burrow in as they get wetter. However, there is evidence that third-instar grubs are capable of surviving prolonged water logging conditions, Robertson and Walker (2001) showed that larvae can be found at shallower depths when soil is saturated, while Illingworth and Dodd (1921) noted that grubs move upwards in the soil profile under very wet conditions, and in some cases come out of the soil completely and hide under trash or plant material. Moreover, Jarvis (1933) found that grubs were able to survive for up to 32 h, with a few grubs surviving for 41 h under water when individual third-instar grubs were placed in glass test tubes containing water and then removed after different time intervals.

The average grub number/stool in canefields varies significantly due to several factors, including time of year, pesticide use, soil type, cane cultivar, soil moisture, grub pathogens and infestation pressure in a particular year. Jarvis (1933) estimated an average of 70 000 grubs per acre, which approximately translates to 15 grubs per stool or more. He also estimated a mortality rate of 5–6% in the late larval stage due to ‘parasitic and predaceous insects and other enemies’. Ward and Robertson (1999) recorded a range of 3–6 grubs/stool as the ultimate number of grubs that eventually settle around a stool towards the end of the season. Other studies record up to 14 grubs/stool late in the season (Volp 1947), while the unusually high mean figure of 30 grubs/stool was recorded by Bell (1934).

It is possible that fields with a high presence of weeds could have a greater larval carrying capacity than weed-free fields (Ward & Robertson 1999). This has also been observed by Jarvis (1933), who noted that a luxuriant growth of weeds among the stools is strongly attractive to egg-laying females. Most studies on grub numbers are conducted during March–May. This measures the functional number of grubs that damage cane, but the figure that determines the following beetle population will be the number present in September–October. A decline in grub densities is expected to occur later in the season but is difficult to quantify. This is due to the difficulty of finding late larval stages or pupae in August–September as grubs dig deep in soil in preparation for pupation. It is well established that grub densities decline in soil over time (Robertson et al. 1997a; Robertson & Walker 2001), and this could be due to dispersal, combat mortality, or death due to pathogens and other unknown natural mortality factors. Bell (1934) also reported a gradual decline in grub numbers during winter months.

Illingworth and Dodd (1921) stated that grubs mainly feed on organic matter, and when they do not get their requirements from soil they start attacking cane roots. Hence, the authors concluded that grubs are only excessively destructive to sugarcane when it is growing in soils deficient in organic matter. It could be argued, however, that richness in organic matter may also induce good plant growth and enhance grub and disease tolerance, masking the grub caused damage.

Pupae

By May–June, fully grown third-instar grubs stop feeding and start digging deeper in the soil in preparation for pupation. Jarvis (1933) found third-instar larvae preparing to pupate at depths as great as 60 cm, but pupae are more frequently found between 15 and 40 cm below the surface, depending on soil type and soil moisture. Jarvis (1933) also recorded that the pupal duration is between 4 and 5 weeks. The same author gives varying pupating depths ranging from 10 to 60 cm depending on soil type. For example, the pupal chamber could be shallow in sandy loams or in light soils with a clay or stony subsoil, while it tends to be deeper in volcanic soils. In addition, pupae can be found at a shallow depth if the soil was still moist during July–August at the time when third instars started to pupate. However, very dry conditions afterwards (August–December) will result in high mortality among these shallow positioned pupae (Buzacott 1947). Much deeper pupation depths of up to 120 cm and more were recorded by Girault (1914) and Illingworth and Dodd (1921).

Logan and Kettle (2007) estimated developmental zero for the pupae to be 12.0°C and the thermal constant to be 476 D°, with pupal duration ranging from 26 to 75 days at 30°C to 18°C, respectively. Simulation of development using temperatures recorded hourly at different depths indicated that pupal development would take 2–10 days longer at 20 cm than at 40 cm depth depending on location and date. When pupation was simulated in late August, as is likely in the field, pupal development at 40 cm depth took 48–56 days at Ayr and 58–62 days at Sarina. Illingworth and Dodd (1921) noted that grubs pupate faster in well-drained soil.

Adult beetles are formed by the month of October. Adults usually remain in the pupal cell for up to 4 weeks, and they may also emerge but remain ‘arrested’ in surrounding soil for weeks until rain commences (Illingworth & Dodd 1921; Jarvis 1933).

Effect of weather conditions on greyback populations

The impact of weather on grub dynamics is an area of debate. The following is an attempt to collate all historical observations with more recent studies to understand the possible effect of weather on the different grub stages.

Adults

There are several historical observations on the impact of temperature and rain on beetle emergence and fecundity. Long continued dry weather will sometimes cause very heavy mortality among adult beetles (Jarvis 1933; Bell 1936). Mungomery (1931) stated that ‘droughty conditions, if prolonged into late spring and summer, may cause beetles to die in their cells, or they may be so weakened that reproduction does not take place’, while Buzacott (1947) reported an unusual number of dead beetles being ploughed up during an exceptionally hot and dry summer in 1946–1947. Also, Bell (1935) asserted that:

the great reduction in damage in the far northern section of the cane belt can be attributed mainly to the abnormally hot and dry weather conditions which prevailed during the latter part of 1934, and in the early months of 1935. This hot weather had the effect of killing most of the beetles that had emerged during November, while those that were still in their pupal cells underground were unable to emerge.

Bell (1935) also added that ‘when a fall of rain eventually did occur in late January, the beetles were too weak to oviposit, and a further period of dry weather likewise accounted for them’. Furthermore, the same author recorded isolated small grub spots during that very dry season, and these were attributed to local thunderstorms which had occurred during late September and October. In addition, Volp (1947) reported heavy mortality among gravid females due to dry and hot soil conditions. The same author also attributed lower fertility among females collected in February (compared with those collected in January) to prolonged dry soil conditions during the adult emergence season, which may have caused female sterility. While Mungomery (1952) stated that:

The very dry spring and early summer had a profound effect on populations of the greyback beetle by almost entirely eliminating major flights in the Mulgrave. Further depression of the populations appears to have been affected by desiccation of the eggs and early larval stages.

Furthermore, Bell (1938) reported large beetle emergence following heavy rainfall in the Burdekin and along the Townsville–Ingham line. However, dry conditions followed and significant damage was only seen in irrigated fields, while unirrigated fields remained comparatively grub free.

Eggs

Wilson (1956) stated that:

temporary depletion of grub populations was experienced in some years prior to the use of BHC (Benzene hexachloride) due to desiccation of the eggs when the soil rapidly dried out in the intense summer heat, after beetle emergence and oviposition had been stimulated by an initial, brief, heavy rain.

Similar remarks were also made by Illingworth and Dodd (1921) who confirmed the severe negative impact of soil dryness and high temperature on adult longevity and egg viability.

Larvae and pupae

Buzacott (1947) attributed the shallow depth at which larvae pupated in 1946 to very dry conditions that made it difficult for larvae to move any deeper in soil, and resulting in high mortality during summer months as the drought continued. Similar observations were also made by Mungomery (1947). It is probably the case that extreme dry or wet weather conditions might result in shallow pupation depth.

Effect of weather conditions on canegrub-induced damage

Horsfield et al. (2008) correlated a number of 3-month average climatic factors with subsequent grub damage using data collected between 1987 and 2003 in the Burdekin district. Of the climatic variables evaluated, only pan evaporation was significant, and it was inversely related to the subsequent area of grub damage. On the contrary, Robertson et al. (1997a) regressed the area of canegrub damage against annual rainfall in the preceding 1, 5 and 10 years and against seasonal rainfall over the period from 1921 to 1994 in the Innisfail and Tully districts, and found no relationship between rainfall and subsequent grub damage. The authors therefore attributed significant declines in grub populations to disease epizootics.

In an attempt to explain the role of climatic conditions on grub populations, it is hypothesised that a considerable amount of rain will cause the homogenous bulk emergence of a regional population, including grubs surviving outside sugarcane fields (i.e. in river banks and under wild grasses, etc.). This would result in the emergence of a ‘critical mass’ of beetles, enabling successful mating and ensuring an abundance of egg-laying females (K Chandler pers. comm. 2003). Alternatively, irrigation alone is usually conducted in separate fields and on separate days, which may cause beetle emergence to become unsynchronised, therefore reducing the egg-laying capacity in an area with beetles emerging in low densities. However, it is obvious that rainfall patterns alone do not show a clear correlation with subsequent grub numbers or ultimate crop damage (Robertson et al. 1997a; Horsfield et al. 2008). Alternatively, pan evaporation, which combines the effects of both rainfall and temperature as well as solar radiation and wind, may better explain the impact of climatic conditions on beetles' survival and fecundity. However, it needs to be stated that Robertson et al. (1997a) and Horsfield et al. (2008), by using historical crop damage data rather than grub numbers (which are unfeasible to obtain), assume a strong positive correlation between grub numbers and subsequent damage under all climatic conditions. With the absence of solid data demonstrating this relationship, and bearing in mind the impact of dry weather on the crop itself which may reduce its tolerance to grubs, further work is required to demonstrate a statistically significant relationship between pan evaporation and actual grub densities, especially in areas mainly relying on rain fall rather than regular irrigation.

Effect of diseases, natural enemies, larval combat and dispersal on canegrub populations

Pathogens

Canegrub larvae are prone to a number of pathogens that could in some cases be responsible for complete destruction of local populations. The most frequently encountered diseases are those caused by the fungus Metarhizium anisopliae, the protozoan Adelina sp. and the bacterium Paenibacillus popilliae which causes milky disease. Other less common diseases are caused by the protozoan Nosema sp. and unidentified entomopox viruses (Dall & Logan 2001). In addition, high larval mortality is usually encountered in grubs from FNQ for unknown reasons (Sallam et al. 2003) and further work is required to identify and account for the factors responsible for this unexplained mortality.

Out of all the detectable diseases, the grub pathogen Adelina sp. is the only mortality factor that appears to act in a density-dependent manner (Robertson et al. 1998). Adelina is a disease frequently encountered in FNQ where it is responsible for variable rates of mortality up to 100%. Robertson et al. (1998) showed that over half of the variance in annual mortality in certain plots at Tully (FNQ) could be explained by the level of incidence of Adelina alone. The same authors suggested that Adelina infection levels are strongly influenced by grub density, with high pathogen incidence following high grub populations, and this causes the grub population to crash which then negatively impacts on subsequent Adelina levels. This agrees with Sallam et al. (2003) who recorded complete disappearance of the disease from a block in Tully where it had formerly been present. Sallam et al. (2003) attributed this to either the sharp decline in grub numbers during 1995–1998 following a peak in 1994, which was recorded by Robertson et al. (1998), or soil disturbance resulting from plough-out of the plot, or, perhaps more likely, a combination of these two factors. Robertson et al. (1998) monitored the same block for 4 years (1995–1998), and showed a steady decline of Adelina infection from 40% to 5%, coincident with the decline in grub numbers from an average of 1.5 to 0.1 grubs/stool. Similar results were also obtained by Sallam et al. (2008).

Sallam et al. (2003) also recorded a higher rate of Adelina survival and carry-over under grass-covered soil compared with that under bare fallow. They attributed this to a combination of high soil moisture and relatively cooler temperatures under cover.

It has been suggested that, based on the absence of Adelina in the more alkaline Burdekin soils, high pH soils may not favour the pathogen. However, Dall and Logan (2003) and Sallam et al. (2003) found no clear relationship between disease incidence and pH or levels of organic matter in soil. Soil moisture proved to be a crucial factor in the persistence of Adelina in soil while soil dryness is detrimental to the pathogen. Dall and Logan (2003) showed that 85% of Adelina cadavers (maintained for 20 months under adequate moisture conditions and used as a disease inoculum) were able to infect new grubs, compared with only 22% when the inoculum was kept in dry conditions prior to the test. This could explain the abundance of the disease in the ‘super-wet’ belt of FNQ. This, however, does not explain the frequent abundance of Adelina at Mutarnee south of Ingham, where very dry winters occur and soil moisture drops significantly between seasons.

In the Burdekin, factors such as burning crop residues, long-term intensive cultivation, high soil temperature and the abrupt change in soil moisture between seasons as a result of irrigation cycles could be important in limiting the occurrence of Adelina (Allsopp 2010). The Adelina pathogen is also rarely encountered in Mackay. It might be the case that moist rain forest soils in FNQ are conducive to Adelina occurrence, and this may have resulted in a long-term association between the grub species and Adelina and led to well-established levels of the pathogen in that region. However, more work is needed to examine this theory.

Another frequently encountered disease, especially in FNQ, is that caused by the soil dwelling fungus M. anisopliae. Different strains of the fungus infect different grub species, with the specific isolate FI-1045 infecting greyback canegrub larvae (Robertson et al. 1997b; Samson et al. 1999). The fungus can be grown on rice grains, and this is used as the method to mass-produce it commercially. The product is currently available under the name BioCane™. Application of BioCane™ gives moderate rates of grub control (50–60%), with the possibility of an increase of infection in subsequent crops when cadavers act as a source of infection (Logan et al. 2000b). Metarhizium spore levels following BioCane™ application decline in soil if not augmented by conidia from infected grubs, and may fall below a level incapable of grub suppression after 3 years from initial treatment (Milner et al. 2003). The same authors determined a mean monthly decay rate of spore viability to be between 0.0309 and 0.0835 (mean = 0.0512, or about 60% per year). The exact level of Metarhizium spores in soil that will result in good grub control is unknown, however, under laboratory conditions, Milner et al. (2002) showed an average LC50 of FI-1045 to be 8.7 × 104 conidia/g of soil against third-instar grubs. Bearing in mind the controlled nature of laboratory work, higher spore levels/g of soil will most certainly be needed in the field to have an impact on the grub population.

Sallam et al. (2006) determined Metarhizium spore levels in soils at varying depths from different regions in Queensland with a history of BioCane™ application. Across all plots, Innisfail and Tully showed a significantly higher spore abundance than Bundaberg, with the highest spore count recorded in Tully (6.35 × 105 spore/g of soil), while the Burdekin had significantly lower counts than all other locations, with the lowest spore level recorded there (3.7 × 102 spore/g of soil). It is possible that harvesting cane by burning negatively impacts on spore levels in Burdekin soils, while soils in Tully and Innisfail are probably more conducive to spore viability due to trash retention, high moisture levels and natural richness in organic matter (Lai-Fook et al. 1997; Samson & Milner 1999; Sallam et al. 2003). It is not clear if the alkaline nature of the Burdekin soils has a negative impact on spore levels compared with the more acidic soils of Tully, Innisfail and Bundaberg, especially when no clear impact of soil type or pH on M. anisopliae spores has been proven in Australia or elsewhere (Bidochka et al. 1998; Milner et al. 2003).

Moreover, Metarhizium spores were more frequently found 10–30 cm below the soil surface, possibly indicating where grubs are mostly active (Sallam et al. 2006). Similar observations were made by Samson et al. (2002), who recorded the highest spore levels at depths between 20 and 40 cm in a BioCane™ trial plot in the Burdekin, where the majority of Metarhizium cadavers and late third-instar grubs were found.

Unlike Adelina, Metarhizium does not act in a density-dependent manner. In areas naturally rich in Metarhizium (i.e. Tully), it is responsible for a relatively constant mortality rate (20–30%) over time, despite changes in grub density (Sallam et al. 2003).

Sallam et al. (2003) recorded that Metarhizium spores exhibit a degree of tolerance to soil disturbance, enabling the fungus to act as a long-term regulating factor of low to moderate grub populations.

A third disease caused by the bacterium P. popilliae (Milky disease) kills a small proportion (2–5%) of third-instar grubs late in the season in far north and central regions (Sallam et al. 2008). The role of this disease in regulating grub populations is unknown.

Predators

Illingworth (1921) and Illingworth and Dodd (1921) give a long list of natural predators including birds, bandicoots, lizards, frogs, rats, flying foxes and even hogs and dogs that attack the beetle and larval stages. The authors refer to two species of Ibis (Carphibis spinicollis and Ibis molucca) as very useful predators of grubs. The same authors also list several insect predators such as the larvae of the Asilid fly (Promachus doddi), the larvae of a giant Elaterid beetle (Agrypnus mastersi), a Pentatomid bug (Amyotea hamata) and the omnivorous ant (Pheidole megacephala). The combined impact of these predators is unknown, but not regarded as being significant.

Parasitoids

Illingworth (1921) and Illingworth and Dodd (1921) list a number of parasitic wasps belonging to the families Scoliidae and Thynnidae. However, no established records of the latter family are available to support their role as parasitoids of greyback grubs. Illingworth (1921) refers to two Scoliids in particular as useful parasitoids of the larval stage of D. albohirtum, and these are Campsomeris tasmaniensis and C. radula. Female wasps dig deep in soil and lay an egg on the scarab larva after paralysing it. Illingworth (1921) and Illingworth and Dodd (1921) give varying rates of parasitism ranging between 25% and 60% at Meringa (FNQ). However, the authors refer to a number of hyperparasitoids that may negatively impact on the effectiveness of the digger wasps. In addition, these wasps are rarely encountered in areas lacking in nectar sources. It also seems, based on observations by Jarvis (1926), that these wasps are not host specific, and may exploit other scarab larvae for their development. The wasps are very rarely encountered in the field in recent times, and therefore unlikely to have any impact on the grub population (N Sallam unpubl. data 2010).

Parasitisation of adult beetles by an unidentified Tachinid fly has been noted by several authors (Illingworth 1921; Illingworth & Dodd 1921; Jarvis 1933). Jarvis (1925) refers to an incidence where he collected 200 adult beetles of which 31% produced the adult Tachinid. However, the impact of parasitism on subsequent grub population is probably insignificant.

Density-dependant larval combat and dispersal

In their work on density-stabilising mechanisms of canegrubs in soil, Ward and Robertson (1999) found no correlation between the counts of adults caught in intercept traps or light traps and subsequent counts of larvae in the same or adjacent fields. The same authors also noted that adult mortality is largely replaceable, with grub populations arriving at a similar density per plant late in the season despite significant variations in numbers of egg-laying adult beetles. They attributed this to larval combat that increases with an increase in grub density. This would explain why targeting adult beetles to control the pest has always proven futile (Bell 1938, 1940, 1943; Allsopp 2010). However, Logan and Kettle (2002) did not find any evidence of larval combat between first instars, with availability of food being a critical factor in early instar survival. They recorded high mortality (87%) in high-larval-density situations in combination with low abundance of food in laboratory containers, compared with a mortality rate of 53% when food was abundant. In addition, they noted that groups of six first-instar grubs dispersed quickly within the first 48 h of introduction, with fewer than two larvae remaining in the zone of introduction therefore avoiding aggressive combat. It might be the case that larvae compete in conditions of very high densities per plant, a condition which will limit the availability of food. Larval combat may inflict a degree of mortality, but it may also lead to active dispersal among competing grubs (Allsopp 2010). It is well known that, when a number of larvae are confined to a closed arena in the laboratory, high mortality usually results due to aggressive combat. Such mortality has not yet been confirmed in field conditions. Yet in the absence of larval combat, it is difficult to explain, for example, how similar larval densities can establish early in the season from a much lower beetle population in FNQ compared with that in the Burdekin. Also, how can similar grub densities per plant in the Burdekin and FNQ result in very different beetle numbers in the following season?

The different conclusions by Ward and Robertson (1999) and Logan and Kettle (2002) may be reconcilable considering the fact that larval combat is only one of several factors contributing to density stabilisation. If there is high larval mortality in FNQ, larval densities will decline over time until the beetle stage, while populations in the Burdekin usually suffer far less mortality due to the absence of key pathogens, resulting in densities of immatures that remain high until the beetle stage. Bell (1934) asserts that, to estimate the probable beetle population emerging in a given area, it is necessary to conduct the diggings during the prepupal, pupal and early beetle stage, which is during the months of August–September. Work by Ward and Robertson (1999) relies on grub counts taken in March–April, but these may not be representative of the final grub densities. The actual functional density from which adult populations eventuate is what is in the soil just prior to emergence (usually in October–December), and this is bound to be less than the number recorded earlier in the year, and is also bound to be a higher figure in the Burdekin compared with FNQ. In addition, while grub infestation in FNQ is usually patchy and scattered, the Burdekin suffers greater wide-scale infestations that would result in a significantly higher beetle population on a regional level. It is proposed that, even though the density per stool remains similar in localised areas on a farm or a ‘micro-population’ level, beetle densities on a larger regional scale will be significantly higher in the Burdekin compared with FNQ, resulting in larger areas of infestation with a higher grub ‘macro-population’.

Role of pesticides in population management

Pesticides have been an effective means of suppressing the greyback canegrub larval population and eventually crop damage, and since it is now widely accepted that no beetle killing method is capable of significantly and cost effectively reducing the overall beetle population on a regional scale, all chemical applications currently target the larval stage.

The history of greyback canegrub damage patterns can be divided into three distinct eras: (1) before 1947, when no effective control measures were available and there was widespread crop damage; (2) 1947–1987, when widespread usage of organochlorines (OCs) brought canegrubs under control; and (3) after 1987, an era marked by the loss of OCs and the introduction of less persistent synthetic and biological insecticides (Logan & Allsopp 2000; Logan 2001).

During Period I, grub control relied on the use of fumigants in combination with tolerant varieties (Jarvis 1933; McDougall 1940). However until the introduction of OCs in 1947, growers were unable to effectively control greyback and suffered widespread damage and significant losses. During Period II, there was widespread and routine use of ‘Gammexane’ (gamma-isomer benzene hexachloride), and heptachlor. These products were relatively cheap and persisted well in soil, and effective control of canegrubs was achieved in all cane growing districts in Queensland for almost 40 years (Logan & Allsopp 2000). Towards the end of Period II, increasing environmental pressure led to the banning of organochlorine usage in sugarcane, and suSCon® Blue, a controlled-release formulation of the organophosphate chlorpyriphos was introduced. However, suSCon® Blue proved ineffective at providing long-term control of greyback in the alkaline soils of the Burdekin. Work by Chandler (1997) and Robertson et al. (1998) showed that alkaline hydrolysis and microbial degradation result in accelerated breakdown and premature loss of active ingredient from suSCon® Blue granules. This led to multiple control failures against greyback grub in the Burdekin, which in turn resulted in a rapid reduction in suSCon® Blue application due to lack of confidence in the new product, aggravating the situation. Significant outbreaks of canegrub damage through the 1990s and early 2000s are documented as a result of lack of insecticide application and multiple failures of suSCon® Blue in the Burdekin (Robertson et al. 1998).

Currently, a number of controlled-release and knockdown insecticides are available for canegrub control. It is widely accepted that, while OCs gave 90–95% control of greyback grubs and lasted for several years, suSCon® Blue gives 80–90% control in plant cane, with the possibility of achieving some control in first and second ratoons (50–60% and 20–30%, respectively) depending on soil conditions and product placement in the field, with high-pH soils negatively impacting its effectiveness. Another controlled-release granule (suSCon® Maxi), which contains Imidacloprid as the active ingredient, gives 85–90% mortality in plant cane and possibly 70% and 40–50% mortality rates in first and second ratoons, respectively. Imidacloprid liquid (e.g. Confidor® Guard) gives 85–90% mortality the year after treatment of plant or ratoon cane, with impact doubtful in subsequent years (Chandler et al. 1993; K Chandler pers. comm. 2005).

Role of farming systems in population management

There is evidence that greyback grub populations suffer relatively higher mortality under green cane trash blanketing (GCTB) in comparison with populations under burnt cane (Robertson et al. 1995; Robertson & Walker 1996; Allsopp et al. 2002). Trash retention was also found to increase densities of the earthworm (Pontoscolex corethrurus) and improves the diversity of soil fauna (Robertson & Bakker 2004). In addition, Allsopp et al. (2002) showed that grub numbers were higher under ‘invasive farming strategies’ such as the combination of intensive cultivation + suSCon application + harvesting by burning. This may be attributed to high disease incidence under ‘softer farming strategies’ such as GCTB and reduced tillage of soil. In addition, Robertson et al. (1999) showed that although pesticide application will suppress grub numbers initially, it ultimately results in population outbreaks. The authors attributed this to the fact that pesticides, by rapidly reducing pest numbers, also disrupt the natural cycle of the range of entomopathogenic pathogens, thus making it difficult for natural control to resume its course of action. Incidences of population outbreaks of canegrubs could possibly be minimised if systems conducive to beneficial pathogens were to be implemented on a wide regional scale (Robertson 1998). Other agricultural practices, such as the use of tolerant varieties, manipulating planting and harvesting dates, thorough management of weeds and improving soil health, are all expected to contribute to a long-term suppression of grub populations (Robertson et al. 1995; Robertson 1998). Independent reviews compiled by Potter (1997) and Dent (1997) emphasised the importance of long-term strategies that integrate all aspects of pest control and crop management. The current review agrees with previous reports that relying on one method of control, though effective in the short term, does not offer long-term suppression of the pest's populations. Integrated pest management and comprehensive crop management strategies are required to maintain a sustainable level of grub population suppression.

GrubPlan

Robertson et al. (1995) highlighted the need for a comprehensive decision-support system for canegrub management which takes into consideration alternative grub management strategies, sequential sampling and Economic Injury Levels guidelines to ultimately develop reliable cost–benefit analysis schemes. More recently, a number of growers have experimented with alternative management methods such as trap cropping and altering planting and harvesting dates with some success (Ward & Cook 1997; Horsfield et al. 2002; Ward 2003a). However, recent work by Sallam et al. (2008) showed that grower's attitude towards grub infestation tends to be reactionary rather than proactive, whereby levels of pesticide usage usually decrease following grub populations decline and vice versa. To address this issue, a training program under the name of GrubPlan was developed and delivered to industry in 2001 (Hunt et al. 2002, 2003; Samson et al. 2005). GrubPlan is an integrated pest management approach incorporating all farming practices that contribute to reducing grub damage while using insecticides strategically. This approach proved successful in managing grub populations, and is increasingly gaining favour among cane growers. The GrubPlan program, by relying on a thorough monitoring system and delivering good advice to growers, ultimately minimises the gap between a rise in grub populations and the grower's response (N Sallam unpubl. data 2010).

CONCLUDING REMARKS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BIOLOGY AND LIFE CYCLE
  5. CONCLUDING REMARKS
  6. ACKNOWLEDGEMENTS
  7. REFERENCES

Understanding the biology and behaviour of D. albohirtum is essential to achieving successful management in Queensland cane crops. The greyback canegrub population dynamics are governed by a complex of factors that include climatic conditions, soil types, pathogen levels, farming practices and pesticide use. As none of these factors acts separately, attempts to model greyback dynamics will have to account for the collective impact of all the different factors. With the knowledge currently available on grub biology and ecology, more research is required to study several aspects of the greybacks biology and behaviour. Areas of research to be addressed include:

  • • 
    Emergence rate of adults under different climatic conditions and rainfall and its impact on the proportion of females that mate and then oviposit
  • • 
    Dispersal distance and direction of beetles (and what proportion oviposits in the same field from which it had emerged)
  • • 
    Reasons why adult beetles are attracted to certain fields and not others
  • • 
    Role of crop height in attractiveness in non-Burdekin areas
  • • 
    Proportion of females that mate and then successfully oviposit
  • • 
    Decline in grub densities over time in absence of pathogens and at different grub densities
  • • 
    The role of larval combat as opposed to dispersal
  • • 
    Disease carry-over rates; a transmission coefficient is needed to describe the relationship between pathogen levels and subsequent disease probability and further inoculum production
  • • 
    Identification of unknown mortality factors
  • • 
    Impact of tillage on pathogen levels in soil

ACKNOWLEDGEMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BIOLOGY AND LIFE CYCLE
  5. CONCLUDING REMARKS
  6. ACKNOWLEDGEMENTS
  7. REFERENCES

This work was conducted as a part of a project funded by the Sugar R&D Corporation (SRDC). I thank Drs Peter Allsopp and Peter Samson (BSES Limited) and Dr Les Robertson (formerly SRDC) for commenting on an earlier version of this paper.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. BIOLOGY AND LIFE CYCLE
  5. CONCLUDING REMARKS
  6. ACKNOWLEDGEMENTS
  7. REFERENCES
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