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.
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.
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.
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).