The integration of biological and chemical control components of an integrated pest management strategy for the Larger Grain Borer in Africa

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P. Golob and K.M. Dick
NRI Chatham Maritime United Kingdom

INTRODUCTION

Faced with the sudden increase in losses to their most important staple food crop as a result of the accidental introduction of the Larger Grain Borer Prostephanus truncatus into, East Africa, the Tanzanian Government initiated an emergency control programme which relied heavily on the widespread use of insecticidal dusts by farmers storing maize. The initial control recommendations involved the application of a synthetic pyrethroid permethrin to shelled maize ideally at time of harvest. However other major storage pests such as Sitophilus spp. are less susceptible to pyrethroids than organophosphorus insecticides and to prevent an increase in the pest status of these species a second compound had to be added to the dust applied by farmers. In Tanzania a "cocktail" containing 0.390 permethrin and 1.6% pirimiphos-methyl is now used by farmers, at a rate of 100g/90kg of grain to protect maize and the annual requirement for the dust (given the trade name ''Actellic-Super") has risen to between 350-500t (Mushi A. pers. comm.).

The Government of Tanzania's control campaign has been successful in showing down the spread of P. truncatus but has not prevented neighbouring countries from becoming infested. A similar situation exists in West Africa and it seems probable that over the next decade the pest will spread to most of the maize and cassava producing areas of sub-Saharan Africa. To control and contain the pest using the same methods employed in the emergency programmes would require a massive expansion in the level of insecticide use among rural African communities. For reasons of cost alone this is an unattractive and probably unrealistic prospect. Additionally there is a need to reduce reliance on chemical control in the near future in order to prevent the development of resistant strains from threatening the long term effectiveness of the currently recommended insecticides.

This paper discusses the implications of continued insecticide usage as a component of an IPM strategy both for direct control of the pest and for the success of a classical biological control programme.

DEVELOPMENT OF RESISTANCE TO INSECTICIDES BY STORAGE PESTS

In the only authoritative review of the subject, Champ and Dyte (1976) provided evidence for the widespread development of resistance to insecticides among populations of most of the major post-harvest insect pest species. Although P. truncatus was not included in this study, resistance to malathion, lindane and phosphine was common in strains of the related bostrichid pest, Rhyzopertha dominica. Strains of the major pests of stored maize, such as Sitophilus spp., which were resistant to lindane were found in almost all the countries in which samples were collected, including several in East Africa.

Sample collection for this survey was, however, heavily biased towards large scale storage sites. It can be argued that at the farm storage level, the selection pressure on pest populations is less intense, as few small-scale farmers use prophylactic treatments of insecticide or use insecticide as regularly as managers of centralized storage facilities. In addition, species such as P. truncatus, Sitophilus zeamais and Sitotroga cerealella are relatively mobile. The occurence of significant field infestations and cross infestation between farm stores should result in dispersal and dilution of pest populations and this in turn should reduce the selection pressure on individual populations exposed to insecticide.

In those areas of East Africa where the Larger Grain Borer (LGB) has become the dominant pest of stored maize and cassava, the use of insecticide at farm level has greatly increased (Golob, 1988; Gilman and Nyakunga, 1988). Many of the conditions that now prevail in farm stores where control measures have been taken against LGB go against the conventional wisdom for managing insecticide resistance. The onset and spread of resistance is predicted to be slowest where the insecticide achieves a high kill very quickly and then decays rapidly (Longstaff, 1988). in contrast, the current recommendations for control of LGB at farm level are that maize be prophylactically treated with a long lasting residual insecticide at intake.

The development of resistance is also encouraged if a significant proportion of individuals are subjected to a sub-lethal dose of the insecticide. There is good evidence that this is occurring where farmers are applying insecticides to control LGB. Surveys carried out in LGB infested districts of Tabora region in Tanzania in 1986 revealed that although the majority of farmers were applying permethrin dust to shelled maize at the correct dose, a significant proportion (12 and 30%) were either under-dosing or over-dosing (Table 1). While overdosing is particularly undesirable because of the danger to human health, under-dosing subjects P. truncatus populations to sub-lethal doses of insecticide which they may be able to tolerate and survive.

These surveys also showed that, although the great majority of farmers expressed satisfaction with the effectiveness of the insecticide, in approximately half the stores sampled live LGB were still present and large numbers of LGB appeared to have survived the treatment in a significant proportion of the stores (Table 2).

DEVELOPMENT OF INSECTICIDE RESISTANCE BY THE LARGER GRAIN BORER

The potential for rapid development of resistance to both the pyrethroid and organophosphorus components of a cocktail aimed at controlling LGB and other pests of stored maize has been clearly demonstrated in the laboratory (Golob et al., in press a). Experiments were undertaken with strains of the insect from Mesoamerica, West Africa and East Africa; the original field populations may thus have been exposed to different levels of insecticide application before being cultured in the laboratory. Adults of successive generations confined on maize grain treated with increasing dosages of insecticide dusts survived application rates which would be expected to produce complete kill in the field (Table 3). Up to F5, survival on increasing doses of permethrin treated grain was at least 60%. Survival was reduced in the F7 generation exposed to 7ppm permethrin but still averaged 33% across the three strains tested. The populations were equally successful in developing on pirimiphos-methyl treated grain and at 30ppm there was no indication that this tolerance was becoming reduced.

In order to quantify the change in tolerance of the population which survived exposure to treated grain, adults from the F6 generation of the selected population were exposed to filter papers treated with a range of insecticide concentrations. The dose response curves produced were compared to similar curves produced when the adults from the parent population were similarly exposed. LD₉₉s of selected populations, for both permethrin and pirimiphos-methyl, were greater than those of susceptible parent populations, the difference being greatest for the Tanzanian strain for both insecticides (Tables 4 and 5).

The Tanzanian insects used in this experiment were from the same stock as those used in a previous experiment in which Golob et al. (1985) reported 100% mortality when P. truncatus adults were exposed to maize treated with 2.5ppm permethrin for periods up to 24 weeks after application. When the application was reduced to 1ppm but used in combination with 4ppm pirimiphos-methyl the effect was the same, though the mortality was due principally to the permethrin component. Thus the dose which caused complete mortality of unselected individuals rapidly became ineffective under laboratory induced selection pressure with no immigration of susceptible individuals.

Under most African farm conditions the potential for misuse of insecticide is relatively high and it is not unreasonable to assume that both under-dosing and over-dosing occurs. This situation increases the total length of time for which residues of active ingredient, which P. truncatus may be able to tolerate and survive, will remain in treated stores. Gradual selection of resistant populations will occur unless there is sufficient immigration of individuals which escape exposure to insecticide application i.e. those insects existing either in untreated maize stores, on maize cobs in the field, on dried cassava which remains untreated, or on wild host species in the wider environment.

The use of a mixture of different chemical components to control insects should also act to reduce the potential rate at which an exposed population develops resistance. Nevertheless the demonstrated ability of P. truncatus to rapidly develop resistance to both pyrethroid and organophosphorus insecticides should be taken into account in planning future pest management strategies. These findings reinforce NRI's belief that it is essential to stimulate the development of alternative control procedures which reduce the reliance on contact insecticides.

A number of other control options are under consideration which it is hoped will reduce this reliance (Dick, 1990; Laborius, 1990; Markham, 1990; Rees, 1990). Studies are most advanced with regard to the possible use of Teretriosoma nigrescens, a histerid predator which appears to be closely associated with P. truncatus in Mesoamerica and which has been shown to be capable of markedly reducing populations of the pest in laboratory studies (Rees, 1985). It is the view of NRI and its collaborating institutes that, in order to be effective, a classical biological control campaign involving T. nigrescens will have to be compatible, at least initially, with on-going chemical control measures taken in farmers' stores.

THE EFFECT ON T.NICRESCENS OF INSECTICIDES USED TO CONTROL LGB

The susceptibility of T. nigrescens to insecticides currently used to protect maize in areas of LGB infestation in East Africa has been determined in laboratory experiments (Golob et al in press b). Both in topical application tests (Table 6) and filter paper tests (Table 7) adult T. nigrescens proved much more susceptible to pirimiphos-methyl than P. truncatus. In addition, the predator was as susceptible to permethrin and more susceptible to deltamethrin than LGB.

The greater susceptibility of the predator to pirimiphos-methyl was not unexpected as P. truncatus is controlled only with difficulty by organophosphates. That the predator is also more susceptible than the pest to pyrethroid insecticides is, however, disappointing since it is this class of compounds which are specifically recommended for control of the pest.

The implications of the results are clear. Field strains of T. nigrescens are unlikely to survive within stored maize stocks which have been treated with insecticide in accordance with the current recommended practice for control of P. truncatus.

DISCUSSION

In some parts of East Africa the use of insecticides m protect stored maize has been a well established practice for many years, although resource-poor farmers rarely use insecticides. As a result cases of insecticide resistance have become more common. In Kenya, for example, resistance to insecticides used on stored maize among field strains of Sitophilus spp., Sitotroga cerealella and Tribolium castaneum was already being reported in the nineteen-fifties and sixties (Muhihu, 1986). Given the increased use of insecticides in those regions of East Africa where LGB is now endemic the development of resistance to the recommended pyrethroid and organophosphorus "cocktail" is a distinct possibility. It may be several years, however, before control failures resulting from genuine resistance are commonplace. Until this happens farmers in LGB-infested areas are unlikely to discontinue their use of chemicals as they become accustomed to their use and are satisfied with their effectiveness (even when complete eradication of insect populations is not achieved: Table 2). in this situation, any LGB-specific biological control agent which is to form an effective component of an IPM strategy must be able to cause mortality of the pest without itself being killed by insecticides applied to protect stored maize.

Greathead (1989) lists the options for combining biological control measures in three main categories:

- application of resistant natural enemies;
- selective use of broad-spectrum insecticides;
- use of selective pesticides.

If, to be effective in reducing losses to stored maize, T. nigrescens has to be able to survive and multiply actually within maize stocks, then ability to tolerate the chemicals currently in widespread use will be at least advantageous and perhaps even necessary. NRI is investigating the viability of selecting for resistance to pyrethroids and organophosphorus insecticides in T. nigrescens populations.

The second category, selective use of broad-spectrum insecticides, may represent the most feasible mechanism by which both T. nigrescens and insecticide applications can contribute to the control of the pest. Given that insecticides used at farm level to control storage pests must have a residual action, the opportunity for achieving selectivity through careful timing of applications would appear slight. However, if the predator can reduce LGB infestation levels in maize stores indirectly, by reducing the pest population on hosts outside of the immediate maize store environment then biological and chemical control measures may complement each other.

In Mesoamerica, P. truncatus and T. nigrescens have been caught in pheromone traps in many different habitats apparently unconnected to maize production or storage (Rees et al., 199 0a; Key et al., unpublished data). Given the recent advances in our knowledge of the range of attraction of the pheromone traps (Rees et al., 1990b; Farrell, 1990), it is now clear that P. truncatus has at least one and probably a range of hosts in the wider environment. It is impractical to think of treating wild host plants with insecticide both for reasons of cost and the potential adverse environmental consequences. However, predation by T. nigrescens on P. truncatus populations utilizing hosts other than maize (and this will include dried cassava until recommendations on the need to protect this commodity can be implemented: Golob, unpublished report) may reduce the numbers of the pest which would otherwise be available to colonize maize in fields or in stores.

The current chemical control recommendations advise farmers in Tanzania to shell and treat their maize before placing it in storage. If the introduction of T. nigrescens resulted in a reduction in the frequency and scale of pre-harvest infestations then prophylactic treatment may cease to be necessary (even in areas where maize is heavily attacked by LGB, some farmers have typically waited until significant damage was evident before treating with insecticide (Golob, unpublished data)). Under these circumstances, the maize store itself might initially represent an untreated environment, suitable for the predator, increasing the potential contribution of biological control to the overall management of the pest.

The third category listed by Greathead (1989), the use of selective pesticides such as insect growth regulators or selective strains of Bacillus thuringiensis, is a long term goal for many pest management programmes. These and other new products may ultimately allow a much more sophisticated combination of biological and chemical control components of an IPM strategy. However, to date there have been few examples of successful use of such products in farm storage situations and much basic research remains to be done before their use can be considered.

CONCLUSIONS

In East Africa, the use of contact insecticides will remain a necessary part of the control campaign against P. truncatus for the immediate future. For this reason, at least initially, Teretriosoma nigrescens most effective contribution as a biological control agent may come through its ability to locate and prey on the pest outside of maize stores. This supposition, however, depends on the degree to which an increased level of mortality among populations of the pest on alternative, unprotected hosts acts to reduce the build up of the pest in stored maize. The relationships between populations of the pest attacking stored maize, dried cassava or other hosts may then become an important determinant of the success of the biological control programme, just as they may be an important factor in determining the rate at which insecticide resistance develops in populations of P. truncatus. These relationships remain essentially untested and are thus an important topic for future ecological research.

REFERENCES

Champ, B. R. and Dyte, C.E. (1976) Report of the FAO global survey of pesticide susceptibility of stored grain pests. FAO Plant Production and Protection Series No. 5, FAO, Rome

Dick, K.M. (1990) Biological control of the Larger Grain Borer in Africa: a component of an integrated pest management strategy. pp. 5-15 in Markham, R.H. and Herren, H.R.(Eds.) Biological Control of the Larger Grain Borer - Proceedings of an IITA/FAO Coordination Meeting, Cotonou, Republic of Benin, 2-3 June 1989. IITA, Nigeria.

Farrell, G. (1990) An investigation of the flight behaviour of the Larger Grain Borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae ) in response to synthetic pheromone. M.Sc. Thesis, University of Birmingham, United Kingdom.

Gilman, G.A. and Nyakunga, Y.B. (1988) Control and containment of the Larger Grain Borer - the Tanzanian experience. pp. 147160 in G.G.M. Schulten and A.J. Toet (Eds.) Proceedings of the Workshop on the Containment and Control of the Larger Grain Borer. Arusha, Tanzania, 16-21 May,1988.FAO,Rome Report 2, 209pp.

Golob, P., Changjaroen, P., Ahmed, A. and Cox, J. (1985) Susceptibility of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) to insecticides. I. Stored Prod. Res. 21, 141150

Golob, P. (1988) Current status of the Larger Grain Borer, Prostephanus truncatus (Horn) in Africa. Insect Sci. Appl. 9, 737-745.

Golob, P., Wright, M. and Broadhead, P. (in press, a) Development of resistance to insecticides by populations of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae). in Proceedings of the 5th Working Conference on Stored-Product Protection, Bordeaux, France, 9-14 September 1990.

Golob, P., Broadhead, P. and Wright, M. (in press, b) Susceptibility of Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) to insecticides. in Proceedings of the 5th Working Conference on Stored-Product Protection, Bordeaux, France, 9-14 September 1990

Greathead, D.J. (1989) Prospects for the use of natural enemies in combinations with pesticides. Technical Bulletin No. 117, Food and Fertilisation Technology Center, Taipei, Taiwan, 7pp.

Laborius, G.-A. (1990) Biologically-integrated control of the Larger Grain Borer: GTZ project research program. pp. 16-24, in Markham, R.H. and Herren, H.R. (Eds. ) Biological Control of the Larger Grain Borer - Proceedings of an IITA/FAO Coordination Meeting, Cotonou, Republic of Benin, 2-3 June 1989, IITA, Nigeria.

Longstaff, B.C. (1988) Temperature manipulation and the management of insecticide resistance in stored grain pests: a simulation study for the rice weevil, SITOPHILUS ORYZAE (L.). Ecological Modelling, 43, 303-313.

Markham, R.H. (1990) The role of quantitative ecology and systems analysis in the biological control of the Larger Grain Borer pp. 35-50, in Markham, R.H. and Herren, H.R. (Eds. ) Biological Control of the Larger Grain Borer - Proceedings of an IITA/ FAO Coordination Meeting, Cotonou, Republic of Benin, 23 June 1989, IITA, Nigeria.

Muhihu, S.K. (1990) The relative effectiveness of various insecticide dusts in the control of insect infestation on maize stored on the cob and as shelled maize. pp. 119- 125 in Maize Conservation on the Farm - Proceedings of a Seminar at Kisumu, Kenya, 2123 January, 1986. DPRA-MOALDUSAID, Maseno, Kenya.

Rees, D.P. (1985) Life history of Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) and its ability to suppress populations of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae ). J. Stored Prod, Res. 21, 115- 118,

Rees, D.P. (1990) Teretriosoma nigrescens Lewis (Coleoptera: Histeridae), a predator of the Larger Grain Borer, Prostephanus truncatus (Horn) (Col.: Bostrichidae): current status of knowledge. pp. 103-111 in Markham, R.H. and Herren, H.R. (Eds. ) Biological Control of the Larger Grain Borer - Proceedings of an IITA/FAO Coordination Meeting, Cotonou, Republic of Benin, 2-3 June 1989, IITA, Nigeria.

Rees, D.P, Rodriguez, R. and Herrera, F. ( 1990a). Observations on the ecology of Teretriosoma nigrescens Lewis (Col.: Histeridae) and its prey Prostephanus truncatus (Horn) (Col.: Bostrichidae) in the Yucatan peninsula, Mexico. Trop. Sci. 30, 153-165.

Rees, D.P., Rodriguez, R. and Herrera, F. and Ofosu, A. (1990b) Advances in monitoring Prostephanus truncatus (Horn) (Col.: Bostrichidae) and Teretriosoma nigrescens Lewis (Col.: Histeridae) populations. in Proceedings of the 5th Working Conference on Stored-Product Protection, Bordeaux, France, 9-14 September 1990.

Table 1. Application of permethrin by farmers in Tanzania

* Correct application rate = 50 g /90 kg grain

Tab - Tabora (n=314)
Bib - Biharamulo (n=280)

Table 2. Farmer perceptions of permethrin efficacity

** = % of farmers who thought that permethrin was effective

Tab - Tabora
Bih - Biharamulo

Table 3. Survival of adult Prostephanus truncatus exposed to treated maize grain (expressed as a percentage)

Each datum represents mean of four replicates.

Table 4: Response of susceptible parent adult Prostephanus truncatus and of F6 progeny reared on maize treated with permethrin to filter papers treated with permethrin.

* Expressed as % active ingredients in non-volatile solvent: W / V

A comparison of probit regressions by maximum likelihood programme provided the following analysis:

CHI SQ at which a 5 % probability is greater than CHI SQ

For 1 d. f. = 3.84
For 24 d.f = 36.4
For 30 d.f. = 43.8

Table 5. Response of susceptible parent adult Prostephanus truncatus and of F6 progeny reared on maize treated with pirimiphos-methyl to filter papers treated with pirimiphos-methyl.

* Expressed as % active ingredients in non-volatile solvent: W / V

A comparison of probit lives by maximum likelihood programme provided the following analysis:

Table 6: Susceptibility of Teretriosoma nigrescens and Prostephanus truncatus + to insecticides applied topically.

* P. truncatus were of Tanzanian origin. Maximum likelihood method was used to compare regressions.
** CHI² at which a 5% probability is greater than CHI² for 1 df = 3.84. In all cases. T. nigrescens was significantly more susceptible than P. truncatus.

Table 7: Susceptibility of Teretriosoma nigrescens and Prostephanus truncatus exposed for 5 hours to filter papers impregnated with insecticides.

* Data relating to Tanzania strains: a composite mean of four samples collected from different locations within the country.

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