Costs and benefits of fighting infection in locusts

Abstract

Locusts and grasshoppers are truly cosmopolitan pests. In an effort to reduce the environmental side-effects of current chemical control practices, several programmes around the world are developing biopesticides, based on fungal entomopathogens, for locust and grasshopper control. Unfortunately, these biocontrol products have achieved mixed success. One of the principal reasons is that locusts are active behavioural thermoregulators, enabling them, under certain environmental conditions, to elevate their body temperatures to levels where fungal growth is suppressed. Here we develop a dynamic behavioural model to predict how locust thermoregulatory behaviour influences disease development. We use the model to explore what the overall consequences of infection might be (i.e. the net effect of disease capturing elements of both pathogen development and host defence) under different conditions in terms of locust mortality, fecundity and crop damage. We modelled two empirical fungal entomopathogens, Metarhizium anisopliae var. acridum and Beauveria bassiana, together with two hypothetical pathogens representing a temperature generalist and a temperature specialist. The model leads to predictions that the effects of a fungal biocontrol agent are strongly mediated by environmental temperature and host behaviour. The positive control effects are manifested through direct mortality and also sub-lethal effects on feeding and fecundity that result from modifications in behaviour associated with host defence and optimization of locust fitness. M. anisopliae var. acridum is predicted to provide the best control of locusts and the specialist fungus to provide the worst. Under hotter conditions, B. bassiana is predicted to provide substantially worse biocontrol than the other fungal strains. These predictions match well with empirical data. In addition, the model reveals the possibility for locusts to balance the costs of host defence through selective expression of behavioural fever in response to individual fungal diseases. We conclude that models like this one may facilitate prospective evaluation of biocontrol and advance our understanding of the role of behaviour and thermal ecology in insect– pathogen interactions.

Citation

Gardner, S.N.; Thomas, M.B. Costs and benefits of fighting infection in locusts. Evolutionary Ecology Research (2002) 4: 109-131.

Costs and benefits of fighting infection in locusts

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