The evolution of pathogen defence in insect hosts: whitefly resistance to biocontrol agents in horticulture

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The control of insect pests in agriculture using conventional chemical insecticides has led to widespread insecticide resistance evolution and triggered extensive biodiversity loss. In response to these problems many agricultural sectors are increasingly switching to biological control agents (such as pathogens and parasitoids) as part of an integrated pest management approach. However, in contrast to the volume of research into chemical insecticide resistance, the possibility that target pests may evolve resistance to these biological control agents has received relatively little attention.Insect populations harbour substantial genetic variation for infection susceptibility, providing strong potential for resistance evolution [1]. Evolutionary responses to parasites are often assumed to be due to changes in components of the immune system coded for by the nuclear genome. However, other factors often strongly influence the susceptibility of insect hosts to infection and may provide novel routes to resistance evolution. For example, behaviour can have major impacts on infection susceptibility [2]: insects that avoid contact with parasites may be at a significant evolutionary advantage. Also, insect resistance to parasitism is often strongly influenced by the presence of maternally inherited symbiotic microbes that live inside the insect's tissues: these "influential passengers" can be the principal determinant of infection susceptibility in many systems, meaning that resistance can evolve through the spread of a novel symbiont rather than via genomic evolution [3].The glasshouse whitefly Trialeurodes vaporariorum is a major pest of agriculture worldwide; in the UK it especially affects greenhouse horticulture. It damages plants by removing resources through sap-sucking, depositing honeydew on foliage thereby promoting fungal growth and by vectoring a range of viral infections. This insect is particularly problematic for commercial greenhouse tomato growers in the UK; growers deploy a range of control agents against whitefly, two of which are the parasitoid wasp Encarsia formosa and biopesticides formulated from the entomopathogenic fungus Beauveria bassiana. Whilst agricultural monocultures can rapidly select for resistance phenotypes in pests, we are especially interested in how ecological heterogeneity can be enhanced in horticulture by planting multiple plant species/varieties and whether this can impede resistance evolution.