Insect Odorant-Binding Proteins: Do They Offer an Alternative Pest Control Strategy?
Effective control of insect pests often depends on several complementary approaches that combine to form an integrated pest management strategy. Recent efforts to find new pesticides have not been able to prevent the evolution of resistance to insecticide treatment. The most successful insecticidal control method used to date has been the development of crops transformed with toxin genes of Bacillus thuringiensis (Bt). However, Bt toxins mostly target lepidopteran pests and have only been licensed in certain countries to control insect pests of corn and cotton. It is imperative that other non-insecticidal strategies be developed to alleviate the current reliance on chemical pesticides. One potential approach to this is to interfere with the ability of pest insects to find suitable mates and hosts which they do through volatile chemical signals (semiochemicals). Such olfaction-based approaches have been applied successfully in 'push-pull' pest control strategies using plants which are capable of producing attractants and repellents. Recently an aphid alarm pheromone synthase gene was successfully transformed into Arabidopsis thaliana, resulting in plants that both repelled aphids and attracted beneficial insects. Efforts to introduce such genes into crops are currently ongoing. There is also a need for efficient eco-friendly control strategies for the insect vectors of human and animal diseases, such as mosquitoes, tsetse flies and horn flies. For example, despite concerted efforts to develop vaccines, and attempts to interfere with parasite development in the arthropod vector through mosquito transgenesis, to date, malaria control relies mainly on repressing the insect vector–human target interaction allied with insecticide treatments. Insects use their sense of smell to find mates for reproduction and to locate hosts for nutrition by detecting semiochemicals. These enter the antennae by diffusion through pores on the cuticle and are transported to the olfactory neurons across the sensillum lymph. Several functional components of the insect olfaction system, which are involved in olfactory signal recognition and transduction in insects, are currently under investigation at the molecular level. These include odorant-binding proteins (OBPs), sensory neuron membrane proteins (SNMPs) and olfactory receptors (ORs). Insect OBPs are thought to provide the first filtering mechanism for semiochemicals and to mediate the activation of the ORs. Recent studies at Rothamsted now provide a starting point for the potential use of OBPs as targets to interfere with insect host location and mating behaviour. Such non-insecticidal approaches could play an important role as part of integrated pest management strategies and broaden the arsenal of available tools for insect pest control.
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