Host finding behavior of two gall midges specializing on wheat

Wheat midge, Sitodiplosis mosselana (Gehin), and Hessian fly, Mayetiola destructor (Say), are major pests of wheat, Triticum aestivum L. Studies of the wheat midge were undertaken to identify the plant traits that influence the host selection behavior of the adult egg-laying female. Studies of the Hessian fly were carried out to determine the fitness consequences of the host selection behavior of the neonate larva. Mechanisms contributing to selection of seed heads were investigated in the wheat midge in both choice and no-choice tests. The three grass genotypes were: two hard red spring wheat (T. aestivum) genotypes, 'Roblin' and 'Key 97-24', and one barley (Hordeum vulgare ) genotype, 'Robust'. Pre-anthesis 'Roblin' seed heads were ranked highest by adult females while post-anthesis 'Roblin' seed heads, pre-anthesis barley, and pre-anthesis 'Key' seed heads ranked lower. Results indicated that both plant genotype and developmental stage of the seed head influenced proportions of females observed on the head and proportions of females probing with the ovipositor. The wheat midge female detects chemical and physical cues while examining the seed head, with this detection determining how many eggs are deposited before she flies away. As a result of her sensory and behavioral mechanisms, the female can distinguish between high- and low-ranked hosts within seconds after landing. This is a useful trait for an insect with a short-lived adult stage.;Hessian fly female showed a strong preference for the youngest fully extended leaf blade for oviposition. The larvae that hatched from eggs placed here migrated to the plant base and moved to attack the adjacent younger leaf. By following what happened to larvae hatching from these eggs, I discovered that the larva does not simply remain in the site chosen by the adult female. Instead the larva moved between leaves at the base of the plant, eventually settling at a younger site than that chosen by the adult female. This suggests that the tiny neonate larva (approximately 500 mum in length) has the requisite sensory and behavioral mechanisms to allow selection of a suitable feeding site.