Recognition and Modulation of Olfactory Signals in the Noctuid Moth Spodoptera littoralis

Food, mates and host plants are essential resources for plant-feeding insects. Optimal resource localization that fulfils the physiological need and reproductive goals of an insect is of utmost importance. Insects, like most animals, rely largely on the sense of smell to locate and evaluate potential resources that enhance their reproductive fitness. Confronted with a multitude of stimuli ranging from food to conspecifics, insects execute behavioural strategies that are strongly modulated by internal physiological factors such as hunger and reproductive status. In this thesis, I address how an insect’s reproductive state modulates olfactory perception and ultimately behaviour towards adult food, female sex pheromone and larval host plant odours. The cotton leafworm Spodoptera littoralis is an appropriate species for studying modulation of olfactory perception and behaviour, since the chemical ecology and olfactory physiology of mate finding and host seeking is being thoroughly studied. Here, I show that mating in female S. littoralis causes transient reduction in the sexual receptivity, and a reduction in longevity. Mating induces physiological changes that strongly influence olfactory coding and preference in males and females that match their current physiological need. Following mating, female S. littoralis switches olfactory preference from adult food to egg-laying cues. Unmated female S. littoralis are highly attracted to lilac flowers (Syringa vulgaris). After mating, females switch their olfactory preference to the host plant cotton (Gossypium hirsutum). Remarkably, the behavioural switch from floral to green odours is also mirrored in the primary olfactory centre, the antennal lobe (AL). Further, I have identified odorants from cotton that elicit robust upwind flight responses in gravid female S. littoralis. In male S. littoralis, mating transiently abolishes attraction to pheromone-releasing females and cotton volatiles that signal larval habitat and mating sites. This behavioural modulation is reflected in the peripheral and central olfactory system, the antenna and the AL. In contrast, behavioural and neuronal responses to lilac flowers that signal an adult food source are not influenced by mating status. These findings provide an excellent substrate to examine how neuronal circuits integrate external sensory information with physiological state to shape behaviour.