Influence of herbivore-induced changes in host plants on reproductive behaviours in Spodoptera littoralis

Insect herbivores orient towards host plants using sensory cues and olfaction plays a major role, especially in nocturnal herbivores, during selection of host plants suitable for feeding, mating and oviposition. Plants defend themselves from herbivore feeding by producing volatiles as well as non-volatiles chemical compounds. Volatile compounds produced in response to feeding damage by herbivores are commonly referred to as herbivore-induced plant volatiles (HIPVs). Emissions of HIPVs are ecologically important as they can increase plant resistance by repelling herbivores and by attracting the natural enemies of the herbivores. We observed a significant reduction in mating when male and female Spodoptera littoralis moths were allowed to mate in the presence of damaged cotton (Gossypium hirsutum) plants. Male activation and attraction towards female sex pheromone was reduced in the presence herbivore-damaged cotton plants. Similarly, females kept individually with damaged cotton plants spent less time in calling compared to females on undamaged plants. These results provide first evidence that herbivore-induced changes in host plants can affect calling and mating behaviours of an insect herbivore. \nIn behavioural studies, in the laboratory as well as in the field, we showed that HIPV emissions from damaged cotton plant neighbours provide resistance to undamaged plants within both conspecific and heterospecific plant patches. Furthermore we found that associational resistance through HIPVs is unidirectional and is highly specific among the tested plant species. Undamaged cotton and alfalfa (Medicago sativa) plants in patches with damaged cotton plant neighbours received fewer eggs, whereas we found no associational resistance when damaged alfalfa and clover (Trifolium alexandrinum) neighbours were present. Electrophysiological (GC-EAD) studies showed that the antennae of the mated female S. littoralis moths detected 18 compounds among the headspace collections of HIPVs from damaged cotton plants. Behavioural studies showed that a blend of seven de novo synthesized volatile compounds among GC-EAD active compounds were sufficient to repel ovipositing S. littoralis. Our results suggest that de novo synthesized volatile compounds provide signalling cues to ovipositing female moths that the plants are under herbivore attack and can be used as reliable cues to avoid plants of low food quality and to reduce risk for competition and predation. Our studies show that HIPVs can have large effect on both male and female reproductive behaviours and that it can affect ecological interactions.