Response to olfactory stimuli in gregarious Pieris brassicae caterpillars

Abstract: Insects search primarily to find food, egg-laying sites, resting sites or conspecifics. Understanding the search behaviour of insects at the point when they are not in contact with the host is relevant to for instance the control of pest species. There are many studies on how adult butterflies respond to different olfactory signals when distanced from the host, but less is known of how caterpillars use and respond to such signals. This study was conducted to investigate what olfactory signals attract gregarious Pieris brassicae caterpillars. Are damaged host plants more attractive than undamaged plants, or will signs of conspecifics such as frass attract the caterpillars? Furthermore, will caterpillars respond differently to different genotypes of the host?Using four-way olfactometers, preference experiments were conducted on the genotypes broccoli (Brassica oleracea L. var. cymosa), cabbage (Brassica oleracea L. var. capitata) and cauliflower (Brassica oleracea L. var. botrytis) of the larval host Brassica oleracea. One preference experiment tested the caterpillars’ response to undamaged plants of the genotypes. In the other three experiments the response of gregarious P. brassicae caterpillars to insect-induced damage (by conspecifics), no damage, and frass treatments of each of the genotypes were tested. The results showed no difference in preference for any of the undamaged genotypes. There was also no difference in the caterpillars’ preference for any of the treatments of broccoli or cabbage, but a significant preference for damaged cauliflower over frass and the control (air). This could be explained by an attraction to the damaged plant tissue itself, or an attraction to conspecific presence because of the species-specific combination of compounds released by the plant. If the lack of difference between the undamaged genotypes is because the genotypes are equally preferred cannot be concluded before further studies have been made, comparing damaged treatments of the genotypes. It is clear from this study that caterpillars of P. brassicae respond to olfactory signals.