The use of oviposition-induced plant cues by Trichogramma egg parasitoids

Female parasitoids have evolved various foraging strategies in order to find suitable hosts. Egg parasitoids have been shown to exploit plant cues induced by the deposition of host eggs. 2. The tiny wasp Trichogramma brassicae uses oviposition-induced cues from Brussels sprouts to locate eggs of the cabbage white butterflies Pieris brassicae and Pieris rapae that differ in their egg-laying behaviour. These plant cues are elicited by male-derived anti-aphrodisiac pheromones in the accessory reproductive gland (ARG) secretions of mated female butterflies. However, the closely related generalist species Trichogramma evanescens does not respond to Brussels sprout cues induced by the deposition of P. brassicae egg clutches. 3. Here we showed in two-choice bioassays that T. evanescens wasps respond to Brussels sprout cues induced by (i) the deposition of single eggs by P. rapae ,a nd (ii) the application of ARG secretions from either mated P. rapae females, or from virgin female butterflies in combination with P. rapae's anti-aphrodisiac compound indole. The wasps only associatively learned to respond to Brussels sprout cues after applying indole alone by linking those cues with the presence of P. rapae eggs. 4. Our results indicate that Trichogramma wasps more commonly exploit oviposition-induced plant cues to locate their host eggs. Generalist wasps show less specificity in their response than specialists and employ associative learning.