Unusual mechanisms involved in learning of oviposition-induced host plant odours in an egg parasitoid?

Parasitoids of herbivorous larvae are well known to be able to associatively learn a response to feeding-induced plant volatiles. However, little is known of the abilities of egg parasitoids to learn a response to herbivore oviposition-induced plant volatiles (OVIV). The eulophid wasp Chrysonotomyia ruforum is known to be attracted by OVIV from Scots pine released 3 days after host egg deposition. This study aimed to elucidate (1) the impact of the nutritional state of the parasitoid on its response to OVIV and (2) the effect of learning and response to the temporal change of OVIV. Supply of food as a reward during associative learning trials inevitably influences the nutritional state and thus leads to an experimental constraint. Nevertheless, the results indicate that the parasitoids have to associate OVIV with host presence and that they need to be supplied with food to learn and to respond to OVIV. Furthermore, the parasitoids' positive response to OVIV from pine twigs laden with eggs for 3 days was independent of the induction time of pine twigs experienced. However, a positive response was given only to OVIV from pine twigs with 3-day-old eggs, while OVIV from twigs with other induction times (1, 2, 4 days) were not attractive. Thus, our findings indicate an unusual learning process in this specialized egg parasitoid. We suggest that the parasitoid is using a learning strategy by which common information is filtered out of similar odour blends (here: 1- to 4-day-long induced pine twigs), but a behavioural response to this learned information becomes evident only in a specific odour context (here: 3-day-long induced twigs).