The effect of terpenoid extracts from 15 pine species on the feeding behavioural sequence of the late instars of the pine processionary caterpillar Thaumetopoea pityocampa

The feeding behaviour of the pine processionary (PPC) caterpillar Thaumetopoea pityocampa Den. and Schiff. (Lepidoptera, Thaumetopoeidae) in L3-L4 stages was explored by means of laboratory arena feeding trials and natural substrates. In the bioassays, volatile extracts of 15 pine species, 8 of which are naturally growing in Greece, were incorporated. An analytical model was developed based on the principle of multinomial logit regression with five outcomes on the basis of the behavioural feeding sequence of the caterpillars. The outcomes were the five steps in which the feeding behavioural sequence was decomposed. The model's suitability (MacFadden's rho(2)=0.229, P<10(-4)) was examined when including 10 terpenes that were judged significant through a stepwise canonical discriminant analysis. The proposed model was superior to a random one and the two models resulting from the addition and subtraction of 4 terpenes to the already 10 existing compounds. The most informative model was built on the terpenes caryophyllene oxide, terpinolene, myrcene, germacrene D, eudesmol, limonene, beta-pinene, beta-caryophyllene, alpha-pinene and manoyl oxide. The background terpenes were present in the model and of particular importance. No special behavioural role, either as promoter or inhibitor could be assigned to the individual volatile metabolites, since no constant pattern among behavioural steps was observed. For instance, beta-caryophyllene while acts as promoter of attraction and trial bite it is a suppressor of partial feeding and strongly inhibits complete needle consumption. The monoterpene limonene, on the other hand, seems to be a suppressor of partial and complete feeding. The overall methodological scheme and the analytical modelling tool could be proved a suitable research protocol in unfolding the ecological role of a complex mixture of secondary metabolites. Those who develop safe practical systems can use this.