Fig. 3. Mechanism of synomone induction. Responses of egg parasitoid female Chrysonotomyia ruforum to volatiles released from differently treated (A–C) twigs of Pinus sylvestris offered in a test field (T) consisting of a four-arm olfactometer with three control fields supplied with clean air (1, 2 and 3). Mean values for the time the parasitoid females spent walking in the test and control fields are given over an observation period of 600 s. Values are means + S.D. The dashed line is the mean time if the duration of walking had been equal in the test and control fields. *** indicates a significant (P<0.001) and NS a non-significant (P>0.05) difference evaluated by a Friedman ANOVA. Different letters indicate significant (P<0.05) differences evaluated by the Wilcoxon–Wilcox test.