Plants respond to grazing by herbivorous insects by emitting a range of volatile organic compounds, which attract parasitoids
to their insect hosts. However, a positive outcome for the host plant is a necessary precondition for making the attractionbeneficial or even adaptive. Parasitoids benefit plants by killing herbivorous insects, thus reducing future herbivore pressure,but also by curtailing the feeding intensity of the still living, parasitised host. In this study, the effect of parasitismon food consumption of the 5th instar larvae of the autumnal moth (Epirrita autumnata) was examined under laboratory conditions. Daily food consumption, as well as the duration of the 5th instar, was measuredfor both parasitised and non-parasitised larvae. The results showed that parasitism by the solitary endoparasitoid Zele deceptor not only reduced leaf consumption significantly but also hastened the onset of pupation in autumnal moth larvae. On the basisof the results, an empirical model was derived to assess the affects on the scale of the whole tree. The model suggests thatparasitoids might protect the tree from total defoliation at least at intermediate larval densities. Consequently, a potentialfor plant–parasitoid chemical signalling appears to exist, which seems to benefit the mountain birch (Betula pubescens ssp. czerepanovii) by reducing the overall intensity of herbivore defoliation due to parasitism by this hymenopteran parasitoid.
