Bottom‐up cascading effects in a tritrophic system: interactions between plant quality and host‐parasitoid immune responses

Abstract

Abstract 1. Little is known about underlying mechanisms by which plants indirectly affect parasitism success in hymenopteran endoparasitoids. The hypothesis that host‐plant effects can challenge the innate immune system of an insect host was experimentally tested in this study using a model tritrophic, crucifer – lepidopteran [ Plutella xylostella (L.)] – parasitoid [ Cotesia plutellae (Kurdjumov)], system. 2. The effects of host‐plant suitability on herbivore performance and parasitism were examined. The bottom‐up effect of plant suitability on host‐parasitoid immune responses was then evaluated using measures of cellular and humoral effectors. 3. Host‐plant quality showed a significant effect on the encapsulation response of P. xylostella to first instar but not to second instar parasitoid larvae. Encapsulation was never sufficient to prevent parasitoid emergence. 4. Poor host‐plant suitability suppressed phenoloxidase activity in the absence of the parasitoid. The suppressive effect of C. plutellae on phenoloxidase activity was much greater and no plant effects were detectable after insects had been parasitized. 5. Despite strong plant effects on parasitism, those on immune effectors of the host were transitory or overwhelmed by the effect of the parasitoid. 6. These results demonstrated that plant‐mediated variation in parasitism success by C. plutellae were not as a result of plant nutritional status or other attributes affecting the immune function of P. xylostella , nor to host‐plant effects on superparasitism. 7. In these experiments, P. xylostella was a fully permissive host to C. plutellae and host‐plant‐mediated effects on the innate immune response appeared to play no part in parasitoid survival within hosts.