Host plant identity and host plant chemistry have often been shown to influence host finding and acceptance by natural enemies but comparatively less attention has been paid to the tritrophic effects of host plant and host plant chemistry on other natural enemy fitness correlates, such as survivorship, clutch size, body size, and sex ratio. Such studies are central to understanding both the selective impact of plants on natural enemies as well as the potential for reciprocal selective impact of natural enemies on plant traits. We examined the effects of host plant and host plant chemistry in a tritrophic system consisting of three apiaceous plants (Pastinaca sativa, Heracleum sphondylium and H. mantegazzianum), the parsnip webworm (Depressaria pastinacella) and the polyembryonic parasitic wasp Copidosoma sosares. All of these plants produce furanocoumarins, known resistance factors for parsnip webworms. Furanocournarin concentrations were correlated neither with the presence nor the number of webworms on a given plant. Concentrations of two furanocoumarins were negatively associated with C sosares fitness correlates: isopimpinellin with the likelihood that a given webworm would be parasitized and xanthotoxin with both within-brood survivorship (of all-male and mixed-sex broods) and clutch size. Brood sex ratio and body sizes of individual wasps were not correlated with furanocoumarin chemistry. Because additive genetic variation exists in P. sativa for furanocoumarin chemical traits, these are subject to selection by webwormis through herbivory. Third trophic level selective impacts on furanocoumarin traits may include selection for reduced production of those chemicals that affect parasitoid survivorship yet do not influence host plant choice by the herbivore. That such might be the case is suggested by patterns of furanocoumarin production in populations of P. sativa with different histories of infestation; in the Netherlands, where parasitism rates of webworms by C sosares are high, plants produce lower levels of all linear furanocoumarins and proportionately less isopimpinellin than do midwestern U.S. populations of P sativa, where natural enemies of the webworm are effectively absent.
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