Abstract
Attraction of parasitoids to plant volatiles induced by multiple herbivory depends on the specific combinations of attacking herbivore species, especially when their feeding modes activate different defense signalling pathways as has been reported for phloem feeding aphids and tissue feeding caterpillars. We studied the effects of pre-infestation with non-host aphids (Brevicoryne brassicae) for two different time periods on the ability of two parasitoid species to discriminate between volatiles emitted by plants infested by host caterpillars alone and those emitted by plants infested with host caterpillars plus aphids. Using plants originating from three chemically distinct wild cabbage (Brassica oleracea) populations, Diadegma semiclausum switched preference for dually infested plants to preference for plants infested with Plutella xylostella hosts alone when the duration of pre-aphid infestation doubled from 7 to 14 days. Microplitis mediator, a parasitoid of Mamestra brassicae caterpillars, preferred dually-infested plants irrespective of aphid-infestation duration. Separation of the volatile blends emitted by plants infested with hosts plus aphids or with hosts only was poor, based on multivariate statistics. However, emission rates of individual compounds were often reduced in plants infested with aphids plus hosts compared to those emitted by plants infested with hosts alone. This effect depended on host caterpillar species and plant population and was little affected by aphid infestation duration. Thus, the interactive effect of aphids and hosts on plant volatile production and parasitoid attraction can be dynamic and parasitoid specific. The characteristics of the multi-component volatile blends that determine parasitoid attraction are too complex to be deduced from simple correlative statistical analyses.
Highlights
Natural enemies of herbivorous insects must find their hosts or prey in habitats that are often structurally and chemically heterogeneous (Meiners 2015; Schoonhoven et al 2005)
When offered volatiles from plants infested with hosts alone versus hosts plus aphids, the effect of aphid infestation period on proportional preference differed for the two parasitoid species (GLM, parasitoid-infestation period interaction, χ21 = 7.63, P = 0.006)
When plants had been infested with aphids for 14 days, D. semiclausum preferred volatiles emitted by plants that had been infested with only hosts (Fig. 3b; t-test; t23 = 2.66, P = 0.014)
Summary
Natural enemies of herbivorous insects must find their hosts or prey in habitats that are often structurally and chemically heterogeneous (Meiners 2015; Schoonhoven et al 2005). Plant volatiles are important during foraging behavior of natural enemies of insect herbivores (Dicke and Baldwin 2010; Hare 2011; Heil 2008). HIPVs may play an important role in structuring plant-associated insect communities as they mediate multitrophic interactions between plants, herbivores and their carnivorous natural enemies (Dicke and Baldwin 2010; Hare 2011; Poelman et al 2012; Vet and Dicke 1992). Herbivory by multiple species is the norm in nature This may affect the induction of HIPVs and as a result the attraction of natural enemies of insect herbivores (De Rijk et al 2013; Dicke et al 2009; Ponzio et al 2013). Reviewing the existing literature, De Rijk et al (2013) found that when given the choice between volatiles emitted by plants infested with hosts plus non-hosts vs. plants infested with hosts alone, roughly
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