Natural enemies often use volatile cues, released by plants during herbivory, to locate prey. However, exploitation of this host-tracking mechanism could be complicated by variation in herbivore communities across landscapes. Indeed, in a regional survey of phloem-feeding aphids, we found that prey-tracking by the aphids’ specialist parasitoid was disrupted where chewing caterpillars were common.
Manipulative field experiments at multiple sites revealed a general disruption of aphid parasitism on caterpillar-damaged plants; at some locations, this led to higher aphid densities. This was in stark contrast to findings in parasitoid-free laboratory arenas, where caterpillar feeding suppressed, rather than promoted, aphid population growth. Foliar-chemistry analyses revealed reduced concentrations of gluconapin, a precursor to a key plant-volatile used by parasitoids to find aphids, in caterpillar-damaged plants. Our study suggests that placing chemically-mediated interactions in a community context of plants, competing herbivores, and natural enemies can provide an understanding of much broader patterns at the scale of diverse landscapes.