Mixed blends of herbivore-induced plant volatiles and foraging success of carnivorous arthropods

Food webs are overlaid with infochemical webs that mediate direct and indirect interactions. Behavioural ecologists have extensively documented that carnivorous arthropods exploit herbivore-induced plant volatiles during foraging for herbivorous arthropods. Most studies on the role of infochemicals in multitrophic interactions have been conducted against an odour-free background, although field studies show that carnivores also use herbivore-induced plant volatiles under more complex conditions. Here we investigated the effect of mixing the blends of volatiles emitted by two plant species on the foraging behaviour of the predatory mite Phytoseiulus persimilis. This was done in an olfactometer under laboratory conditions and in a semi-field setup under greenhouse conditions. The olfactometer setup ensured directed mixing of the two odour blends, while odour mixing in the greenhouse setup was much less controlled and resulted from diffusion. In 4 out of 5 olfactometer experiments the behaviour towards volatiles from spider-mite (Tetranychus urticae) infested Lima bean plants was not affected by mixing with volatiles from caterpillar (Pieris brassicae) infested Brussels sprouts plants. In the fifth olfactometer experiment the response shifted significantly towards the volatiles from infested Lima bean leaves without volatiles from infested cabbage leaves. In the greenhouse setup no effect of infested cabbage plants or their volatiles on the location of spider-mite infested bean plants was recorded. The two odour blends used in this study, i.e. those from spider-mite infested Lima bean leaves and from caterpillar-infested Brussels sprouts plants, are very different and there is no overlap in compounds that are known to attract the predators. The results are discussed in the context of other types of odour-blend mixing and the effects on food web interactions.