Arthropods use odours associated with the presence of their food, enemies and competitors while searching for patches of food. Responses to these odours therefore determine the spatial distribution of animals, and are decisive for the occurrence and strength of interactions among species. We studied odour-mediated indirect interactions in an artificial arthropod food web that occurs on cucumber plants in greenhouses where biological control is applied. We investigated indirect effects in very simple subsets of this food web, considering (a) plant-mediated indirect interactions between two herbivores, the two-spotted spider mite Tetranychus urticae and the western flower thrips Frankliniella occidentalis, (b) responses of the two herbivores mediated by their predators (the phytoseiid mites Phytoseiulus persimilis, Neoseiulus cucumeris, and N. californicus) and (c) interactions between predators mediated by their prey. We found that odour-mediated indirect interactions are an important organising force in this arthropod food web. Spider mites and thrips can interact indirectly both through their common host plants and through predators, whilst predators may interact indirectly through prey. Indirect interactions as these may produce effects in food webs that are as important as those of direct predator-prey interactions, which may cause both positive and negative effects on biological control of herbivores. We will argue that the importance of odour-mediated indirect interactions in determining community-level interactions both in artificial and in natural food webs should not be underestimated.

Index terms: Frankliniella occidentalis, Tetranychus urticae, Phytoseiulus persimilis, Neoseiulus cucumeris, behavioural responses.

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