Species- and density-dependent induction of volatile organic compounds by three mite species in cassava and their role in the attraction of a natural enemy

Upon damage by herbivores, plants induce an array of volatile organic compounds (VOCs) that mediate ecological interactions involving communication with organisms of the second and third trophic levels. VOC-mediated tritrophic interactions have largely been studied in various systems, including cassava (Manihot esculenta), but little is known about the chemical nature of herbivore-induced VOCs in this crop and the response they evoke in natural enemies. Several tetranychid and predatory mites are associated with cassava. Here, VOC emissions from uninfested plants and plants infested with 200 or 400 Mononychellus tanajoa, a specialist herbivore on cassava, and the generalists Tetranychus urticae and T. gloveri were measured. Dual-choice experiments were also conducted to assess the preference of inexperienced (reared on prey-infested bean plants) and experienced (adapted on prey-infested cassava plants) predatory mites, Neoseiulus idaeus (Phytoseiidae), between odors of uninfested plants versus odors of plants infested with M. tanajoa, T. urticae or T. gloveri. Two hundred individuals significantly increased the emissions of (Z)-3-hexen-1-ol, (E)-β-ocimene, β-caryophyllene, alloaromadendrene and (E)-geranyl acetone in T. urticae-infested plants, and (E)-β-ocimene and methyl salicylate (MeSA) in T. gloveri-infested plants. Four hundred individuals significantly increased the emissions of (Z)-3-hexen-1-ol, MeSA, α-pinene and D-limonene in M. tanajoa-infested plants. In addition, T. urticae at this density induced (E)-β-ocimene, D-limonene, (E)-geranyl acetone and six compounds that were not detected in other treatments. Tetranychus gloveri-infested plants induced the emissions of (E)-2-hexenal and D-limonene. Regardless of the infesting species, inexperienced N. idaeus did not discriminate between uninfested or infested plants. Upon experience, they discriminated between the odors of uninfested and T. urticae-damaged plants. Our findings reveal that mite infestations in cassava result in density-dependent and species-specific emission of VOCs, and that N. idaeus relies on associative learning to forage for its prey.