Suppression or differential induction? Indirect defence of tomato plants attacked by Tetranychus evansi

Plants have developed sophisticated mechanisms of defence against herbivores. These defensive traits may acts directly against herbivores, thus reducing their performance, or indirectly, by attracting the herbivores' natural enemies. However, herbivores can adapt to plant defence in many different ways. The red spider mite Tetranychus evansi is known to suppress direct defences of tomato plants to their own benefit. Here, we investigated the interference of T. evansi with the indirect defences of tomato plants consisting of the attraction of odours from plants attacked with spider mites to their natural enemies. In olfactometer experiments, we observed that volatiles from tomato plants infested with approximately 350 T. evansi were not attractive to the predatory mites Phytoseiulus macropilis, P. longipes and P. persimilis. Whereas, volatiles from plants infested with 3.5 times more herbivores were significantly attractive. We also observed that odours from tomato plants infested with the inducer Tetranychus urticae (both with low and high densities) were attractive to and preferred by these predatory mites. Additionally, we identified the volatile chemicals released by clean plants and plants attacked by herbivores. The compounds of the blends were identified and quantified. We observed that T. evansi did not induce those volatile compounds that are often observed in attractive blends from plants attacked by the inducers. Despite the absence of attraction of predatory mites to odours from plants infested with low densities of T. evansi, these blend contained some compounds induced by T. evansi (but not by T. urticae). These results indicate that T. evansi may manipulate the indirect defence of tomato plants by inducting and suppressing the production of different volatile organic compounds.