Effect of Sequential Induction by Mamestra brassicae L. and Tetranychus urticae Koch on Lima Bean Plant Indirect Defense

Attack by multiple herbivores often leads to modification of induced plant defenses compared to single herbivory, yet little is known about the effects on induced indirect plant defense. Here, we investigated the effect of sequential induction of plant defense by Mamestra brassicae caterpillar oral secretion and an infestation by Tetranychus urticae spider mites on the expression of indirect plant defense in Lima bean plants. The effect on indirect defense was assessed using behavior assays with the specialist predatory mite Phytoseiulus persimilis in an olfactometer, headspace analysis of 11 major herbivore-induced plant volatiles (HIPVs) including (E)-beta-ocimene, and transcript levels of the corresponding gene Phaseolus lunatus (E)-beta-ocimene synthase (PlOS). Predatory mites were found to distinguish between plants induced by spider mites and caterpillar oral secretion but not between plants with single spider mite infestation and plants induced by caterpillar oral secretion prior to spider mite infestation. Indeed, the volatile blends emitted by plants induced by spider mites only and the sequential induction treatment of caterpillar oral secretion followed by spider mite infestation, were similar. Our results suggest that plant indirect defense is not affected by previous treatment with oral secretion of M. brassicae caterpillars.