Costs and benefits of plant defence suppression by Tetranychus evansi spider mites

Herbivores eat plants, and plants defend their tissues. To overcome these plant defences, herbivores have evolved a variety of offensive strategies. Some herbivores use enzymes to detoxify defensive compounds of their hosts, and others suppress plant defences by manipulating a plant’s physiological processes. How and why has this diversity of herbivore offensive strategies evolved? In this thesis, I studied evolutionary costs and benefits of plant defence suppression by the herbivorous spider mite Tetranychus evansi.
By quantitatively reviewing past evidence, I confirmed that T. evansi can attain a considerable fecundity benefit by suppressing the inducible defences of their tomato hosts. However, competing herbivores can impose considerable costs on this offensive strategy, because they can also benefit from the suppressed defences of a shared host plant, and can subsequently induce these defences to the disadvantage of T. evansi. Furthermore, I investigated T. evansi populations from several locations around the world, and found that suppression of plant defences is variable within and among T. evansi populations. Given this variation, I allowed a genetically diverse T. evansi population to evolve on plants with different levels of defence, and expected T. evansi to evolve lower levels of suppression on plants without functional defences. However, I found that T. evansi retained its level of defence suppression on such plants, indicating that metabolic costs are likely low.
I discuss the evolution of defence suppression from the perspective of costs and benefits, highlight gaps in our knowledge of defence suppression by T. evansi, and provide detailed suggestions to tackle remaining questions.