Why Do Herbivorous Mites Suppress Plant Defenses?

Abstract

Plants have evolved numerous defensive traits that enable them to resist herbivores. In turn, this resistance has selected for herbivores that can cope with defenses by either avoiding, resisting or suppressing them. Several species of herbivorous mites, such as the spider mites Tetranychus urticae and Tetranychus evansi, were found to maximize their performance by suppressing inducible plant defenses. At first glimpse it seems obvious why such a trait will be favored by natural selection. However, defense suppression appeared to readily backfire since mites that do so also make their host plant more suitable for competitors and their offspring more attractive for natural enemies. This, together with the fact that spider mites are infamous for their ability to resist (plant) toxins directly, justifies the question as to why traits that allow mites to suppress defenses nonetheless seem to be relatively common? We argue that this trait may facilitate generalist herbivores, like T. urticae, to colonize new host species. While specific detoxification mechanisms may, on average, be suitable only on a narrow range of similar hosts, defense suppression may be more broadly effective, provided it operates by targeting conserved plant signaling components. If so, resistance and suppression may be under frequency-dependent selection and be maintained as a polymorphism in generalist mite populations. In that case, the defense suppression trait may be under rapid positive selection in subpopulations that have recently colonized a new host but may erode in relatively isolated populations in which host-specific detoxification mechanisms emerge. Although there is empirical evidence to support these scenarios, it contradicts the observation that several of the mite species found to suppress plant defenses actually are relatively specialized. We argue that in these cases buffering traits may enable such mites to mitigate the negative side effects of suppression in natural communities and thus shield this trait from natural selection.

Tetranychusevansi spider mite populations suppress tomato defenses to varying degrees

“…Moreover, in the presence of cues associated with P. longipes , T. evansi females choose to more often oviposit in their web, where their eggs are less prone to predation by P. longipes than on the leaf surface (Lemos et al, 2010). Although these traits may also entail costs (e.g., web production), they provide protection against competitors and natural enemies and thus “buffer” (Frank, 2007) T. evansi against the negative biotic consequences of defense suppression (Blaazer et al, 2018). In this context, future research could investigate if the degree to which T. evansi populations engage into such buffering behavior correlates with the variation in defense suppression observed in this study, because this may point toward ecological costs of defense suppression.…”

“…Understanding why defense suppression is a successful herbivore offense strategy requires insight into its evolutionary costs and benefits (Blaazer et al, 2018). A benefit of defense suppression for herbivores is that it prevents expression of plant defense, which would otherwise have resulted in reduced herbivore performance (Kant et al, 2015; Musser et al, 2002).…”

“…Such biotic interactions introduce new costs. A prime example of such costs can be found in Tetranychus evansi spider mites (Blaazer et al, 2018). By suppressing tomato defense, T. evansi increase not only their own performance but also that of competing Tetranychus spp.…”

Plant Defenses Against Tetranychus urticae: Mind the Gaps

“…Many phytophagous organisms, and particularly the generalist T. urticae, have acquired traits to overcome plant defenses through three main strategies: avoidance, metabolic resistance and suppression [4,93]. The avoidance of induced plant defenses entails a behavioral feature while the other two strategies make the herbivore cope with ingested plant metabolites.…”

Rapid specialization of counter defenses enables two-spotted spider mite to adapt to novel plant hosts

“…Suppression of induced plant defenses was proposed to be the common and most efficient 183 mechanism of TSSM adaptation to new host plants (Jonckheere et al, 2016;Villarroel et al, 184 2016; Blaazer et al, 2018;Jonckheere et al, 2018). Indeed, interference with either JA-185 biosynthesis or its signalling appears to be an effective way to attenuate a whole range of 186…”