Plant defence suppression is an offensive strategy of herbivores, in which they manipulate plant physiological processes to increase their performance. Paradoxically, defence suppression does not always benefit the defence-suppressing herbivores, because lowered plant defences can also enhance the performance of competing herbivores and can expose herbivores to increased predation. Suppression of plant defence may therefore entail considerable ecological costs depending on the presence of competitors and natural enemies in a community. Hence, we hypothesize that the optimal magnitude of suppression differs among locations. To investigate this, we studied defence suppression across populations of Tetranychus evansi spider mites, a herbivore from South America that is an invasive pest of solanaceous plants including cultivated tomato, Solanum lycopersicum, in other parts of the world. We measured the level of expression of defence marker genes in tomato plants after infestation with mites from eleven different T. evansi populations. These populations were chosen across a range of native (South American) and non-native (other continents) environments and from different host plant species. We found significant variation at three out of four defence marker genes, demonstrating that T. evansi populations suppress jasmonic acid- and salicylic acid-dependent plant signalling pathways to varying degrees. While we found no indication that this variation in defence suppression was explained by differences in host plant species, invasive populations tended to suppress plant defence to a smaller extent than native populations. This may reflect either the genetic lineage of T. evansi - as all invasive populations we studied belong to one linage and both native populations to another - or the absence of specialized natural enemies in invasive T. evansi populations.

Spider mite leaf damage measurements

GUS activity assays

qPRC measurements

All the necessary information can be found in the Knegt et al. paper