Suppression of Plant Resistance May Be a Common Trait Among Adapted Herbivores

Herbivores have evolved distinct strategies to cope with plant defenses prior to becoming a pest. Some evolved resistance to toxic compounds; others suppress host toxin production. These traits seem to co-occur within herbivore species, particularly among spider mites, which are major pests in many crops. The intraspecific variation within the spider mites is a model for adaptive pest evolution on crops such as tomato. Empirical data collected from nonsolanaceous wild host plants suggest that natural populations predominantly comprise individuals capable of inducing tomato defenses, while a small proportion suppress these defenses. Additionally, resistant individuals are observed only rarely within these populations. This study aimed to investigate the presence of these traits within populations adapted to tomato plants. Here, Tetranychus urticae populations sampled from tomato at three field sites were compared. To discriminate between mites that induce/suppress defenses and to assess their degree of resistance, the magnitude of induced defenses was measured and aligned with oviposition data. The expression of effector 84 was also assessed to determine whether its magnitude of expression is a good predictor of suppression with the magnitude of suppression. Surprisingly, we observed that suppression is the dominant phenotype in mite populations collected from field-grown tomatoes. Our results suggest that suppression may be rare only at the beginning of an herbivore's adaptive trajectory after colonization of a novel host but may rapidly become common due to natural selection. This suggests the possibility that suppression potentially represents a prevalent phenotype among host-adapted herbivores and, consequently, among pests.