Spider mites collectively avoid plants with cadmium irrespective of their frequency or the presence of competitors

Abstract

AbstractPlants can accumulate heavy metals from polluted soils on their shoots and use this to defend themselves against herbivory. One possible strategy for herbivores to cope with the reduction in performance imposed by heavy metal accumulation in plants is avoidance of contaminated tissues. Such avoidance, however, may hinge upon the specific conditions faced by herbivores.Here, we tested whether the spider-miteTetranychus urticaeavoids tomato plants contaminated with cadmium in presence of conspecifics or heterospecifics and depending on the frequency of contaminated plants.We show that individual spider mite females do not discriminate between discs with or without cadmium. However, in a set-up where 200 mites were simultaneously given the choice between four plants with or without cadmium, they collectively avoided plants with cadmium. This happened when 50% of the plants were uncontaminated, but also when only a single plant contained no cadmium. In addition,T. urticaedid not discriminate between plants infested with its competitorT. evansiand other uncontaminated plants but they preferred plants with competitors when the other plants contained cadmium.Our results show that aggregation may be an important mechanism through which spider mites avoid contaminated plants. They also indicate that cadmium accumulation in plants is a stronger selective pressure than interspecific competition withT. evansi. This suggests that non-accumulating plants will suffer more from herbivory than accumulating plants in metal polluted environments.Synthesis and applications. We show that collective avoidance of metal-accumulating plants by herbivores is robust to environmental conditions and may have important consequences for species distribution and interactions in metal contaminated sites.