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

Accumulation of heavy metals by plants can serve as a defence against herbivory. Herbivores, in turn, may avoid feeding on contaminated tissues. Such avoidance, however, may hinge upon the specific conditions faced by herbivores. Here, we tested whether the spider mite Tetranychus urticae avoids 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 preferentially move to leaf tissues with or without cadmium, despite clear costs on their performance. 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, irrespective of the proportion of plants with cadmium. In addition, T. urticae did not discriminate between plants infested with its competitor T. evansi and other uncontaminated plants but they preferred plants with competitors when the other plants contained cadmium. Our results show that aggregation may facilitate avoidance of contaminated plants. They also indicate that cadmium accumulation in plants is a stronger selective pressure than interspecific competition with T. evansi. Therefore, 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.