The effect of predation risk on an acarine system

Current theory on trait-mediated interactions in tri-trophic food chains shows that antipredator behaviour of the middle species can cause similar indirect effects giving rise to trophic cascades as mediated by density changes. In this article, the effect of predation risk in a tri-trophic food chain (bean plant–two-spotted spider mite prey–predatory mite, Phytoseiulus persimilis) is measured, both on plants (changes in leaf damage) and spider mites (changes in egg numbers, mortality rate and dispersal behaviour), under two risk scenarios. In the predator risk treatment a predatory mite was caged in an experimental cell that was placed above a leaf disc with spider mites. This prevented the predator from subduing prey while the predator was perceived by spider mites as a potential threat. In the predator cues treatment a predatory mite was introduced to the leaf disc for 24 h before placing the spider mites on the disc. Compared to control without predators, after four days we observed in both risk treatments a nonsignificant increase in plant damage per spider mite and a significant decline in spider mite fecundity. No significant effect of predation risk on spider mite mortality was observed. We also showed that the above effects are not uniformly distributed in time. For example, the spider mite fecundity and dispersal rate in the predator cues treatment were significantly different from the control only the first day. We also found that under predator cues treatment, walking activity of the spider mites increased significantly.