Tiny predators avoid herbivorous caterpillar traces to prevent accidental predation

We report the first example of predators having a strategy to avoid accidental predation by herbivorous insects. The predatory mites <i>Neoseiulus womersleyi</i> and <i>Euseius sojaensis </i>are smaller than 0.5 mm and lay eggs on plant leaf surfaces; thus, their immobile eggs would be incidentally consumed along with leaves by voracious lepidopteran caterpillars. We experimentally demonstrated that eggs of both mite species were preyed upon by tested hawkmoth caterpillars (<i>Theretra oldenlandiae</i>, <i>Theretra japonica</i>) along with leaves. Therefore, the ability to avoid such accidental predation should confer a selective advantage to mites. We further demonstrated that adult females of both mite species avoided laying eggs on leaves with traces of all tested caterpillars (<i>T. oldenlandiae</i>, <i>T. japonica</i>, <i>Papilio xuthus</i> and <i>Bombyx mori</i>), indicating that eggs may avoid accidental predation by voracious caterpillars that may be nearby. This is the first demonstration of a repellent effect of herbivorous insect traces on carnivores. Considering previous studies showing that spider mites as small as predatory mites also avoid caterpillar traces, the same need to avoid predation by huge caterpillars may have led to the development of the same solutions for both spider mites and predatory mites.

本研究首次报道了捕食者演化出规避植食性昆虫意外捕食的策略案例。体长不足0.5毫米的捕食螨（predatory mites）——Neoseiulus womersleyi与Euseius sojaensis——会将卵产于植物叶面；因此其无法移动的卵会被贪食的鳞翅目幼虫连同叶片一同误食。本研究通过实验证实，两种捕食螨的卵均会被受试的天蛾幼虫（Theretra oldenlandiae、Theretra japonica）连同叶片一同取食。因此，具备规避此类意外捕食的能力，应当能为捕食螨带来选择优势。本研究进一步证实，两种捕食螨的雌成螨均会避开带有所有受试幼虫痕迹的叶片产卵，受试幼虫包括Theretra oldenlandiae、Theretra japonica、Papilio xuthus与Bombyx mori，这表明其卵可通过这种方式规避周边贪食幼虫的意外捕食。本研究首次证实植食性昆虫痕迹对肉食动物具有驱避效应。结合此前针对体型与捕食螨相仿的叶螨（spider mites）同样会避开幼虫痕迹的研究结果，规避大型鳞翅目幼虫捕食的相同演化压力，或许推动了叶螨与捕食螨演化出趋同的应对策略。