Carnivorous predatory mites avoid laying eggs near herbivorous caterpillar tracks; Discovered by Kyoto University

A research group at Kyoto University has discovered that predatory mites (Phytoseiidae), which feed on the vegetable-eating pest known as the spider mite, avoid laying eggs near the "footprints" (tracks) left by butterfly and moth larvae (caterpillars). This behavior is believed to be a strategy to avoid "accidental predation," being swallowed unintentionally by the overwhelmingly larger herbivorous insects as they consume leaves. The group suggests that using the chemical components of these tracks as a biological control in agriculture could steer both the pest spider mites and their natural enemy, the predatory mites, to the same locations, potentially reducing crop damage.

In the natural world, the relationship between "eater" and "eaten" typically involves carnivores eating herbivores that consume plants. However, when a large herbivore eats a plant, it may accidentally consume tiny carnivores living there. Related to this, Graduate Student Shiori Kinto at Kyoto University's Graduate School of Agriculture, who specializes in ecology, discovered in 2023 that herbivorous spider mites, which are less than 1 mm long, avoid the chemical components left by the tracks of caterpillars several centimeters in length. This time, working with Assistant Professor Shuichi Yano at Kyoto University (also ecology), she decided to investigate whether carnivorous predatory mites, which eat spider mites, also avoid these tracks.

According to Yano, predatory mites are fast for their size and can escape an approaching caterpillar, but their eggs cannot. In an experiment using two species (Euseius sojaensis and Neoseiulus womersleyi), the researchers offered leaves of Causonis japonica and kidney bean that caterpillars had walked on and leaves they had not to find out on which type of leaf each species chooses to lay eggs. Both species of predatory mites almost never laid eggs on the leaves with caterpillar tracks.

The researchers also tested stems. Using kidney bean stems, they observed whether the mites laid eggs on stems crossed by T. japonica versus untouched stems. The mites avoided the stems with the caterpillar's footprints.

According to Kinto and Yano, this is the first study to show that a predator (carnivorous insect) possesses a defensive strategy to avoid being accidentally preyed upon by an animal from a lower trophic level (herbivorous insect). The 2023 study confirmed that spider mites avoid the chemical components of caterpillar tracks. Since predatory mites appear to avoid the same chemical components, identifying the specific chemicals in caterpillar tracks that both the pest spider mites and their natural enemies dislike could be a game-changer. By using those components, farmers could potentially herd both types of mites into the same location. This would make it easier for the predatory mites to find and eat the spider mites, leading to more efficient biological pest control.

The study was published on April 7 in the electronic version of the American international academic journal Ecosphere.