Kyoto, Japan -- Predators don't expect to be preyed upon, and especially not by herbivores such as caterpillars. The slow-moving, leaf-eating larvae may only intend to consume plants, but sometimes tiny creatures making their homes under leaves unwittingly become dinner: cue the mites.
Tiny herbivores can be subject to intraguild predation, or IGP, when larger herbivores accidentally consume them along with a host plant. Vegetarian spider mites are intentionally preyed upon by predatory mites, which climb onto leaves to eat spider mites and to lay eggs in sheltered spots. Adult predatory mites can evade approaching caterpillars, but eggs laid on leaves cannot escape.
Inspired by their previous finding that spider mites avoid traces left by large caterpillars, a team of researchers at Kyoto University investigated whether female predatory mites avoid caterpillar traces when laying their eggs. While other previous research has shown that some predators avoid counterattacks by herbivores, there are few reports of predators evading large herbivores to avoid being accidentally eaten.
"Because such incidents are unintended and accidental, their ecological importance has often been overlooked; however, the consequences are fatal for the prey," says corresponding author Shiori Kinto.
First, to assess whether caterpillars would consume predatory mite eggs, the scientists offered leaves of a vine plant -- containing eggs from two mated female mites -- to the caterpillars of two different moth species. Then separately they conducted a choice test to see if female predatory mites would avoid caterpillars. The team induced a caterpillar to walk across one leaf a few times, then positioned that leaf next to another with no caterpillar traces and introduced a female predatory mite to a piece of film in contact with both leaves, recording the number of eggs laid on each leaf. The team then conducted a similar experiment using the stems of kidney bean plants.
The results revealed that caterpillars accidentally consume predatory mite eggs along with leaves, the first report of herbivorous insects accidentally consuming carnivores. Furthermore, the team found that predatory mites avoid laying eggs on plant leaves bearing traces of caterpillars, providing the first evidence of a predator avoiding the traces of an herbivore.
The researchers were also surprised to find that predatory mites even avoid the traces of caterpillars they're unlikely to encounter in the wild. "For tiny mites with limited information-processing capabilities, a rough-and-ready yet practical risk avoidance strategy -- such as 'if it smells dangerous, stay away from it' -- is likely the best approach," says Kinto.
Predatory mites are widely used in agriculture as biological control agents against spider mites, which are major crop pests. If both mites avoid caterpillar traces, they are more likely to encounter each other in areas where such traces are absent, which could potentially enhance the effectiveness of biological control. Future research will involve identifying the chemicals present in caterpillar traces that predatory mites avoid, and investigating whether caterpillar traces influence the effectiveness of predatory mites.
