Individual spider mites cooperate with outsiders to cope with natural enemies

Herbivorous and carnivorous arthropods on a leaf form deadly predator-prey interactions, just like herbivorous and carnivorous mammals of the savanna. Spider mites are well-known agricultural pests that are as small as < 0.5 mm. They live together in the space between leaf surfaces and thin silk webs. Since individuals living in a group generally incur the costs of increased competition for food or space among group members, group living does not pay unless it has a considerable benefit. This study revealed that spider mites live in a group for cooperative defense against natural enemies.

Spider mite webs act as refuges that most natural enemies cannot access. However, spider mites have to build new webs every time they move to a new food leaf, and they are exposed to natural enemies (e.g., the predatory mite Euseius sojaensis) until the new webs are completed. During this period, the per capita predation on spider mites is diluted in larger groups seemingly because webs were completed while the initial prey was eaten. An individual spider mite that has to build a web alone gets a free ride by joining webs established by others. Conversely, an original inhabitant mite that had labored building the web is never kicked out, but readily hosts the newcomer mite. This is because it is advantageous for the hosting mite to cooperate with the newcomer mite against natural enemies.

Surprisingly, this interaction is consistent even when it involves different mite species (Tetranychus urticae and Tetranychus kanzawai). This apparent cooperation between species may be a byproduct of within-species cooperation. However, since the two species can discriminate mates for copulation, they may share webs to benefit from cooperation, while being aware that the residents are different species. It is difficult for humans to form alliances with very different people; however, this appears to be common behavior for mites that are always faced with life-and-death decisions.

The finding of Assistant Professor Shuichi Yano of the Graduate School of Agriculture was published in Behavioral Ecology and Sociobiology online.

Provided by Kyoto University