Variation in social behavior within a spider mite genus, Stigmaeopsis (Acari : Tetranychidae)

The spider mites belonging to the genus Stigmaeopsis construct extremely dense oval woven roofs (web-nests) over depressions on the lower surface of host leaves and are known to have a kind of sociality. The four species that occur on bamboo plants in Japan show different nest areas. The nest area of Stigmaeopsis longus is the largest, followed by that of S. celarius, S. takahashii, and S. saharai in decreasing order. Smaller nests effectively prevent adults of several predator mite species from intruding. We hypothesized that variation in nest size reflects different anti-intruder adaptations of this mite group in relation to their sociality. The larger nest makers may adopt an alternative anti-intruder strategy, namely, counterattack by a large group, so as to compensate for the disadvantage of large nests. S. longus and S. celarius adults effectively defended their large nests against potential predators, and the effects of nest defense increased with the number of individuals in a nest. S. takahashii and S. saharai revealed no counterattack effect. Counterattack abilities that increase with the adult density, and thus, sociality may compensate for the vulnerability of larger nests.