Cooperative web sharing against predators promotes group living in spider mites

I examined spider mite cooperative web sharing against predation as a factor promoting group living. Tetranychus urticae and Tetranychus kanzawai infest leaf surfaces under webs made of silk threads. Experimental observation of predation by the predatory mite Euseius sojaensis on spider mites of different group sizes revealed that fewer spider mites were preyed upon when the web-building period before the attack was prolonged, suggesting that established webs help protect spider mites. Moreover, per capita predation on spider mites was diluted in larger groups. This was not due to predator satiation but seemingly because webs had been completed while the initial prey was consumed. Spider mites lived more closely together in the presence of a predator, showing that the degree of group living is facultative. In the presence of a preceding spider mite with an established web, a newcomer spider mite gain protection by taking residence in the established webs; sharing the web was not disadvantageous for the preceding mite. The proportion of individuals preyed upon did not differ between preceding and newcomer mites, suggesting that there was no interference against the latter. These interactions were consistent between heterospecific spider mites. Because there was no detectable indirect interaction between mites sharing fresh webs, cooperative web sharing seemed to be a major force promoting group living in the spider mites. Moreover, the distances between spider mites did not differ between heterospecific and conspecific groups, demonstrating that mites living together do not distinguish between species; hence, heterospecific mites may cooperate and live together in the same manner as conspecifics.