Waste management behavior is essential to achieve nest sanitation that is highly inferential on the evolution of group living because nest waste is an inevitable cost. However, how group living animals dispose of waste has not attracted much attention. Schizotetranychus miscanthi Saito is a social spider mite infesting a perennial grass (Miscanthus sinensis Anderss), in which all nest members tend to defecate at specific sites. We investigated the mechanisms by which the individuals select the site of defecation. The results show that nest members defecate at only one site inside the nest, and that waste management is maintained by two simple rules. First, mites defecate near the nest entrances if no volatile chemical cues are available, and secondly, when chemical cues are available from feces deposited previously, they defecate at this site. We discuss the adaptive significance of these mechanisms, as well as their role in the evolution of sociality in mites.

The nest‐weaving spider mite Schizotetranychus miscanthi Saito showed a cline in male‐to‐male aggression intensity with minimum winter temperature. The altered kin structure in spring nests, which might be caused by winter harshness, was hypothesized to be a key factor responsible for the cline in male‐to‐male aggression. In Japan, we found two forms (high‐ and low‐aggression forms) that showed different clinal trends with similar regression slopes and different intercepts. The former is characteristic of local populations from milder climates, whereas the latter occurs in harsher climates. We hypothesized that the intensity of female diapause, which may determine whether males are produced during late winter, was one of the factors separating these two clinal forms. In the four populations studied, short day length during the developmental period (egg to adult emergence) induced diapause in females. Diapause intensity varied greatly among populations, and particularly between the high‐aggression and low‐aggression forms. These data, together with data on winter harshness between the localities, led us to believe that female diapause is one of the factors affecting relatedness between interacting males in nests and can explain why the two forms of S. miscanthi have different clinal trends in male aggression in Japan.

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Similar nest-weaving habits and sociality characterize all the species belonging to the recently reinstated spider mite genus Stigmaeopsis. Molecular phylogenetic analysis using 28S rDNA shows that Stigmaeopsis forms a distinct clade and that there is another unrecognized clade involving Schizotetranychus species with comparable nest-weaving habits distributing in tropical regions. Furthermore, the phylogenic hypothesis suggests that Stigmaeopsis miscanthi (Saito) inhabiting Miscanthus sinensis Anderss. shared a common ancestor with Stigmaeopsis longus (Saito) inhabiting Sasa senanensis (Franch. et Sav.), both mite species having very similar social systems. We confirmed reproductive isolation between them and tested the suitability of the two plant species as hosts for these two mite species. The result that S. miscanthi immatures could survive on Sasa bamboo for a long period, but S. longus immatures could not survive on Miscanthus grass suggested that the former has evolved from a common ancestor living on Sasa bamboo. This is also supported by a third piece of indirect evidence obtained from observing mite responses to the plant-originated chemical cues involving feces used for waste management in these species. Therefore, we conclude that the host plant shift of S. miscanthi from Sasa bamboo to M. sinensis played a key role in speciation.

We previously determined that certain recessive genes decrease female fecundity in a haplo-diploid spider mite, Stigmaeopsis miscanthi (Saito). However, whether the depression was caused by the breakdown of heterosis or the expression of deleterious genes retained in a population could not be determined, because we had started our inbreeding experiment from a mixture of two isolated populations. In order to answer this basic question, inbreeding effects on survival and fecundity were measured for eight small populations occurring far from the two initial populations. There was little depression of immature survival of inbred lineages in all populations. On the other hand, in two inbred lineages, both originating from the smallest populations, female oviposition decreased significantly with the increase of Wrights f-value, showing that mildly deleterious genes are actually retained even in natural populations of haplo-diploid organisms.