Variation in nesting behavior of eight species of spider mites, Stigmaeopsis having sociality.

Nesting behavior is considered to be an important element of social living in animals. The spider mites belonging to the genus Stigmaeopsis spend their lives within nests produced from silk threads. Several of these species show cooperative sociality, while the others are subsocial. In order to identify the origins of this social behavior, comparisons of nest sizes, nesting behaviors (making nests continuously or separately), and their associated traits (fecal deposition patterns) were made for eight cogeneric Stigmaeopsis species showing various levels of social development. All of these species inhabit bamboo plants (Poaceae). We initially addressed the proximate factor of nest size variation. The variation in nest size of the eight species corresponded well with the variation in dorsal seta sc1 length, suggesting that nest size variation among species may have a genetic basis. The time spent within a nest (nest duration) increased with nest size on the respective host plants. Nest arrangement patterns varied among species showing different sized nests: Large nest builders continuously extended their nests, while middle and small nest-building species built new separate nests, which resulted in different social interaction times among species, and is thought to be closely related to social development. Fecal deposition behaviors also varied among Stigmaeopsis species, suggesting diversity in anti-predatory adaptations. Finally, we discuss how the variation in sociality observed within this genus is likely the result of nest size variation that initially evolved as anti-predator strategies.

Inbreeding depression of female fecundity by genetic factors retained in natural populations of a male-haploid social mite (Acari: Tetranychidae).

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.