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.

Counterattack success of a social spider mite against two predominant phytoseiid predator species.

A survey was conducted of the predator fauna occurring in and around the nests of the two forms (LW: low male aggression and HG: high male aggression) of Stigmaeopsis miscanthi (Saito) that occur in Japan. Two phytoseiid species, Neoseiulus womersleyi (Schicha) and Typhlodromus bambusae Ehara predominated in S. miscanthi nests and their respective occurrence frequencies were the same in LW form nests as in HG form nests. We examined the counterattack success of S. miscanthi LW form males against these two phytoseiid predators. It was shown that while LW form male(s) could kill or effectively drive the larvae of both predator species out of their nests, there were no significant differences in the male counterattack success rate between 1-male and 2-male defended nests, or against the two predator species. On the other hand, there was a significant difference between the two predator species' behavioral response to male-defended nests.

The parapatric distribution and contact zone of two forms showing different male-to-male aggressiveness in a social spider mite, Stigmaeopsis miscanthi (Acari: Tetranychidae).

Two forms showing different male-to-male aggressiveness, different male morphologies and different diapause attributes are known in Stigmaeopsis miscanthi (Saito), a social spider mite infesting Chinese silver grass (Miscanthus sinensis Anderss). Reproductive isolation exists between the forms, although it is not always complete, and the details of their distributional patterns are unknown, but expected to be parapatric. We searched for the contact zone at Mt. Unzen (Nagasaki Pref., Japan) where both forms are known to occur at different altitudes. We found the two forms together in several M. sinensis stands, suggesting there is frequent contact between the forms in their boundary area. We discuss the mechanism(s) that maintain the parapatry related to their frequent contact and the pattern of reproductive isolation between them.

Ambulatory migration in mites (Acari: Tetranychidae, Phytoseiidae) to new leaves of moso bamboo shoots.

The migratory behaviour of two tetranychid pest species, Aponychus corpuzae and Schizotetranychus nanjingensis, and one phytoseiid, Typhlodromus bambusae, was studied in several monocultural bamboo forests in Fujian Province, China. The aim of the study was to assess how the ambulatory immigration of tetranychid and phytoseiid mites from the ground to new leaves is affected by a sticky barrier around the stem, by the age of bamboo shoots or by shoot density. The results show that while the sticky barrier is particularly effective at disrupting the ambulatory immigration from the ground to new leaves of S. nanjinigensis to 1-year-old shoots and of A. corpuzae to 3-year-old shoots, it has no significant effect on the immigration of the phytoseiid mite.