‘Harem’ and ‘non‐harem’ type mating systems in two species of subsocial spider mites (Acari, Tetranychidae)

Summary The mating system of a subsocial spider mite, Schizotetranychus miscanthi Saitō, which is closely related to Schizotetranychus longus Saitō (the long seta form of Schizotetranychus celarius (Banks) is a synonym of the latter) was studied in comparison with that of S. longus . Comparisons between nesting patterns of the two related species, S. miscanthi and S. longus revealed a difference in distribution of males among nests. Although more than one male sometimes occurred in the large nests of S. miscanthi , most nests were occupied only by a single male. On the other hand, many nests of S. longus included several males. Behavioral experiments revealed that the male and females of S. miscanthi which cohabited in a nest defended their offspring from phytoseiid predators. Observations and a census of the nesting pattern in a wild population indicate that this is the second example of biparental defense and of a subsocial life‐pattern in spider mites. Differences in mating systems were experimentally demonstrated in the two species. Only a single male of S. miscanthi survived in a nest, as a result of highly aggressive male‐male combat, while two males of S. longus cohabited in a nest. The mating system of the former species is thus considered as harem polygyny, while that of the latter as scramble type polygyny. Furthermore, observations by video recording and scanning electron microscopy showed that the winning male in the male‐male combat in S. miscanthi often preyed on the loser, suggesting cannibalism among them.