Extraordinary effects of fertilization status on the reproduction of an arrhenotokous and sub‐social spider mite (Acari: Tetranychidae)

Summary Experimental observations on the arrenotokous reproductive patterns of two spider mite species (Acari: Tetranychidae), the long‐seta form of Schizotetranychus celarius ( Banks ) and Tetranychus urticae Koch , revealed that reproduction of unfertilized females of the former is very differnt from that of the latter. Unfertilized females of S. celarius , which has a subsocial life, laid a few eggs and then became inactive. In contrast, the fecundity of unfertilized T. urticae females was only slightly reduced as compared with fertilized females. Mother‐son matings may thus sometimes occur in natural S. celarius populations. A two‐year field survey revealed that, in the absence of males, overwintering females of S. celarius occasionally remain unfertilized until early spring. Furthermore, nest foundation observed in late spring indicated that most of the season's first nests were founded by single females. These two sets of observations strongly suggest that motherson mating takes places in nature, corresponding to the reproductive trait seen in the experiment. Mother‐son mating inevitably increases the relatedness between nest members. The estimated father's relatedness to its offspring is extraordinarily high under such condition. The possibility that kin‐selection in the long seta‐form of S. celarius led to subsociality, especially paternal care, is suggested.