Evaluation of counterattack efficiency against predators in Schizotetranychus brevisetosus using small glass beads and phytoseiid eggs

Counterattacks against predators are reported in several web-nesting spider mites (Acari: Tetranychidae) that display communal sociality. For example, adults of Stigmaeopsis longus (Saito) kill phytoseiid larvae that invade their nests. We recently found similar defensive behaviours in various life stages of Schizotetranychus brevisetosus Ehara, a specialist of evergreen oak Quercus glauca Thunb. (Fagales: Fagaceae), which attack larvae of Feltiella acarisuga (Vallot) (Diptera: Cecidomyiidae) in groups outside the nest. However, due to the limited number of observations, we could not establish the factors inducing this counterattack or its efficiency against general predators. We conducted two experiments to evaluate the degree of S. brevisetosus female attacks against abiotic substances and living organisms. First, we assessed the behaviours towards predator-mimicking glass beads (50–400 μm in diameter) randomly spread over the leaf surface. Second, we monitored the 24-h survival of phytoseiid eggs (Amblyseius swirskii Athias-Henriot, Acari: Phytoseiidae) evenly placed near S. brevisetosus nests under various experimental conditions, including the presence or absence of adult mites, intact or removed webs, and the presence or absence of a phytoseiid larva to stimulate females (because phytoseiid larvae easily enter web nests, their presence may provoke counterattacks). In the first experiment, females responded immediately to the glass beads of all sizes, although their activity gradually declined over the subsequent 3-day period. Three types of behaviours were observed: patrolling around the nest, pecking the glass beads, and grasping them. In the second experiment, approximately 20% of phytoseiid eggs died when females were present, whereas no eggs died in their absence. There was no effect of webs and phytoseiid larvae on the mortality of eggs. These results demonstrated that physical stimuli play a role in evoking counterattacks in S. brevisetosus females and thereby reduce potential predation risks.