Lethal attack of Schizotetranychus brevisetosus Ehara (Acari: Tetranychidae) on predatory midge larva

Abstract

Spider mites (Acari: Tetranychidae) display an anti-predatory strategy by constructing a web using silk threads secreted from the palp tips to distract predators. Several studies have shown the defensive effects of the web itself as well as web-associated ovipositional behaviours (Mori et al. 1999, Oku et al. 2003, Horita et al. 2004, Lemos et al. 2010, Yano 2012). Conversely, alternative behavioural strategies, such as counterattack against predators, have been only reported in Stigmaeopsis spp. (Saito 2010). Stigmaeopsis spp. densely weave nests over depressions on the surface of the host leaves (known as WN species; Saito 1983, 1985), and several generations overlap in the same nest among species that make larger nests. In S. longus (Saito) that live on the Sasa bamboos, adults attending to their offspring increase the survival rate of the offspring by driving out or killing the phytoseiid larvae that enter the nest (Saito 1986, Mori & Saito 2004). Behaviours such as these are also reported in S. miscanthi that inhabit the Chinese silver grass (Saito et al. 2011, Yano et al. 2011) and S. nanjingensis (Ma and Yuan) that reside on the Moso bamboo (Saito & Zhang 2017). One possible factor allowing the evolution of this somewhat risky behaviour in these species is the high genetic relatedness among the nest members in the colony. This relatedness is because the individuals develop in the same nest where they were born and end up mating with their siblings or even their parents and such inbreeding can be repeated for several generations (Saito 2005, Saito 2010, Sato et al. 2013). Although the extent of genetic relatedness has not been directly measured in any WN species, if the relatedness is very high, similar counterattack behaviour may evolve in other WN species as well. However, there have been no reported observations of counterattack behaviours since the findings in S. longus more than 30 years ago (Saito 1986). This is a brief report of a new type of attacking behaviour in Schizotetranychus brevisetosus Ehara. This species (subtype WN-s) inhabits the bottom surface of leaves of the evergreen oak Quercus glauca Thunb. (Ehara 1989), builds a dense web near the junction of leaf veins and deposits faeces on the nest roof (Saito 1983). Recent phylogenetic studies revealed that S. brevisetosus and the other WN-type congeneric species inhabiting evergreen oaks consist of a separate clade, and this clade is distinct from the monophyletic clade of the genus Stigmaeopsis (Matsuda et al. 2014, Matsuda et al. 2018). Thus, it is believed that the former species independently evolved the WN life type from Stigmaeopsis spp. In S. brevisetosus, multiple adult females co-operatively make a nest and reproduce in a group. As a result, S. brevisetosus might have a primitive form of sociality (Y. Saito, personal communication). The sex ratio at maturity is highly female-biased (Tamura & Ito 2017) where males are observed to be engaged in lethal combat for mating precedence (Masuda et al. 2015). Therefore, the nest members comprise of many females and a few males that are occasionally in a harem (consisting of one male per nest) as in S. miscanthi (Saito 1990). Curiously, some of the adult females in the nest community venture out, walk around the nest and return home as if they were patrolling. These individuals sometimes limp legs or vigorously spin threads (identified by shaking their anterior body). However, the meaning of these behaviours is unknown.