SOCIAL FAMILIARITY AFFECTS GROUP-JOINING DECISIONS OF THE PREDATORY MITE PHYTOSEIULUS PERSIMILIS Muluken G. MULETA1 & Peter SCHAUSBERGER1 1Group of Arthropod Ecology and Behavior, Division of Plant Protection, Department of Crop Sciences, University of Natural Resources and Life Sciences, Peter Jordanstrasse 82, 1190 Vienna, Austria mgoftishu@haramaya.edu.et, peter.schausberger@boku.ac.at BACKGROUND and HYPOTHESES Group membership decisions are of critical importance to group-living animals (Krause & Ruxton 2002). When approaching a group of conspecifics, individuals are faced with the decision to join or not to join and search for another group. Joining behaviour occurs when the individual responds to a stimulus resulting in its non-transient association with the group (Prokopy & Roitberg 2001). In previous studies we found that in mixed-age groups of familiar and unfamiliar individuals of the group-living predatory mite P. persimilis, familiar individuals preferentially grouped together (Strodl & Schausberger 2012). Familiar mites foraged more efficiently, i.e. needed less prey at similar developmental speed and body size at maturity, than unfamiliar mites (Strodl & Schausberger 2012). Here, we assessed whether social familiarity affects group joining decisions of adult P. persimilis females. THE MITES Phytoseiulus persimilis lives in groups and is a highly specialized predator of tetranychid mites such as Tetranychus urticae. Spider mites are patchily distributed on their host plants and so are the predators that forage, reproduce and develop in the spider mite webbings. These circumstances lead to repeated encounters of the predators, raising opportunities to familiarize. The ability of P. persimilis to discriminate familiar from unfamiliar conspecifics has been observed in various contexts such as kin cannibalism (Schausberger & Croft 2001), egg placement (Schausberger 2005, 2007), foraging and development (Strodl & Schausberger 2012). RESULTS No matter whether the design was balanced or not with respect to prey egg density at the two connected leaflets, P. persimilis females had a highly significant preference for joining the familiar group (GEE; balanced: Z = -3.435, P < 0.001; little biased to unfamiliar: Z = -3.578, P < 0.001; little biased to familiar: Z = -4.025, P < 0.001; moderately biased to unfamiliar: Z = -3.578, P < 0.001; moderately biased to familiar: Z = - 4.025, P < 0.001; strongly biased to unfamiliar: Z = -3.628, P < 0.001; strongly biased to familiar: Z = -4.025, P < 0.001) (fig. 1). Overall, attraction to socially familiar individuals was not counterbalanced by attraction expected from higher prey densities. Preference for joining the familiar group did not differ between designs (Wald-x2 = 3.548, P = 0.738) and did not change over time (Wald-x2 = 5.275, P = 0.135) (fig. 1). Fig. 1. Joining behaviour of P. persimilis females given a choice between a socially familiar and an unfamiliar group. Prey egg distribution: B-balanced; LBUF-little biased to the unfamiliar; LB-little biased to the familiar; MBUF-moderately biased to the unfamiliar; MBF-moderately biased to the familiar; SBUF- strongly biased to the unfamiliar; SBF-strongly biased to the familiar (see experiments for details). Phytoseiulus persimilis Tetranychus urticae TU © P. Schausberger EXPERIMENTS Group joining decisions were assessed in choice situations of two bean leaflets infested with spider mites and connected via a wax bridge. Four different prey egg densities were used: 90/90 (balanced), 75/100 (little biased), 75/125 (moderately biased), and 75/150 (strongly biased) on each leaf disc. Before the test P. persimilis female was released, 3 familiar gravid females were placed onto each of the two leaflets. One group was familiar to the test female, the other group was unfamiliar. After release, the position, number of prey eggs eaten, and number of eggs laid were assessed. CONCLUSIONS Group joining behaviour was significantly influenced by social familiarity. Preference for socially familiar groups was stronger than preference expected from prey density, i.e. attraction to socially familiar individuals in low prey density patches more than outweighed attraction to socially unfamiliar individuals in high prey density patches. REFERENCES Krause, J. & Ruxton, G.D. (2002). Living in groups. Oxford University Press, Oxford, UK. Prokopy R.J. & Roitberg B.D. (2001). Joining and avoidance behavior in non-social insects. Annu. Rev. Entomol. 46, 631– 665. Schausberger, P. (2005). The predatory mite Phytoseiulus persimilis manipulates imprinting among offspring through egg placement. Behav. Ecol. Sociobiol. 58, 53-59. Schausberger, P. (2007). Kin recognition by juvenile predatory mites: prior association or phenotype matching? Behav. Ecol. Sociobiol. 62, 119-125. Schausberger, P. & Croft, B.A. (2001). Kin recognition and larval cannibalism by adult females in specialist predaceous mites. Anim. Behav. 61, 459-464. Strodl, M. & Schausberger, P. (2012). Social familiarity modulates group-living and foraging behaviour of juvenile predatory mites. Naturwissenschaften 99, 303-311. Funded by oead-appear and