Altruistic defenders in a Japanese gall-forming aphid, Quadrartus yoshinomiyai (Homoptera : Aphididae : hormaphidinae)

We found defensive behavior in a nipponaphidine aphid, Luadrartus yoshinomiyai, which forms conspicuous large galls on its primary host plant Distylium racemosum. When moth caterpillars were introduced into natural galls or experimental arenas, the aphid nymphs aggressively attacked them with their stylet. First-instar nymphs were the main attackers, while some second-instar nymphs also showed attacking behavior. No remarkable morphological differences were observed between the attacking and non-attackingfirst-instar nymphs. Some attacking first-instar nymphs had the next instar cuticle inside the body. These results indicate that Q.yoshinomiyai produces non-specialized, monomorphic first-instar defenders. When disturbed, wingless adult aphids discharged a large amount of adhesive droplets from their cornicles, while nymphs and winged adults discharged none or only a small amount of the droplets. It is conceivable that the cornicle droplets mayhave a direct defensive function against predators and/or contain an alarm pheromone for recruiting defender nymphs. When most of the gall inhabitants developed to winged adults and migrated to the secondary host plant, wingless adults were still alive and many first-instar nymphs remained in the galls. These patterns may suggest the strategy of Q yoshinomiyai for effectively defending the migrant individuals, thereby maximizing the colony-level fitness.