Soldier behaviour and division of labour in the aphid genus Pemphigus (Hemiptera, Aphididae)

Summary. We devised a test for quantitatively measuring the defensive behaviour of individual aphids of the gall generations of the genus Pemphigus. Five species (P. spyrothecae, P. gairi, P. bursarius, P. phenax, and P. protospirae) were shown to have soldiers, but the early instars of Pemphigus populi showed no aggressive behaviour. A higher proportion of the first instars of P. spyrothecae showed aggressive behaviour, compared with the other four soldier-producing species. Although the first instars were always the most likely to be aggressive, later instars also showed significant levels of aggression in all the soldier-producing species. Soldier behaviour was elicited by a range of stimuli. A live, restrained, predator was the most effective stimulus tested, but physical stimulation (with a paintbrush) enhanced the attack rate. The aphids could distinguish between lures soaked in water and those soaked in aphid haemolymph, which elicited a higher level of response. First instars of the early and late gall generations of P. spyrothecae were equally likely to attack predators. Although the species with the most specialized soldiers (P. spyrothecae) also had the most prolonged gall phase, this is clearly not the only factor that is important in the evolution of soldiers in this genus.