DEFENDERS IN THE NORTH-AMERICAN APHID PEMPHIGUS-OBESINYMPHAE

Gall-inhabiting individuals of the aphid Pemphigus obesinymphae act as defenders, protecting other colony members against attack by dipteran and neuropteran larvae that are the primary predators of this species . As first instar nymphs, the progeny of the fundatrix patrol surfaces of galls and adjoining leaves. These first instar nymphs attack poten tial predators by mounting and grasping them and inserting their style ts. This defensive behavior, which is not exhibited by nymphs in later instars, appears to be effective in reducing predation. The fundatrix typically produces defenders throughout the extended gall-inhabiting phase, and her progeny delay development beyond the defensive first in star stage. By August, galls contain an average of 101 defenders. Earl y death of the fundatrix reduces the number of defenders in the gall a nd advances maturation of defenders into winged migrants. which otherw ise mature in September and October. In Pemphigus, defensive behavior by first instar nymphs appears to have evolved in the context of sever al types of derived life cycle, each involving an extended gall-inhabi ting phase.