2011 Annual Report 1a.Objectives (from AD-416) Although aphid endosymbionts are known to confer resistance to attack by some parasitoid species, very little is known about the impacts such endosymbionts have on biological control of pest aphids. We will determine whether the aphid endosymbiont HAMITONELLA DEFENSA affects parasitism by APHELINUS species being considered for introduction against pest aphids. 1b.Approach (from AD-416) We will test whether parasitism of APHIS CRACCIVORA by APHELINUS species that are candidates for introduction against soybean aphid is affected by the presences of HAMITONELLA DEFENSA. 3.Progress Report In experiments with APHIS CRACCIVORA, we found that the presence of a symbiotic bacteria of aphids (HAMITONELLA DEFENSA) known to affect parasitism by aphidiine parasitoids of aphids had no effect on resistance to parasitism by two species of APHELINUS, one with a narrow host range and another with a broad host range. APHELINUS has large, heavily chorionated eggs that absorb nothing from their hosts during development, whereas aphidiine parasitoids heavy small porous eggs that absorb nutrients from their hosts, and thus allow uptake of the toxin produced by the bacteriophage in HAMILTONELLA. This may explain the difference in impact of HAMITONELLA on APHELINUS versus aphidiine parasitism.