Monitoring Laboratory and Field Biotypes of the Walnut Aphid Parasite, Trioxys pallidus, in Population Cages Using RAPD-PCR

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Abstract
A discriminate analysis, based on the frequencies of occurrence of six random amplified polymorphic DNA (RAPD) fragments, distinguished individuals of a pesticide-resistant laboratory biotype (SEL) of the walnut aphid parasite, Trioxys pallidus Haliday (Hymenoptera: Aphidiidae), from individuals of a wild orchard biotype (RB). In mixed populations in the laboratory, the SEL biotype was heavily favored, irrespective of pesticide treatment, suggesting a high degree of laboratory adaptation. The resistance levels in untreated and pesticide-treated mixed (SEL + RB) populations were not different after eight generations. Analyses of RAPDpolymerase chain reaction (PCR) data suggested that the selective advantage of the SEL biotype may have been aided by a partial mating incompatibility between the biotypes. The data can be explained by either of two types of reproductive incompatibility: (1) RB females mating with SEL males produced only male offspring; or (2) the RB biotype could not successfully find mates under laboratory conditions. Successful discrimination of T. pallidus biotypes and their hybrid progeny in population cages suggests that RAPD-PCR offers a method to determine the fate of the pesticide-resistant SEL biotype after release into orchards populated by the RB biotype.