MONITORING LABORATORY AND FIELD BIOTYPES OF THE WALNUT APHID PARASITE, TRIOXYS PALLIDUS, IN POPULATION CAGES USING RAPD-PCR

A discriminate analysis, based opt the frequencies of, occurrence of s ix random amplified polymorphic DNA (RAPD) fragments, distinguished in dividuals of a pesticide-resistant laboratory biotype (SEL) of the wal nut aphid parasite, Trioxys pallidus Haliday (Hymenoptera: Aphidiidae) , from individuals of a wild orchard 'biotype (RB). In mixed populatio ns in the laboratory, the SEL biotype was heavily favored, irrespectiv e of pesticide treatment suggesting a high degree of laboratory adapta tion. The resistance levels in untreated and pesticide-treated mixed ( SEL + RB) populations were not different after eight generations. Anal yses of RAPD-polymerase chain reaction (PCR) data suggested that the s elective advantage of the SEL biotype may have been aided by a partial mating incompatibility between the biotypes. The data can be explaine d by either of two types of reproductive incompatibility: (1) RB femal es mating with SEL males produced only male offspring; or (2) the RB b iotype could not successfully find mates under laboratory conditions. Successful discrimination of T. pallidus biotypes and their hybrid pro geny in population cages suggests that RAPD-PCR offers a method to det ermine the fate of the pesticide-resistant SEL biotype after release i nto orchards populated by the RB biotype.