Stability of Malathion Resistance in Two Hymenopterous Parasitoids

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Malathion resistance documented in a field strain of Anisopteromalus calandrae (Howard) (Hymenoptera: Pteromalidae) collected in September 1992 near Bamberg, SC, was stable after 23 generations of laboratory rearing with no selection pressure. F23 progeny of this parasitoid strain were 2,800-fold resistant to malathion relative to a susceptible laboratory (Savannah) strain and 200-fold more tolerant of malathion than the field strain of the insect host Sitophilus oryzae (L.) (Coleoptera: Curculionidae) with which they were collected. Seven cycles of laboratory selection with malathion, chlorpyrifos-methyl, and pirimiphos-methyl did not increase resistance levels in the Bamberg strain of A. calandrae to these grain protestants. Malathion resistance in F21 male and female progeny of a field strain of Bracon (Hahruhracon) hebetor Say (Hymenoptera: Braconidae) collected in South Carolina was not significantly different from that of F6 progeny. However, LT50s determined in a serial-time bioassay were 0.2-fold lower and may indicate a slight decline in resistance following 15 generations of laboratory rearing. Little is known about the mechanisms or genetics of the malathion resistance in these two parasitoids. Nevertheless, the stability of the resistances documented in this study, particularly in A. calandrae, will facilitate the incorporation of these parasitoid strains into integrated biological-chemical control programs for stored-product insect pests.