Resistance in Field Strains of the Parasitoid Anisopteromalus calandrae (Hymenoptera: Pteromalidae) and Its Host, Sitophilus oryzae (Coleoptera: Curculionidae), to Malathion, Chlorpyrifos-methyl, and Pirimiphos-methyl

A glass vial bioassay was used to evaluate and compare the sensitivities of laboratory and field strains of the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) and an associated parasitoid, Anisopteromalus calandrae (Howard) (Hymenoptera: Pteromalidae) to three grain protectants, chiorpyrifos-methyl, pirimiphos-methyl, and malathion. The laboratory strain of A. calandrae was very sensitive to the three pesticides. Lethal dose ratios at the LD50 indicated that, relative to the laboratory strain of the host rice weevil, A. calandrae was 22.4-fold more sensitive to chiorpyrifos-methyl and about 8-fold more sensitive to both pinmiphos-metbyl and malathion. However, despite the sensitivity of the laboratory strain of this pteromalid to the pesticides, a field strain of A. calandrae collected near Bambeng, South Carolina in September 1992, survived doses of malathion of 172 μg/vial and was > 1240-fold resistant to malathion at the LD50, relative to the laboratory strain. This field strain of A. calandrae was about 90-fold more resistant to malathion than the field strain of its weevil host. Relative to the laboratory strain, the field strain of A. calandrae was also 5.1-fold resistant to chlorpyrifos-methyl and 7.3-fold resistant to pirimiphos-methyl. The field strain of S. oryzae (collected with the parasitoid) was only weakly resistant to the three grain protectants. A hypothesis for the development of naturally occurring pesticide resistance in the parasitoid A. calandrae and the lack of significant resistance in its associated weevil host is discussed in terms of a stored grain ecosystem.