Fitness of a Malathion-Resistant Strain of the Parasitoid Anisopteromalus calandrae (Hymenoptera: Pteromalidae)

Biological fitness of a malathion-resistant (R) and a malathion-susceptible (S) strain of the solitary parasitoid Anisopteromalus calandrae (Howard) was compared when the wasps were parasitizing immature rice weevils, Sitophilus oryzae (L.), in stored wheat. Despite having a <2,500-fold naturally occurring resistance, in the absence of insecticide the R strain was equal to the S strain in ability to parasitize hosts and in several developmental parameters. Development times of cohorts of male and female progeny produced over 7 d by Rand S females at 25ºC and 75% RH were not significantly different. There was no significant strain effect on daily fecundity. Parasitization of hosts was not significantly affected by strain-host density interactions. There was no significant effect of strain on total progeny production at different host densities, but more female progeny were produced by the susceptible strain at high host densities. Otherwise, the proportion of females among progeny of the 2 strains was not significantly different. Frequency of the R allele in Hardy-Weinberg populations, set up with an initial R allele frequency of 0.5, was lower than expected but tended to stabilize after 4 generations. Frequency of the R allele in a population started with hybrid females was not Significantly different from values expected under the hypothesis of no fitness costs. Failure to detect fitness costs associated with malathion resistance in A. calandrae could be caused by the lack of negative pleiotropic effects associated with the R allele.