Abstract
Arthropod pests cause economic losses to the agricultural industry through production losses and management costs. Among these pests, filth flies are prevalent in the animal industry, developing in decaying organic matter and can serve as mechanical vectors of disease-causing pathogens. In addition, most filth flies are considered nuisance pests, as their activities often disrupt the day-to-day activities of humans and animals. Filth flies are difficult to manage due to non-specific host-location preferences, their propensity to disperse great distances, and insecticide resistance evolution. However, insecticides continue to be the primary management method utilized by agricultural producers to reduce the impacts of these pests. Although commercially-available insecticides can be effective and fast-acting, they also can detrimentally impact alternative methods such as biological control organisms. Spalangia cameroni Perkins are beneficial parasitic wasps, which often are utilized as biological control organisms to manage filth fly populations such as house flies, Musca domestica L. and stable flies, Stomoxys calcitrans (L.). Because these wasps search for filth fly pupae as hosts in areas likely treated with insecticides, they may be experiencing non-target insecticide selection effects. However, research regarding resistance evolution in parasitic wasps, including S. cameroni, is limited. This study was conducted to 1) determine the potential of S. cameroni to evolve resistance to permethrin, an insecticide often used to manage filth fly populations, under laboratory conditions and 2) determine and compare permethrin susceptibility among several field-collected and long-established S. cameroni strains. Insecticide resistance evolution in this non-target organism has far-reaching implications for potential effects in other agricultural pest management systems. No tendency for increased or decreased susceptibility was observed for the selected strain. However, susceptibility was significantly reduced for the selected strain when compared to that of its unselected parent colony two years later. This was likely due to an absence of insecticidal selective pressures while maintained or an inability to evolve due to fixation of resistance-related genes. Results from geographic comparisons indicated that with the exception of the selected strain, all other strains exhibited similar susceptibility to permethrin as a susceptible strain. A second comparison indicated that an Insectary strain was significantly more susceptible than two field-collected strains. The implications of these findings include potential differences in parasitoid field efficacy and fitness due to differential insecticide susceptibility, and brings to light the potential of unrecognized non-target insecticide exposure effects in other pest systems.