Chlorpyrifos Resistance in Greenbugs (Homoptera: Aphididae): Cross-Resistance and Synergism

Greenbugs, Schizaphis graminum (Rondani), with the pattem-2 resistance factor (enhanced metabolic degradation) were resistant to the three insecticides (carbofuran, chlorpyrifos, and malathion) evaluated in our research. Chlorpyrifos was mixed with selected insecticides and synergists in the laboratory to evaluate the potential of these mixtures for overcoming resistance. The most effective mixture when the chemicals were applied simultaneously in vials was chlorpyrifos + carbofuran, which provided a 14-fold increase in toxicity compared with the chemicals applied individually. Malathion and the synergists piperonyl butoxide and DEF did not increase greenbug mortality when applied Simultaneously with chlorpyrifos in vials. However, when aphids were exposed to the three chemicals 1 h before they were placed in vials treated with chlorpyrifos, mortality increased 20.8-, 10.8-, and 4.5-fold for malathion, piperonyl butoxide, and DEF, respectively. These results suggest that resistance to chlorpyrifos might be overcome if the chemical were combined with carbofuran or if the greenbugs were preexposed to insecticide synergists before application of chlorpyrifos.