Lygus Bugs in Arizona Regain Susceptibility to Key Insecticides

Adult lygus bugs, Lygus hesperus (Knight), were collected from alfalfa fields in 22 different cotton producing areas of Arizona. A standardized, glass vial method was used to estimate susceptibility of the collected populations to the pyrethroid insecticide Captures (bifenthrin) and the organophosphate Orthene® (acephate). Large differences were recorded in susceptibility of Arizona populations to both insecticides. The most susceptible lygus populations continue to be found in the eastern areas of the state and the least susceptible in central Arizona. Lygus from throughout the state were substantially more susceptible to Capture and Orthene in 1996 than in 1995. Lygus bioassayed repeatedly from the same locations in 1996 exhibited moderate-to-small seasonal variability in susceptibility to Capture. However, some of the same populations varied widely in susceptibility to Orthene in 1996. The large changes in susceptibility to Orthene were attributed to episodic movements of lygus from other hosts. Eleven insecticides were evaluated against populations most and least susceptible to Capture and Orthene to identify promising candidates for future lygus field trials. A Safford population was substantially more susceptible than a Maricopa population to Admire®, Curacron®, Cygon®, Malathion® and Vydate®. Most surprising was the greatly reduced susceptibility to Admire of the Maricopa population. The older insecticides Bidrin®, Lannate®, Monitor ®, Naled®, Ovasyn® and Thiodan® were quite similar in toxicity to both the Safford and Maricopa populations and therefore are good candidates for further field evaluations to judge their merit for inclusion in lygus insecticide rotations. Bioassay results were related to field performance of four insecticides in a field trial conducted in Central Arizona. Findings showed that the two insecticides that caused the lowest mortality in vial bioassays, Orthene and Vydate, resulted in the greatest suppression of lygus in the field. These findings underscore that the absolute level of mortality observed in bioassays should not be assumed to reflect relative efficacy in the field. It also confirms that Orthene and Vydate continue to be good choices for lygus control, even in Central Arizona where populations are decidedly less susceptible to these insecticides. We have demonstrated that the adult vial bioassay provides a reliable method for measuring differences between Arizona lygus populations in susceptibility to a broad range of conventional insecticides. We found the method sufficiently sensitive to repeatedly detect significant within-season and regional differences in lygus susceptibility. These new insights will allow us to better manage the insecticides used in Arizona to control this important pest