Resistance to Pyrethroid Insecticides in Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae)

Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), commonly referred to as the bollworm and corn earworm, is a polyphagous pest of important crops including cotton, corn, grain sorghum, soybeans, and tomatoes. One of the most common and relatively inexpensive control measures for this pest is the use of pyrethroid insecticides. The objective of this research was to determine if the current method of resistance monitoring, the male adult vial test, is an accurate predictor of resistance in larvae and ultimately of field failure of pyrethroids. LC₅₀ and LC₉₀ values were estimated for a susceptible H. zea colony using the pyrethroids cypermethrin, esfenvalerate, and bifenthrin in third instar and adult vial tests, which expose insects to different concentrations of insecticide residues on glass vials. Resistance ratios and likelihood ratio tests of equality and parallelism were analyzed using PoloPlus software. Preliminary results indicate that third instar larvae were more sensitive than adult male moths to cypermethrin; however, larvae were less sensitive than moths to bifenthrin and esfenvalerate. These results are important because current resistance monitoring programs utilize cypermethrin to test adult male moths for resistance. Differential responses of larval and adult H. zea to different pyrethroids may greatly affect the accuracy with which resistance can be predicted. This may explain why control of larval populations has been successful in areas where significant levels of resistance have been detected using the cypermethrin adult vial test.