Source: Dominic Reisig, NCSU COOPERATIVE EXTENSION SERVICE, 8/8/17


Pyrethroids (common examples include Baythroid XL, bifenthrin, Declare, Karate Z, and Mustang Maxx) have been used for managing bollworm (corn earworm) in the Southeast for over 40 years. Given this amount of time, and the prevalence of this insecticidal class in field crops, it is not surprising that resistance might crop up. Pyrethroids have been especially useful in Bt cotton, since larvae that survived the Bt were smaller and slower to develop. The effect of Bt cotton increased their susceptibility to insecticides, such as pyrethroids, possibly even in cases where they might have developed some pyrethroid tolerance.
Field failures of pyrethroids have been most common in soybeans. Often it was difficult to tell if this was because tobacco budworm was present (tobacco budworm have been resistant to pyrethroids since the 1980’s), if there was an application issue, or if the bollworm were resistant to pyrethroids. However, in a few cases, we have suspected that pyrethroid failures in soybeans were due to resistance in bollworm.

Adult vial tests– Entomologists across the southern US have been monitoring pyrethroid resistance using a vial test. In this test, male moths are exposed to a single dose of pyrethroid in a vial and survival is recorded. Importantly, survivorship has increased yearly in every state where these tests were done. While it is difficult to connect the vial test to pyrethroid resistance, field failures have also been increasing every year.

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