Considerations for Pyrethroids (Bifenthrin and Others) Used to Manage Bollworm in Cotton and Soybean
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.
Vial test survivorship generally increases across the growing season in a given location. Already in NC during 2017, survivorship has averaged 58%; I expect this to increase. This doesn’t mean that 58% of the fields will experience pyrethroid failures or that 58% of the moths sprayed with pyrethroids won’t die. My interpretation is that we should expect more failures than last year if pyrethroids are used this year.
Diet based bioassays: Sally Taylor, VA Tech Entomologist, has tested three populations collected from North Carolina corn during 2017. These three collections were made from non-Bt corn in three adjoining Coastal Plains counties. All three populations were resistant to Bt cotton (bioassay results from the Reisig Lab). Two populations were susceptible to pyrethroids, but one was resistant to pyrethroids. Because this population was also resistant to Bt cotton, I would expect that a pyrethroid would have little effect if it were sprayed against this population in cotton.
Other considerations: Field effects of resistance are variable for many reasons. Pest pressure is probably the overriding factor. Heavy bollworm infestations can make a problem more apparent, while light infestations can mask a problem that is brewing. Populations are not uniform. Some may be more resistant to Bt cotton, while others may be entirely susceptible. The same is true for pyrethroids. Layer on other environmental factors plus coverage issues (bollworm larvae are ninja level for hiding in cotton) and making a blanket recommendation is next to impossible.
What should growers use to treat bollworms: Based on the above information, pyrethroids are still an option (based on what little information we have (diet based bioassays) susceptible populations are around). Growers choosing to use pyrethroids may experience phenomenal control or lackluster control. Other effective options include Besiege, Blackhawk, Orthene, Intrepid Edge, Prevathon, and Steward. Some of these products bring the added benefit of preserving natural enemies, which can help out with management of other pests or late-season bollworms.