Insecticide Spray Considerations for Worms in Cotton, Sorghum and Soybean

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I have heard a lot of concern about worms (bollworm, fall armyworm, soybean looper, etc.) in our crops this year. From what I can tell, nothing is out of control, unmanageable, or widespread resistant. Included are a few tips to help you make good spray decisions:

1. Correctly identify the pest and don’t mess up your crop by throwing something in the tank. Sprays made at the wrong time or with the wrong insecticide are, at best, a waste of money for that spray, or at worst, can force you to respray unneeded insecticides for the rest of the season.

2. Correctly manage bollworm (aka corn earworm) in cotton. A recommendation change to boost Phytogen yield in heavy bollworm situations has caused an unneeded stir (click here). We should always be prepared to spray Bt cotton, which is not 100% effective for bollworm. This is why we have larval thresholds. You don’t need to switch up any management from the past if you don’t want to. Although bollworms seem to be coming off Bt corn more this year (click here), there is no need to change an insecticide that has worked for you in the past. The same holds true for sorghum and soybeans.

Bollworm (aka corn earworm) resistance to pyrethroids has slowly been creeping up over the years in soybean. That being said, there are plenty of places where pyrethroids still clean them up. Also, in cotton, pyrethroids seem to still work well to kill larvae in Bt cotton before they enter the boll. Once these larvae enter the boll, they are really hard to touch. You will have to make a decision on your own about what product might work for you.

Sorghum thresholds are close to 100% infestation in the whorl stages and one per head after head emergence. You’ll want to target larvae when they are small, before they enter the whorl or before they’ve eaten grain. Be sure you are spraying bollworm (aka corn earworm) and not fall armyworm if you are using a pyrethroid. Also note that pyrethroids will kill natural enemies and could cause you problems with sugarcane aphid.

3. Don’t spray beet armyworm, fall armyworm, or soybean looper with a pyrethroid. Note that bifenthrin is a pyrethroid. Other common examples are Baythroid, Karate, and Mustang. Instead, use a worm-specific material like Blackhawk, Intrepid Edge, Prevathon, or Steward. Belt would also be a good option, but might be hard to find this year. Stocks on hand can be used, but it is not being made and distributed and registration might be canceled (click here).

Soybean loopers have been showing some resistance in the Blacklands that has not been seen in the Coastal Plain (last year’s article). Besiege is a great option when you are trying to kill worms and stink bugs, bean leaf beetles or kudzu bugs since it is a combination of Prevathon and Karate. The downside is that it also kills natural enemies, so it should only be used when non-worm pests are at or near threshold. Please consult the label to be sure you have the correct rate of each component.

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Photo of Dominic Reisig, N.C. Cooperative ExtensionDr. Dominic ReisigAssociate Professor and Extension Specialist (252) 793-4428 (Office) dominic_reisig@ncsu.eduEntomology and Plant Pathology - NC State University
Updated on Aug 3, 2016
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