Agronomic Insect Update – May 25, 2018

David Owens, Extension Entomologist,

Although worms tend not to like prolonged wet weather, both black cutworm and true armyworm are active in fields. How much they are suppressed is the open ended question that is almost like predicting a 40% chance of anything. If you see worms in the field, they were not affected.

As we wrote earlier, if you have a late planted field that has had a recent green cover crop (within the past week), or if you have a corn field that is not yet planted, you will be at greater risk for either pest. We usually don’t worry about these critters in fields destined for soybean. We should expect good protection from cutworms with traits that contain Cry1F or Vip3A. Please see The Handy Bt Trait Table for an excellent breakdown of trait packages that contain them: A small amount of feeding in a field with the above traits is expected, the Bt is a stomach poison that needs to be ingested. But watch the field to ensure that injury doesn’t progress.

True armyworm is a bit different. Only trait packages with the Viptera gene, (Leptra, Viptera, Trecepta and the 5222 Agrisure Duracade – not 5122) are rated for true armyworm protection. It is possible that the other traits may have some efficacy, but scout your fields and look out for worms in other traits. The photo below was taken by Dr. Kelly Hamby, University of Maryland field crops entomologist from organic (no trait package) field corn. Thresholds for armyworm depend on number, defoliation, and size of the worm. Treatment is warranted if 25% of the plants are infested, 50% defoliation has occurred, and worms are less than ¾ inch long.

True armyworm on organic corn

For wheat we use a threshold of 1-2 armyworms per foot of inter-row space. Check the base of the plants. Barley thresholds are a little lower at 1 per row-ft. It seems like harvest is a long way off, but if you have a treatable population be careful with observing the preharvest intervals. The only pyrethroid with less than a 30 day PHI is Mustang Maxx (14 days). The diamide insecticide Prevathon is available for Delaware this year and has a 1 day pre harvest interval.

Some beans went into the ground before the rain. The wet weather we have been having favors slug activity. You will want to watch your soybeans to detect any early issues with slug-induced stand loss. Unlike corn, soybeans can compensate for some stand loss. Fields with prior slug history and no-till fields with heavy residue tend to have greater slug populations.

We are also seeing bean leaf beetle working on seedlings. Beetle activity is concentrated a little more than usual for this time of year due to planting delays and the wet weather suppressing beetle activity last week. Beetle movement really picked up with Sunday’s sunny weather. Beetles may move out, when scouting, check for beetle presence. Count the number of beetles per 6 row-ft in 10 spots per field, and estimate defoliation percentage. Seedling thresholds are 2/ft, 25% stand loss. Once the plants have their second trifoliate, we need to see 2-3 per plant and 25% or more defoliation. Defoliation can be hard to visually estimate, use the below image as a general guide.

Graphic representations of various levels of soybean leaf defoliation.