Agronomic Crop Insect Scouting

David Owens, Extension Entomologist, owensd@udel.edu

Corn
Slugs continue to cause the most feeding injury to young corn. Although recent warm weather has been favorable to corn growth, foggy mornings have helped keep slugs moist and happy. Wireworm injury is also apparent now on V3-4 corn, often as a bright yellow stripe on one side of the leaf blade. This is different from nutrient deficiency which often appears as several interveinal stripes. There have also been reports of sporadic cutworm activity. In addition to the black cutworm, which is migratory, there are a few species that overwinter in the state. We have also hit the necessary degree days for a ‘significant’ (although small) black cutworm flight that occurred in the Seaford area in April to result in larvae large enough to cut plants. Thresholds and treatment information can be found here: https://www.udel.edu/content/dam/udelImages/canr/pdfs/extension/sustainable-agriculture/pest-management/Insect_Management__In_Field_Corn_-2020.pdf. Mid-size armyworms are also sporadically showing up in light numbers in fields planted into late cover crops. Thresholds for armyworm are 25% plants infested.

A significant revision has been made to the North Carolina stink bug threshold in corn. You can see Dr. Dominic Reisig’s post here: https://corn.ces.ncsu.edu/2020/05/important-changes-for-stink-bug-thresholds-in-corn-for-2020/. Essentially, the threshold has been lowered by about half for corn V14 to R2. This is based on recently completed, multiple year studies in North Carolina and in Virginia caging stink bugs for 1 to 3 weeks at various plant developmental stages.

Soybean
Continue scouting for slugs between planting and bean emergence. In last week’s post, I borrowed from Galen Dively’s observations and recommendations regarding Deadline bait. There is another product that was registered last year in DE – Ferroxx AQ. The active ingredient is iron phosphate. An older product containing the active ingredient is Sluggo. The new formulation is as, if not a little bit more, weather resistant than Deadline. In some trial work in 2018 its efficacy was no different than Deadline. One advantage to Ferroxx AQ is lower mammalian toxicity. Labeled rate ranges between 4 and 25 pounds per acre.

Bean leaf beetles are starting to become active, but so far, no significant feeding has been observed. Seedling stage thresholds are pretty high – 40% defoliation with 2 to 3 beetles per plant. Even these thresholds are conservative, recent work from Mississippi demonstrated leaf feeding as high as 60% on seedlings did not reduce yield.

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