Agronomic Crop Insect Scouting

David Owens, Extension Entomologist, owensd@udel.edu

Corn
Pay attention to the silks in fields that have Japanese beetle history. Treatment is warranted if 1-2 beetles per ear are present and they have clipped silks back to ½ inch, and ears are less than 50% pollinated. In the northern third or so of Delaware, and the western side of Kent County on heavier ground you might see western corn rootworm feeding on silks and pollen, especially in fields that have not been rotated in a couple of years. Silk clipping threhsolds are the same as for Japanese beetles but with 5-6 beetles per ear. If you see unusually large numbers for your fields, it may be a sign that the field should be rotated next year out of corn or you might want to consider a Bt variety that will control beetles. You can review trait packages that control beetles using the Handy Bt Trait Table:

https://agrilife.org/lubbock/files/2020/02/BtTraitTable_FEB_2020.pdf.

Also continue scouting for brown stink bugs. If you look at just the ear zone, 10 stink bugs per 100 plants is considered threshold prior to silking, and 28 per 100 after silking. If checking the entire plant, threhsolds are slightly lower: https://corn.ces.ncsu.edu/stink-bug-management-in-corn/.

Soybean
Continue scouting for defoliators and mites. Green cloverworms are the most abundant defoliator, but other notables include Japanese beetle, grasshoppers and the occasional bean leaf beetle. I am seeing more blister beetles out this year, this means we will probably see fewer grasshoppers next year. It also means you need to be cautious when examining your sweep net! If you feel that a mite treatment is needed due to stippling, leaf dropping and heavy mite loads (20 per leaflet), moist soil will improve Dimethoate activity. Pay attention to your water conditions. High pH and high iron can degrade dimethoate. True miticides labeled for soybean are Agri-mek and Zeal. Dectes stem borer is also active, but there are no threhsolds. If you see unusual Dectes activity, prioritize the field for as early a harvest as possible.

Alfalfa
Hopperburn can easily be seen in some alfalfa fields. Do not let your fields yellow up! Leafhopper nymphs are small and can be pale. Empty your sweep net slowly so you can get a good count. The field in this photo should have been harvested before hopperburn became evident. Cutting will destroy a lot of these nymphs and adults will often move elsewhere, but be sure to scout the regrowth. Thresholds are much lower for small plants.

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