Agronomic Crop Insect Scouting

David Owens, Extension Entomologist, owensd@udel.edu

Soybean
Continue scouting for defoliators and for spider mites. I have a spider mite plot at Carvel that is now defoliated. If battling spider mites with Lorsban, Bifenthrin, or Dimethoate, you may need a follow up spray about a week later.

August is worm month, and we are picking up low numbers of earworm, loopers, and armyworm. With earworm counts increasing in pheromone traps, be sure to start scouting for them in reproductive stage soybean. There is a handy threshold calculator on NCSU’s website: https://www.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html.

Corn
If you have corn-on-corn and are planning to put the field into corn next year, be sure to scout for western corn rootworm now. Adults will be concentrated around the ear zone and are easily disturbed and fast moving. They have black stripes on the wings and a yellow-green abdomen. Sometimes the stripes merge into each other resulting in more of a black patch than a stripe. If you have more than 1 beetle per plant, consider either rotating out of corn next season or using a beetle trait for next year. You can use the handy Bt trait table to easily determine what seed package has rootworm traits in it: https://agrilife.org/lubbock/files/2020/02/BtTraitTable_FEB_2020.pdf.

Alfalfa
Continue sweeping for potato leafhopper and in flowering alfalfa, blister beetles.

Sorghum
Be sure to scout for corn earworm in the heads. Earworms generally have a ‘bumpy’ appearance, and in sorghum, a darker striping pattern. The head is orange. Fall armyworm may also be present in heads, and is generally considered to be equally damaging. Prior to heading, armyworm thresholds in sorghum are very high, near 75% infested plants. Fall armyworm are usually gray, although lighter greenish color morphs can be found. Lighter color morphs usually have a dark spot on the side behind the thoracic true legs. The head has an inverted Y faint line and the last abdominal segment has 4 ‘dots’ arranged in a square pattern.

There is a very good threshold calculator available from Texas A&M https://agrilife.org/extensionento/sorghum-headworm-calculator/. This calculator lets you estimate control cost given your insecticide and application method of choice, grain value, and plant population. Note, large larvae are more difficult to kill and some products may not work as well on them. Other worms present include fall armyworm (difficult to kill with pyrethroids), yellow striped armyworm and true armyworm (easiest to control).