David Owens, Extension Entomologist, owensd@udel.edu
Soybean
The next 10 days or so are going to be critical for finding small worms in soybean and sorghum. Playing with some numbers and examining trends between 8, 9, and 10 dollar bushel beans, I think a reasonable threshold for using a lower treatment rate of Besiege is around 2 per 15 sweeps. The soybean threshold calculator can be found here: https://www.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html.
We are seeing unusual soybean aphid activity in some fields. The Midwestern states use a threshold of 250 aphids per plant through R5, 80% of plants infested, and aphid populations increasing. Last year I ran into one field that would have warranted treatment, the plants were slick with honeydew. In our area, a pyrethroid should clean them up, but aphids are resistant to pyrethroids in the upper Midwest.
Sorghum
In sorghum, there tends to be very high mortality in the smallest instars. I was in a field a couple of days ago with 33 small earworms per 25 heads. To look for earworm, bend the stalks into a sweep net or a bucket and whack the head against the sides of the container several times to dislodge worms. A similar threshold calculator to the soybean calculator can be found at Texas AM’s website. Please note that it asks for yield per CWT not per bushel. https://extensionentomology.tamu.edu/sorghum-headworm-calculator/.