David Owens, Extension Entomologist, owensd@udel.edu
Soybean
Continue scouting full season soybeans for stink bugs, particularly those fields that are fairly narrow and with a large, wooded perimeter. We sampled over a dozen fields this week and encountered significant stink bug populations in a couple of them. Soybeans about a hundred feet or so away from a wooded edge may be below threshold for stink bugs.
Reports are coming in of podworms beginning to make an appearance, particularly in southern Sussex and adjacent areas in Maryalnd. As a reminder, the soybean threshold calculator is a good tool to ballpark estimate at what level podworms should be treated:
https://www.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html.
Pyrethroids are inconsistent for us, and may work in some fields, or they may not work at all or well enough in others. In sweet corn, the best pyrethroid has been Hero at its highest rates. Chlorantraniliprole is the best earworm material that we have for soybean and is a component of Besiege, Elevest, Vantacor and Coragen eVo. These materials provide consistency, peace of mind, long residual in case an extended moth flight occurs, and provide some suppression of soybean looper if that critter establishes in large numbers before cool weather in September. Two years ago, 6.5 fl oz of Besiege did very well in soybean under high pressure conditions. Another product worth mentioning is Denim, while not quite as good as the diamides for earworm, it is a far superior looper product with a longer residual.
Let’s go back to the threshold calculator. The first box to estimate is the Control Cost. Besiege is probably going to run about $2 per ounce, or 12-20 dollars in product. Application (machinery, fuel, time, custom ground or aerial spraying) is going to run anywhere from roughly $9-14/acre. How are the beans going to be treated? If you go out with a ground rig, will you be using preexisting wheel tracks? If not, driving down beans can result in anywhere from a 1-6.5% yield loss, which on a double crop soybean field could be anywhere from 0.2 bu to 3 bu/A. This should be factored in on the control cost and might push the needle towards an aerial application if possible.
Assuming soybean at $9.50/bushel, a ballpark estimate for a threshold comes out to about 3 to 3.5 worms per 15 sweeps. There can be high mortality among the small early instar worms. If finding small, itty-bitty worms now, it might be prudent to wait a few days and rescout before pulling the trigger on the field.
Sorghum
Scout for headworm in sorghum by shaking 30-50 heads in a bucket to dislodge worms. Texas A&M has an excellent calculator here: https://extensionentomology.tamu.edu/sorghum-headworm-calculator/. Please note that the value for sorghum is expressed in count weight, not bushels. In 2019 we did a sorghum spray trial (UD spray trail results can be found here: https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/pest-management/insect-management-reports/) and achieved good control with carbaryl, Besiege, and Baythroid XL 2 days after application. BUT 6 days after application, the worm counts were very low (see the notes about high natural mortality) and Baythroid XL had 0.5 worms, carbaryl 0.3 worms, and the untreated check 0.75 worms. No significant difference. What certainly did not work was low rates of pyrethroids. Please note, FMC now has a 2ee recommending the lowest rate of Vantacor for headworm at 0.9 fl oz/A.
Sugarcane aphid appears to be widely scattered right now, but with warm, dry weather that we have had, it would be prudent to scout late sorghum for them. The best material for sugarcane aphid is Sivanto.
Pasture
We encountered true armyworm in grassy weeds during our soybean scouting. Scout pastures for true armyworm and defoliation. If a large number of worms are present, a pyrethroid should clean them up well.