David Owens, Extension Entomologist | owensd@udel.edu
University of Delaware
The month of August is a critical month for scouting field crops. Right now, full season beans are between R3 and R5, while double crop soybean is either just starting to flower or will be soon. Scouting soybean can save money in two ways: finding a problem that wasn’t anticipated and not finding anything worth spraying. This month we have been in fields that fit both categories, as well as fields that an insecticide could have been left out.
The main threat for full season soybean comes from stink bugs. R4 and R5 are the most critical times to scout for stink bug activity. Stink bugs tend to concentrate along field edges, especially those bordering wood lines with wild cherry and maple and those bordering corn fields. If you do not see a sizeable population there, the odds of having one in the field interior are low. If a field edge is above threshold, go out into the field. Its not uncommon for those populations to decline 150-200 feet from the field edge. The most commonly used threshold for stink bugs is 5 bugs (adults and nymphs, all soybean-feeding species combined) per 15 sweeps. We grow an unusual amount of Plenish soybean which garner a bit of a premium, quality is a bit more important, and some folks tell me it takes them longer to dry down for harvest. I think it is justifiable to go with a bit of a lower threshold for those, around 4 bugs per 15 sweeps. Fortunately this year, stink bug blacklight trap captures have not been high.
At some point in August, our main corn earworm flight comes in, putting any full flowering soybean field at risk. Please keep checking with your local extension agent or closest earworm trap counts. So far this year, our flight seems to be a bit late, but that can change in a hurry! Earworm tends to be a quick moving flash in the pan. Moths come in, about 7-10 days later calls come in, and two weeks later there are no earworms to be found, but they can do a lot of pod feeding in that short window. The youngest earworms you may find in a sweep net will be a bright, translucent orange or orange-ish blue color with a dark head capsule and short bristly hairs. Larger earworm can be somewhat variable in color. Some are pink, yellow, yellow with some black on them, and green morphs which can be confused with green cloverworm. The best resource I know for determining if a field is over threshold is North Carolina State University’s corn earworm calculator: https://soybeans.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html. Plug in your sampling method, control cost, soybean price, and row width. For some rough ball park numbers, this generally comes out to between 2.5 and 4.5 earworm in a 15 sweep sample. With low soybean prices it may be tempting to use a cheap pyrethroid, but beware, they are not reliable.
Another factor worth considering is how to treat insect pests this late. Soybeans are going to have less ability to compensate for drive-down damage. If you have a field of tall beans that have good to excellent yield potential and canopy thickness and a smaller sprayer boom, drive down damage can be substantial. An aerial application might be a bit more expensive, but the spray plane wont damage beans.
Before finishing, I wanted to send you other pest updates. Earlier this week I came across a field with a heavy population of soybean aphid which is very unusual. If you come across a field with a lot of broken, flagged trifoliates, it is a sign of possible Dectes infestation. Prioritize those fields for as timely a harvest as possible.
Sorghum: Sugarcane aphid (now called sorghum aphid) is starting to show up in sorghum. Occasionally this yellow-white aphid can build up in such large numbers as to cause crop injury. Look for it underneath mid-canopy leaves. Late sorghum that has not yet pollinated may be attractive to corn earworm for egg laying so be sure to scout heads.
Good insect id and proper product and rate selection are critical. Besides a sweep net, your most valuable scouting tool is your phone. Snap a picture and text it to someone for confirmation. There’s also an app, the MyIPM app for Row Crops which includes pictures, chemicals, rates, and brief overviews. Check it out. And sometimes, by scouting a field, you may see something else that needs addressing, like broken pivot nozzles. Good luck getting your soybean and sorghum to the finish line!
