I have had 10 to 15 calls recently on green and southern green stink bugs in beans that are either vegetative or at R1-R2 stage with no pods. Numbers have ranged from 4-5 to 8-11 per 25 sweeps depending on the field. It is not uncommon to have a field or two on occasion to have threshold numbers of stink bugs before soybeans put on pods, but the number of fields this year being reported is certainly unusual.
Why are they in soybeans this early?
Stink bugs are seed feeders by nature. They strongly prefer to be in soybeans that are filling pods and preferentially feed on developing seed. The fact that stink bugs are being reported on a wide scale across the state may be an indication of the very late spring we had. Wheat dry down was later than normal and most of the state’s corn crop was in that “in between” stage this year when stink bugs were looking for a host to feed on. Since corn is not a preferred host in the mid-whorl stage they may have not had much choice but to move to soybeans. Also, the soybean crop is much later than normal; so many more acres are still at these early stages than normal. This is just my best guess. Nevertheless, they are in the crop and we need to have a plan to deal with them if need be.
What to do?
We have one threshold for stink bugs in Mississippi regardless of growth stage. It is 9 stink bugs per 25 sweeps. Although some of these fields are at threshold, I feel there is no real danger to soybeans that have no pods yet. I would ride these populations as long as you can and hope to make it to the R3 fungicide application. There is some evidence that early infestations can increase the incidence of green stem but the data I reviewed looked to be levels much higher than 9/25 sweeps. One potential watch out is indeterminate beans. Remember in the indeterminate varieties that even if you are at R2-R3 growth stage there may be large pods toward the bottom of the plant that could be taking damage. So take that into consideration if numbers are high. I feel good about our threshold for stink bugs in soybeans sot there is no need to “cheat it down” particularly in this situation.
The danger of spraying too early.
The likelihood of releasing or flaring caterpillar pests such as corn earworm increases dramatically following a broad spectrum insecticide application that eliminates beneficial insects, so let’s not create a problem where one does not currently exist. Also, keep in mind that residual for stink bug materials is short (4-7 days max), so if you make an application early while these stink bugs are still moving into fields it is no guarantee you will not have to make another one in the next 7-14 days.
What to spray?
Should you need to spray you should be able to clean up green and southern green stink bugs easily with pyrethroids or acephate. If you have brown stink bugs I would use acephate for sure.