Certainly this is the time to be checking your soybeans for the presence of stink bugs. We have not noticed economically important problems in the soybeans we've sampled, but stink bugs are certainly common and our light traps have been capturing very large numbers compared to previous years.
At Princeton, Ky., in particular, the increase is greater than three times what we have captured in the past two years and the season is not over. Black light trap data do not translate directly to numbers in the field. Nevertheless, this very large increase begs taking a look at the situation.
Soybeans are at some risk to stink bugs from beginning bloom until R6/R7stage. So while full-season soybeans are probably well advanced, there are lots of double-crop and late double-crop soybeans still at risk. For many growers the damage stinkbugs cause in grain is never noticed until the crop is harvested. This is because these sucking bugs feed directly on the seed inside the pods and cause very little obvious damage to the plants. Losses are due to damaged & shriveled beans and aborted pods. It is therefore necessary to actually sample for the pest to determine if the plants are at risk.
Though we have several species of stinkbugs in our soybeans, it is the green stink bug that is most numerous. Adults are ½-inch long, green (and a few slightly smaller browns) with sucking mouth parts. The body is shield-shaped. Nymphs are wingless and are quite variable in color, but yellows and reds are common. Both adults and juveniles feed on the plants.
Stink bugs are most often found in border rows and against woodlots. One may use a shake cloth to sample, but as we generally have narrow row beans (e.g. less than or equal to 15 inches), a 15-inch sweep is most often used. Using this sweep net, take a series of 25 sweeps in each location and sample in multiple locations within the field. At each location, count the number of stink bugs captured and average this number over the number of locations sampled in the field.
Our threshold for recommending an insecticide treatment is 3 stink bugs per 25 sweeps at R1-R3 growth stage, and 9 stinkbugs per 25 sweeps R4-R6 growth stage. Treatment of more mature beans is not recommended.If insecticidal control is warranted you may find a list of insecticides labeled for this use at: http://pest.ca.uky.edu/EXT/Recs/ENT13-Soybeans.pdf.
In addition to our “normal” stink bugs, brown marmorated stink bug (BMSB) is now well established in the central Kentucky region. Ric Bessin in Lexington has this year for the first time captured BMSB in his black light trap. In addition, he has seen them in soybeans and in the fruit and vegetable plots on the UK farms. BMSB are also quite common on fruiting trees and ornamental plants in and around Lexington.
These changes indicate that BMSB is adapting very well to our state and may yet develop into a major pest of crops and as a home invading pest. BMSB is still quite rare in the western half of the state. As far as grain crops are concerned, BMSB is counted just like any other stink bug for making spray decisions.
Sampling for Kudzu bug indicates that there is little or no risk of economic problems with this soybean pest this year. We've added one western Kentucky county (Christian) to the list of counties where the pest is found. However, in all cases including resampling of last year’s locations along the I-75 corridor, though the pest is present, populations are very small, and thus far, restricted to Kudzu.
Be reminded that stink bugs feed on a wide range of crops and ornamental plants.