It’s nearing the end of growing season and may seem like a waste of time to scout soybeans when we’re still picking corn, defoliating cotton, and digging peanuts. However, I’ve had a number of calls this week on kudzu bugs in soybeans at the R6 growth stage and some stink bug calls.
Normal soybean plants juxtaposed with plants harboring “sooty mold” from kudzu bug. Kudzu bug feeds on the veins of the plant, excreting extra sugars. Mold colonizes these sugars and can result in a black coating. Picture from Phillip Roberts, U. of Georgia.
Kudzu bug: The unfortunate fact is that many of these fields where kudzu bug is above threshold should have been sprayed a month ago. With R7 as our “safe stage” from kudzu bug and other defoliating pests, it becomes a hard call to know whether to spray kudzu bug in beans at R6. Things you might consider are how long the insects may have been out there (hard to say if they haven’t been scouted), how many of them there are, how close you are to R7, and how much yield you think you’ll lose from running over the beans. It takes between two to three weeks move from R6 to R7. Also remember that the R7 growth stage is one brown pod anywhere on the plant in most plants across the field. My advice would be to spray beans with high population abundances (well over the one nymph per sweep threshold) that are closer to R6 than R7. Most fields now will require a judgement call on a case to case basis.
Stink bug: The other thing that we need to watch for this time of year is stink bug. Soybeans are most sensitive to yield loss from stink bugs in the R4-R6 growth stages. You need to be scouting your beans and spray based on the recommended thresholds (click here). New research from VA Tech. suggests that these thresholds are overly conservative, meaning they might tell us to spray even when we don’t have to, but I still think we should use them to preserve our yield. Once beans are in the R7-R8 growth stages, stink bugs can impact quality, which can include visible punctures on the seed and decreased test weight. Seed producers can expect germination to be impacted. Because stink bugs are not impacting yield during R7-R8, I’ve been suggesting that the threshold can be doubled.
Dr. Reisig, Have you seen the flight of adult Kudzu bugs from early beans to beans behind wheat as I have? I have also seen them in ditch banks and suspect they may over winter there.I also had skipper caterpillars in fairly high numbers on one farm along with corn ear worm. I was not expecting the skippers to do that much leaf damage.
Glenn, I have not seen that. We have some tests near double cropped wheat. The beans behind wheat have had consistently low numbers all year. If you’re observing a migration now, it is likely the adults that will go into overwintering. What are you seeing in terms of numbers? I would like to know of your situation. It makes sense to me that kudzu bugs can overwinter in ditch banks. A student of mine found them in kudzu litter this winter.
I would not expect the skippers to do a lot of leaf feeding either. I’ve been getting a lot of questions on them this year, but have not heard of a damaging situation. Sorry to hear that you were the one!
I have since seen them in the woods on oak,gum and other trees on the field borders.I sprayed the late beans and the residual chemical is killing them as they migrate in.I did not get to spray the older beans due to rain and wet ground.I had very high populations of Kudzu bugs in the older beans that are now at late R7 to R8.I will try to get you some pictures of the bugs on trees this week.