Kudzu bug activity has heightened with the warm weather in the past two weeks. Adults are flying from overwintering sites and searching for their reproductive hosts, wisteria, kudzu and soybeans. In the meantime, they can be found nearly everywhere. I have seen photos of these insects on nearly every plant you can imagine, as well as the sides of houses and pickup trucks. Insects likely aren’t feeding or reproducing on these things (certainly not feeding on houses), so property owners will have to remain patient, while growers who planted soybeans on the early side might be getting a bit jumpy with the influx of adults into their field.
Soybeans this early in the season are incredibly resilient. Combining information from at least 20 seed treatment and foliar spray trials over the last 10 years from VA and NC, we have never documented a single yield boost (see previous blog article), even though the seed treatments are effective for reducing things like thrips and threecornered alfalfa hopper (seed treatments are not effective, however, against kudzu bug). The point is that we can document injury to seedlings from things like thrips and protect them with things like seed treatments and foliar oversprays. When you put the combine in the field, it doesn’t make a bit of difference. The soybeans plants always seem to compensate. Every year I become more and more confident of this.
Based on this, my guess is that a treatment threshold for kudzu bug for seedling soybeans is going to be drastically higher than it would be later in the season, when soybeans are producing pods or filling seeds. We do have one bit of datum to back this up. Based on an experiment in GA where kudzu bugs were infested at V2-V3, 5 bugs per plant seemed like a safe number to effects on the plant and to preserve yield. This is the preliminary threshold that GA is recommending for seedling soybeans.
Let me stress that this is a preliminary threshold that will absolutely change as we get more information on this. The preliminary NC threshold is 5 bugs per seedling, until plants are one foot tall. Fields infested at these levels will likely be a rare situation. Then, the threshold will change to 10 bugs per plant for plants from 1-2 feet tall. The established threshold of one nymph per sweep (one swoosh of the net) should be used for plants above 2 feet tall. Plants should be sampled at least 50 feet from the edge of the field. The reason for this is that the adults have an extended migration period (6-8 weeks) and colonize field edges first. If you sample the edges, chances are you will make a spray decision too soon before the migration is over.
Let me also tell you a cautionary tale. An NC grower noticed kudzu bugs on the edge of his April-planted beans in May 2012. They had not yet infested the interior portions of the field. He opted to spray. He then had to spray again in June, as the adults remigrated into the field. Additionally, sprays don’t kill eggs, so these hatched into nymphs. The grower then had to spray a 3rd time in June, as spider mites were flared in the field from the lack of beneficial insects. We want to avoid these costly situations while still preserving our yield.