As kudzu bugs build, follow thresholds to avoid costly resprays

 • Dominic Reisig •

kudzu bugs
Kudzu bug adults on soybean seedlings — photo by Brad Fritz

I tweeted last month that there was no shortage of kudzu bugs around and that I expected some beans to be sprayed. However, I was expecting sprays in July and August, not during June!

We haven’t seen numbers this high and this early since 2012. I received reports of the first kudzu bug migrants into full-season soybeans last week (end of May). We can expect more migration as nymphs continue development on kudzu and migrate to soybeans as an adults.

This migration might peak in a few weeks, so be aware that a few scattered fields might require another application. Kudzu bugs prefer early-planted beans and are more prominent on field edges near good overwintering sites (like woods) or near alternate hosts, like kudzu and wisteria.

Early season kudzu bug infestations can reduce height of soybeans. In good conditions, they will compensate and yield fine. We have never documented a yield loss from these early season infestations, as the soybeans plants can compensate later in the season. However, the insects can reduce plant height.

Furthermore, our early season experiences were in years with generally plentiful rainfall. So there may be some situations where plants can’t compensate in droughty conditions. To avoid any impact on plant height or yield loss, our early season threshold is five bugs per seedling. Fields infested at these levels will likely be a rare situation.

If you are visually sampling seedling soybeans, peel back the plants to reveal the stems. You will see many plants without bugs and some that are loaded (the insect has an aggregation pheromone, making it attractive to other kudzu bugs). Most will congregate toward the middle of the plant.

Base treatments decisions on a per-plant average. Neighboring plants will likely compensate for any stunting that might occur on heavily infested plants. Plants should be sampled at least 50 feet from the edge of the field since this insect is so spotty.

kudzu bug trials
Early-season kudzu bug infestations can reduce height of soybeans. In good conditions, they will compensate and yield fine — photo courtesy NCSU

The established threshold of one nymph per sweep (one swoosh of the net) should be used for plants once we reach mid-July or so. Again, 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 (six-eight 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. One cautionary tale (truth though!)…a North Carolina 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 third 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.

Dr. Dominic Reisig is a professor and Extension specialist, Entomology & Plant Pathology, with North Carolina State University. He may be reached at dominic_reisig@ncsu.edu

Check Also

louisiana soybean planting

Choices, choices

Do your homework and take a ‘1,000-foot look’ when selecting varieties. • By Vicky Boyd, …