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Kudzu Bug in Soybeans

We are starting to receive some questions about Kudzu bugs in Soybeans. This is what the 2014 Soybean Production Guide has to say about this pest.

 

The Kudzu bug is an economic pest of soybeans that was first found in Georgia in the fall of 2009. Since then they have been found in 12 additional states. Yield data has been obtained from 37 soybean trials over the past four years investigating kudzu bug in Georgia soybeans. Average yield loss was 19 percent in unprotected plots with a range from 0 to 60 percent.Kudzu bug adults are oval shaped, small, about ¼ inch in diameter, and greenish brown in color. Eggs are creamy white and are laid in double-rowed batches of about 20 eggs. Nymphs are also oval shaped and are light green to brown in color and have numerous setae or hairs. Both adults and nymphs are most often observed on plant stems and have sucking mouthparts which they use to feed on plant sap. Excessive feeding weakens and stresses the plant which can result in fewer pods per plant, fewer seeds per pod, and smaller seed size.

Kudzu bugs survive the winter under tree bark and in debris on the ground in well drained areas. During early spring adults are active on warm days and are in search of a reproductive host. Although kudzu bug adults may be observed on many plant hosts (i.e. fig trees and others), the primary reproductive hosts in Georgia are kudzu and soybean; reproduction has also been observed on wisteria. Kudzu appears to be an important spring host. Kudzu bugs begin laying eggs on kudzu shoots when kudzu breaks dormancy; this generally occurs in late March in south Georgia and mid-April in north Georgia. Overwintered adults may utilize early planted soybeans as a reproductive host as well. Adults will lay eggs on kudzu for several weeks. The time required to reach the adult stage is about 6-8 weeks depending on temperature. These new adults then disperse to soybeans and other reproductive hosts.

Kudzu bugs may infest soybeans in the seedling stage (V2-V3). Although this is a rare event it tends to occur when overwintered kudzu bugs infest early planted soybeans. We are suggesting a threshold of 5 adult kudzu bugs per plant on vegetative soybeans. We have observed stunting of plant growth and reductions in plant height at the R2 growth stage as a result of seedling infestations of 5 per plant. More information is needed on how vegetative infestations impact final yield.
Adults lay eggs on the underside of soybean leaves and a generation requiring about 6 weeks will be completed on soybeans (oviposition may occur in soybeans for several weeks). Initial field invasions tend to be more concentrated on field margins but will eventually spread throughout the field. In many situations we will begin to see immature kudzu bugs in soybeans at about the R2-R3 stage. Kudzu bugs complete two generations per year.

Planting date has a significant impact on the risk of kudzu bug infestations. Field trials conducted during 2012 and 2013 confirmed field observations in 2011 that early planted soybeans are more likely to experience high kudzu bug infestations compared with late plantings. For example, kudzu bug egg mass counts per 5 plants at the R2 growth (flowering) stage were 49, 23, 8, and 2 for soybeans planted in April, May, June, and July during 2012. Yield data follows the same trend with the greatest yield loss in unprotected plots occurring in April (50 percent) followed by May (28 percent), June (16 percent), and July (7 percent). These data should not be the deciding factor concerning planting date; soybeans should be planted when maximum yield potential occurs. But rather the grower should understand the risk of infestations does vary by planting date. Insecticide application(s) may be needed for kudzu bugs regardless of planting date so thorough monitoring for kudzu bugs and other pests is needed.

Kudzu bugs can be scouted using a 15-inch diameter sweep net. Kudzu bug populations can be extremely high. Current recommendations include interrupting the development of each generation of kudzu bug by applying an insecticide to target the immature stage of the insect. Insecticide should be applied when sweep–net sampling catches one immature insect per sweep. Samples should be taken from all areas of the field, including edges and the middle, taking care not to bias sampling along border rows where population build initially. As an alternative to sweep-net sampling, visual inspections of insect density lower in the canopy will suffice. If immature kudzu bugs are easily and repeatedly found on leaf petioles and/or main stems, treatment is likely warranted. In some situations, a single properly timed insecticide application for kudzu bug based on the 1 nymph per sweep threshold has preserved soybean yield. If insecticides are applied when adults are still actively migrating from kudzu to soybeans; additional applications may be needed. It is important not to spray too early as reinfestation will occur. Multiple classes of insecticides have shown activity on kudzu bugs. Growers actively treating kudzu bugs with broad spectrum insecticides should consider using a preventive application of Dimilin at the R2/R3 growth stage for control of velvetbean caterpillar and green clover worm (see Preventative Insect Control and Damage section). Insecticides used for kudzu bug control will disrupt natural controls such as predators and parasitoids which will increase the risk of foliage feeding caterpillars such as soybean looper, velvetbean caterpillar, and green cloverworm.