Since the kudzu bug showed up on soybeans in Georgia, South Carolina and parts of our state in 2010-2011, there has not been a reported infestation on a commercially planted soybean field until the first generation kudzu bug adults have developed.
From observations in previous years, kudzu bugs emerge from over-wintering and migrate to hosts.
In the spring of this year, we have had reports of kudzu bugs on all sorts of non-hosts (humans, houses, fig trees, grapes, wheat, cotton, and a magnolia tree, to name a few), hosts on which they feed and may or may not reproduce (e.g., legumes such as wisteria), and the major host of this pest, kudzu plants.
In 2010 and 2011 adults moved from over-wintering sites through the landscape until they found kudzu and laid eggs, which developed as nymphs to the adult stage (first generation). These first generation adults then moved onto soybeans, stayed on kudzu, or moved to other legume plants. They then laid eggs, which resulted in the development of a second generation of kudzu bugs for the year.
This year I have seen a glitch in this pattern, as kudzu bugs have been found in low densities on soybean seedlings in scattered areas of North and South Carolina (see previous blog post). These areas are all in areas where kudzu bug was confirmed as present in 2011.
The recommendation has been to focus treatment on the kudzu bugs that arrived in soybeans and the resulting second generation of nymphs. A preliminary threshold of one nymph-stage bug per sweep (i.e., 15 nymphs per 15-sweep sample) is recommended and may help reduce the need for multiple sprays in reproductive stage soybean.
Georgia, South Carolina trials
This is based on multiple trials in Georgia and South Carolina in 2010 and 2011. But how do we deal with kudzu bugs this early in the season?
Remember that adults present in the field during this time of the year are adults that have over-wintered last year. They are laying and have laid eggs that will develop into the first generation of kudzu bugs for the year. This impacts how we manage adults in May:
• Managing populations of kudzu bugs that are found in the homeowner landscape will not impact population densities in soybeans (see homeowner recommendations). Homeowners (especially ones who grow soybeans) may think that by killing kudzu bug adults they find in and around their home (e.g., bugs on a fig tree) will reduce later populations in soybeans. This is simply untrue for this prolific and mobile pest.
First generation kudzu bugs are developing in the surrounding natural landscape and will infest soybeans despite our best efforts.
• I do not know if treating adults on volunteer soybeans or those that are currently present in soybean fields will impact population densities in soybeans later in the season. If soybeans are treated now, there is a chance for re-invasion.
Remember that the adults are moving around and are mobile. Furthermore, in areas of the state surrounded by kudzu, kudzu patches will provide a source for these insects to develop and move into soybean fields.
Finally, we do not know whether the first generation of this insect can complete development (from egg to adult) on soybeans alone. The second generation can easily complete development on soybeans alone.
• I do not know if kudzu bugs can cause a yield loss when feeding on soybeans this early in the season. If you decide to spray, not only do you risk re-infestation, but you may be throwing away money. Soybean plants have the ability to compensate for injury, especially when this occurs early in the season.
In conclusion, I am unsure of the “correct” management action to take against kudzu bugs in soybeans at this point in the season, simply because this behavior is new and there are no data.
My advice is to carefully consider each individual case. Ask yourself about the characteristics of the surrounding landscape, if kudzu bug was an issue on soybeans in your area last year and where the kudzu bug population is in terms of its life cycle before making a decision.
Hopefully, I can give you a better answer than this in the near future.