With the exception of scattered areas in our far eastern counties, tarnished plant bugs (or Lygus) have not been a significant problem for most North Carolina cotton producers.
Since 2002, an average of approximately 5 percent of our cotton acreage has been treated for plant bugs pre-bloom.
Post-bloom damage from plant bugs is much harder to define because their damage to small bolls cannot be distinguished stink bug damage. However, the Southeast region benefits (at least for now) from having plant low bug levels that most Mid-south growers can only dream about.
With the introduction of Bt cotton in 1996, our previous range of 2 to 3 or 4 late-season bollworm applications on conventional cotton has been reduced to an average of just under 1 application, resulting in greater potential late-season plant bug damage to squares, blooms and small bolls.
The increased planting of Bollgard II, WideStrike, and other new Bt cotton lines with greater caterpillar activity is expected to lower worm sprays even more, further increasing the potential damage from boll sucking bugs. So far, that’s happened with stink bugs — but not with plant bugs.
Unlike much of the Mid-south, where plant bugs have become a dominant, yield-reducing, multiple application mid- and late-season pest, boll damage from this species appears to be much more limited in the Southeast (with Alabama probably intermediate).
From 2005 until the present, over 100 replicated tests were conducted from Virginia to Georgia as part of a Cotton Incorporated regional project investigating all aspects of stink bug ecology, biology, crop impact and management.
In these assessments, we were happily surprised to discover very low direct and indirect indications of plant bug activity: 1) an average of almost 90 percent square retention into the fourth to fith week of bloom, 2) dirty bloom levels rarely exceeding the 15 percent threshold, and 3) ground cloth samples averaging only 10 percent to 15 percent of the threshold triggers used in most cotton states.
Although economic infestations of plant bugs are not unheard of here, plant bugs take a distant back seat to stink bugs in most situations.
Most of the recent high quality research into plant bug biology, damage and yield associations, scouting procedures and efficiency, and thresholds is being conducted by USDA (Agricultural Research Service) and university scientists in the Mid-South and far West In these high plant bug potential damage areas, it appears that dirty bloom and small boll assessments alone typically reveal a limited picture of recent plant activity.
Because any one sampling type has its limitations, assessments may include several scouting methods, including sweep net sampling, ground cloth counts, visual observations, external and internal square damage evaluations.
As long as the retention of new squares remains over 80 percent, no additional types of scouting assessments for plant bugs are needed. In most cases, square retention in the low to mid-90 percent range, even through the first 3-plus weeks of bloom, is common here. If square retention drops below 80 percent, then further sampling is needed, with either a sweep net and/or ground cloth.
After blooming begins, plant bugs can damage small bolls. Counting dirty blooms, cutting open or squashing small bolls, and being on the lookout for blackened squares and plant bug activity provide a reasonable approach to making treatment decisions, or in deciding if further scouting is needed.
Although dirty bloom counts represent an indirect scouting method (a direct approach counts the live bugs) and measures plant bug damage made several days earlier when these blooms were medium to large-sized squares, this approach is fast and can at least put the producer in the ballpark to determine if additional scouting is needed.
Fortunately, dirty bloom counts here are often a small fraction of the 15 percent to 20 percent threshold some states employed as a treatment trigger until recently. If dirty bloom counts are very low, as is most often the case, a ‘no treatment’ decision is relatively easy. That is, zero percent to10 percent dirty blooms would be a ‘no-spray’ decision, while a count of 20 percent or more dirty blooms indicates that further, more intense sampling would be advisable via sweep net and/or ground cloth.
Additionally, the examination of small bolls is identical to that done to assess stink bug (typically our more damaging pest complex) damage. The examination of quarter-sized bolls for both stink bug and plant bug damage becomes particularly urgent during the third to fifth or sixth week of bloom.
Because cotton producers in North Carolina do not often treat for plant bugs, and because our overall late-season insecticide use for all cotton pests on Bt cotton still averages approximately one application per acre, plant bug resistance to insecticides appears to be minimal here so far, and the materials listed in the 2009 North Carolina Agricultural Chemicals Manual should provide good plant bug activity.
Unlike producers and consultants in the Mid-South, we’re hoping that the pest status of plant bugs stays right where it is.