For most North Carolina cotton producers, with the exception of scattered areas in our far eastern counties in some years, tarnished plant bugs (or Lygus) have not been a significant early season problem.
Averaged over the last six years, approximately 6 percent of our cotton acreage has been treated annually for plant bugs prior to bloom.
Post-bloom damage from plant bugs is much harder to define because their damage to small bolls cannot be distinguished from that of stink bugs. However, the Southeast region of cotton production benefits (at least for now) from having low plant bug levels and the associated minimal damage 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.
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 on the fence).
In a series of replicated tests carried out in Georgia, North Carolina and South Carolina as part of a Southeast Region Sucking Bug Project, square retention rates of over 90 percent were recorded into the fifth week of blooming at most test sites in 2005 and 2006.
Additionally, dirty blooms only occasionally exceeded 15 percent and averaged less than 6 percent, suggesting minimal plant bug damage at the study sites.
Ground cloth samples further confirmed the low plant bug levels. This is not to say plant bugs are not a significant late season pest in some cotton fields in some years in the Southeast. In our region, however, plant bugs appear to take a 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. In this high plant bug pressure area, it appears 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 that include several scouting methods, including sweep net sampling, ground cloth counts, visual observations, external and internal square damage evaluations, in addition to dirty bloom and small boll sampling, may be recommended in that area.
In North Carolina, 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 is a measure of plant bug damage made several days earlier when these blooms were large squares, this approach is fast and accurate. And fortunately, dirty bloom counts here are often a small fraction of the 15 percent to 20 percent threshold that some states employed as a treatment trigger until recently. Additionally, the examination of small bolls is
identical to that done to assess stink bug (typically our more damaging pest complex) damage. 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.
Because cotton producers in North Carolina do not often treat for plant bugs, and because overall late season insecticide use for all cotton pests on Bt cotton still averages approximately one application per year, plant bug resistance to insecticides should be minimal here, and the materials listed in the 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.