Plant bugs: A late problem in cotton?

For most North Carolina cotton producers, tarnished plant bugs (or Lygus) have not been a significant early season problem, with the exception of scattered areas in our far eastern counties in some years.

Averaged over the last decade, approximately 5 to 10 percent of our cotton acreage has been treated specifically for plant bugs prior to bloom. Post-bloom damage from plant bugs is much harder to define, because the damage to small bolls cannot be distinguished from that of stink bugs.

Bug Increases — With the introduction of Bollgard 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 on Bt cottons, resulting in somewhat greater late season plant bug damage to squares, blooms and small bolls.

The anticipated increased planting of Bollgard II 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.

Plant bugs, a Southeast perspective — 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 and North 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 tests sites in 2005.

Additionally, dirty blooms only occasionally exceeded 15 percent and averaged less that 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 that 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.

Scouting for plant bugs and their damage — The lions' share of the large volume of 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 by university scientists in the Mid-South.

In this high plant bug pressure area, 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 that include several scouting methods, including sweep net sampling, beat cloth counts, visual observations, external and internal square damage evaluations, in addition to dirty bloom and small boll sampling, may be recommended.

Suggestions for North Carolina — Until the above options and their relationship to yield are better understood, 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.

Although dirty bloom counts represent an indirect scouting method, and assess plant bug damage made several days earlier when these blooms were large squares, this approach is fast and dirty bloom counts are often a small fraction of the 15-20 percent threshold that some states employ as a treatment trigger.

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 well outside of the “slot”, treatment decisions are relatively easy. That is, zero to 10 percent dirty blooms would certainly be a ‘no-spray’ decision, while with 50 to 60 percent dirty blooms, treatment would be advisable. The former finding is more common here.

Dirty bloom counts in the 15 to 20 percent range should probably trigger using a sweep net or a ground cloth, the scouting methods of choice in the Mid-South, to help get a better fix on the need to treat.

Chemical options — Because cotton producers in North Carolina do not often treat for plant bugs, and 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 should be minimal here, and the materials listed in the North Carolina Agricultural Chemicals Manual should provide good plant bug activity.

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