Plant bugs: Late threat to cotton?

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 for most North Carolina cotton producers.

Averaged over the last seven years, an average of approximately seven 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 quantify, because the damage to small bolls cannot be distinguished from that of stink bugs.

With the introduction of Bollgard cotton in 1996, our previous range of two to three or four late-season bollworm applications on conventional cotton has been reduced to an average of just under one 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.

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 perhaps Alabama intermediate).

In a series of replicated tests carried out in Georgia and in North and South Carolina as part of a Southeast Region Sucking Bug Project, square retention rates of over 90 percent were recorded into the fourth week of blooming at most test sites in 2005 and in 2006.

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.

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 university scientists in the Mid-South.

In this high plant bug pressure area, it appears dirty blooms 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 and beat cloth counts, are generally recommended.

In North Carolina, if initial scouting reveals small square retention of 90 percent or more and if dirty bloom levels are less than 10 percent, more detailed assessments are probably not needed at that time.

If these initial inspections show either small blackened squares or dirty bloom levels of more than 10 percent, further sampling with a beat cloth and/or sweep net is probably indicated.

Although dirty bloom counts represent an indirect scouting method (a direct approach counts the live bugs) and reveal plant bug damage made several days earlier when these blooms were medium to large squares, this approach is fast, and fortunately dirty bloom counts here are often a small fraction of the 20 percent threshold that some states employ as a treatment trigger.

Additionally, the examination of quarter sized bolls for internal damage does not attempt to separate plant bug from stink bug damage.

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 acre, plant bug resistance to insecticides should be minimal here, and all of the materials listed in the North Carolina Agricultural Chemicals Manual should provide good plant bug activity.

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