Evaluate plant bug control carefully

Plant bugs have never been much of a significant early-season problem on cotton, except for some producers in our northeastern and far eastern counties.

With the introduction of Bollgard cotton in 1996, however, our typical range of two to four late-season bollworm applications has been reduced to an average of 0.8 applications, resulting in potential late season plant bug damage to squares, blooms and small bolls. This has resulted in two rather distinct scouting periods — early-season (pre-bloom), and mid-to late-season.

Populations reduced

In most cases with conventional cotton, pyrethroid treatments have reduced late season plant bugs to low enough levels that anything more than periodic checks of a few selected cotton fields should not be necessary.

In Bollgard cotton, however, at least weekly scouting for plant bug damage is recommended during the bloom period.

Listed below are some suggestions for monitoring of plant bugs:

• Early-season monitoring — In North Carolina, the quickest and most sensible way to assess the need for early-season plant bug treatment is to monitor the percentage of squares being retained by the plants during the pre-bloom period.

If cotton plants are retaining 80 percent or more of their squares, additional sweeping or other monitoring for live plant bugs is unnecessary.

In most years in North Carolina, the percent square retention is often in the low to high 90s until blooming begins. To assure that fruit loss due to plant bugs is recent, be sure to check at least two newly developing squares per plant, selecting one in the terminal area and one on a lateral fruiting branch.

Monitoring procedure

The pinhead squares monitored should be about an eighth inch in total length, including the bracts, and are often the smallest squares which are easy to see. These squares will usually have a basal leaf about the size of a dime through a quarter. Hotter, drier weather seems to push plant bugs down in the canopy, and at times, first and/or second position square losses on lateral fruiting branches can predominate.

Checking two squares each per plant from 25 randomly-selected plants from throughout the field would constitute an adequate sample size in most cases. If square retention is less than 80 percent, and square damage from tobacco budworms has been ruled out, then sweeping is normally advised.

For fields with square retention of less than 80 percent, taking 10, 25 sweep samples throughout the field for adult and immature plant bugs should indicate if treating is advisable (though other factors, such as node of first fruit and stand density, may affect a treatment decision).

Also, because levels of adult plant bugs present in cotton fields are affected by temperature and vary throughout the day, the value of sweeping varies, and is best carried out during the morning or evening hours, or on cloudy days.

In North Carolina, expect adults to make up the vast majority of the population under most circumstances in pre-blooming cotton.

The counts of swept plant bugs can be compared to the appropriate plant growth category in the plant bug threshold table in the 2001 Cotton Information Booklet or the 2001 Cotton Insect Scouting Guide (www.run.to/cotton).

  1. Post-blooming and Bollgard cotton — In Bollgard cotton, in untreated cotton, or in conventional cotton which has either been treated very late or a single time, late-season plant bugs (and stink bugs) cannot be ruled out. In these latter situations, regular assessments of plant bug-damaged blooms and small bolls are now in order in North Carolina (see the 2001 Cotton Insect Scouting Guide for particulars).

    Supplemental sweeping to confirm the presence of an active plant bug population and the presence of nymphs (indicating successful reproduction) is also advised.

  2. Plant bugs in perspective — Despite late-season plant bug increases during the past five years, this species is still a relatively minor cotton pest in North Carolina, even on Bollgard cotton. Because plant bugs have been a significant pest in some fields, and because the impact of their damage to young bolls is only partially understood (and perhaps under-estimated), the monitoring of cotton fields for this ‘new’ late-season pest of Bollgard cotton is strongly encouraged.

Note: June budworm damage to squares is often associated with a tiny penetration hole, webbing, and/or some adjacent feeding on tender leaves. However, budworms can also make a single penetration hole which becomes nearly invisible as the square dries up. These blacked squares can be difficult to differentiate from plant bug injury. Fortunately, budworm damage to squares in the 10 percent range or higher is almost always associated with the more obvious symptoms mentioned above. At least into the foreseeable future, do not expect this budworm damage on Bollgard cotton in North Carolina.

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