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Plant bugs: Early, late threat to cotton?

For most cotton producers, plant bugs (or Lygus) have not been a significant early season problem in North Carolina, with the exception of some areas in our far eastern counties in some years. Averaged over the last decade, our cotton producers have treated approximately 7 percent to 10 percent of our cotton acreage specifically for plant bugs.

Levels so far in 2005 appear to be reasonable, but this could change due to seemingly large amounts of flowering host vegetation this spring.

With the introduction of Bollgard cotton in 1996, our former 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, resulting in greater late season plant bug damage to squares, blooms and small bolls. This has resulted in two rather distinct plant bug scouting periods — early season (pre-bloom), and mid- to late-season (during boll formation).

In most cases with conventional cotton, pyrethroid treatments usually reduce late-season plant bugs to low enough levels that fruit damage is minimal. However, in some cases plant bug damage to blooming cotton has occurred prior to these bollworm applications.

In Bollgard cotton, at least weekly scouting for plant bug damage (along with stink bug damage), is strongly recommended during the bloom period.

Listed below are some suggestions about plant bug monitoring:

  • Pre-bloom: Given our light to sometimes-moderate early season plant bug pressure, the quickest and most sensible way for us to assess the need for an 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, 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 has been under way for about a week, or sometimes two weeks.

    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. The pinhead squares monitored should be about one eighth to one fourth inch in total length, including the bracts, and are often the smallest squares that are easy to see.

    These squares will usually have a basal leaf about the size of a dime through a quarter.

    Count yellowing, brownish and black squares as missing positions. 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 (certainly the case on all Bt cotton), then sweeping is normally advised.

    (June budworm damage to squares in non-Bt cotton is sometimes associated with a tiny penetration hole, webbing, and/or some adjacent feeding on tender leaves. However, this hole often 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, and Bollgard, Bollgard II, and Widestrike cotton can presently be considered “budworm-proof”).

    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 also effect a treatment decision).

    Also, because levels of adult plant bugs in cotton fields are affected by temperature and may vary throughout the day, sweeping is best carried out during the morning or evening hours, or on cloudy days, when adults are more likely to be found in the fields.

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

    Plant bug thresholds are listed at the end of the 2005 North Carolina Cotton Information Booklet, in the 2005 Cotton Insect Scouting Guide, or on line at

  • Post-blooming, Bollgard cotton: In Bollgard cotton, in untreated cotton, or in conventional cotton that has either been treated late or a single time, late season plant bug (and stink bug) damage cannot be ruled out.

    From 1996 to the present, total bug damage (plant bugs and stink bugs combined) in Bollgard cotton was approximately 3-fold higher than bollworm damage, based on extensive fall damaged boll surveys.

    In Bt cotton, regular assessments of plant bug damaged blooms and small bolls are now in order in North Carolina (see the 2005 Cotton Insect Scouting Guide for particulars).

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

  • Plant bugs in perspective: Despite late season plant bug increases during the past 9 years, this species is still a relatively minor cotton pest in most areas of the state, even on Bt 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 underestimated), the monitoring of cotton fields for this ‘new’ late season pest of Bollgard cotton is strongly encouraged.

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