Jack S. Bacheler
North Carolina Extension Entomologist
Southeast Farm Press, July 21
Even though North Carolina cotton producers are aware that stink bugs and plant bugs are often (or should be) their primary target in Bollgard cotton, most post-bloom attention is typically given to bollworms. With stink bugs and plant bugs at far higher levels than in previous years, this would be good year to take this bug complex very seriously, and plan one's scouting efforts accordingly.
In the first eight years of its commercial availability (1996 to 2003), Bollgard cotton acreage received an average of 0.95 late season treatments here, although the range of applications was from zero to two, and in very rare instances, three, and targeted mostly for bollworms. Stink bugs and plant bugs should figure more prominently in Bollgard treatment decisions, as bug damage to bolls is typically two to three or four-fold higher than bollworm damage with this technology, based on extensive fall damaged boll surveys of whole fields managed by producers.
Scouting Priorities: The recommended procedures for scouting Bt cotton for insects are similar to those used in conventional cotton. However, the relative status of our late season insects is different between Bollgard cotton and the more heavily treated conventional cotton (no budworms and fewer European corn borers, and far higher levels of stink bugs and plant bugs in Bt cotton). Because potentially far more late-season stink bug and plant bug pressure is expected on Bt cotton this year; even more attention should be directed toward the bug complex.
Bollworms: Because hatching bollworms must feed on cotton to ingest a lethal amount of the toxin within Bollgard cotton plants, scouts should not use low numbers of eggs or first stage bollworms as a treatment trigger, but only as an indication of potential pressure to help gauge scouting frequency. Scouts should direct their attention to detecting high egg levels, economic square or boll damage and second stage bollworms. Thus, it is essential to both recognize the difference between first and second bollworm stages (a second stage bollworm will have molted once, and be approximately 1/8 inch in length), and to identify the difference between insignificant, superficial square damage and damage which will cause the square to abort. However, if egg levels are in the 75 to 100-range per 100 terminals, or 8 to 10 per 100 fruit, an economically damaging infestation of bollworms is likely and treatment is advised.
Since the introduction of Bollgard cotton here in 1996, most of the bollworms which become established on Bollgard cotton appear to do so on or within blooms, especially dried 'bloom tags' stuck to young bolls. The B.t. protein is not well expressed in the pollen anthers and petals of the flower. Because our thresholds are quite low (protective) for Bollgard cotton, however, flowers must not be over sampled. Flowers and dried bloom tags should be sampled in proportion to their occurrence in the overall boll population. If a higher proportion of pink blooms and bloom tags is sampled, bollworm thresholds must be raised accordingly.
Bollworm Thresholds: The following thresholds are suggested for Bollgard Cotton in 2004: (At the onset of the major bollworm moth flight. Twice per week scouting is recommended). The more aggressive, variable thresholds for responding to fewer bollworms if found on consecutive scouting trips is designed to avoid being hurt by economic boll damage from low, but consistent, levels of bollworms.
# Three live 2nd-stage bollworms (c inches or longer) per 100
squares or bolls. (Pay particular attention to bollworms in or on yellow,
pink or dried blooms stuck to young bolls),
or
# Two 2nd-stage bollworms/100 fruit on 2 consecutive scouting trips,
or
# One 2nd-stage bollworm/100 fruit on 3 consecutive scouting trips,
or
# Six economically-damaged squares per 100, (damage which will cause the square
to abort)
or
# 75 to 100 eggs per 100 terminals, or 8 to 10 per 100 fruit. (This threshold
should not be used within a week of an insecticide application.)
* Twice per week scouting frequency recommended.
(For all other insects, use conventional thresholds.)
Stink Bugs:In late season damaged boll surveys taken here from 1996 to 2003 under grower conditions, stink bug (and to a lesser degree plant bug) damage has been approximately 3-fold higher than bollworm damage in Bollgard cotton, thus showing that bug pests should be the target of significantly greater scouting effort in this low spray environment, especially in 2004.
So far this season, stink bugs and plant bugs have been showing up in pre-blooming cotton at exceptionally high numbers in many areas, suggesting that the bug challenge may be greater in 2004 that in past years. With these high early levels, we expect some threshold levels of bugs to occur on conventional and on Bollgard prior to the onset of the major mid to late July major bollworm moth flight (unusual for us). Stink bug boll damage assessments are best taken within week after the onset of blooming with specific sample of 50 quarter-size bolls inspected for internal damage. These bolls can either be cut open with a pocket knife or crushed open between the thumb and forefinger and counted as damaged if damaged lint or internal boll wall warts are present. If 10% or more of the bolls show internal spotting or lint staining, especially on developing seed, treatment is advised. Plant bugs cause identical damage and is counted along with sink bug damage. As the boll development period continues, the proportion of "bug-safe" bolls increases, and thresholds based upon the examinations of quarter-sized bolls should be increased accordingly. A boll can generally be considered safe from bugs damage once it is approximately 3.5 weeks or has an outside diameter of approximately 1.25 inches. (See Cotton Insect Scouting Guide for details).
Automatic 'over sprays' are not recommended for Bollgard cotton in North Carolina. However, treating in response to the above protective thresholds for late season stink bugs and bollworms often pays for itself several times over. This would be a good year for North Carolina cotton growers to take the bug complex seriously.