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Field and Forage Crops |
The bollworm moth flight is now present throughout North Carolina. This year's flight will probably be marked by it's variability in intensity and perhaps its longevity, although historically flights that started early and more intensely have also tended to be longer in duration. This year's flight has not particularly early or intense so far. However, cotton maturity is generally on the late side, which, barring drought and subsequent early cutout, will likely signal a more protracted boll susceptibility to late season insects.
Producers should be aware that the threat of damage from plant bugs continues to be much higher than usual. At this time of the season, both dirty blooms and internal damage to small bolls should be the main focal point of attention, supplemented by visual assessments of adult and nymphal plant bugs in blooms and blackened small squares. Boll damage from stink bugs is indistinguishable from that of plant bugs; therefore, quarter-sized boll inspections for internal stained lint and warts may be caused by either pest. So far, it is hard to characterize whether this year will be light, heavy or somewhere in between for stink bugs. At this boll-susceptible first 4 weeks of blooming, a bug threshold of 10 percent internally-damaged bolls is recommended. With the plant bug pressure moderate to high in so many cotton fields this year, at least weekly boll assessments should be routine. Plant bug (and possibly stink bug) damage to squares and small bolls will likely be overlooked by many producers this season.
We appear to have at least one and perhaps two very large generations of European corn borer moths ready to emerge, primarily from field corn, in the coming days and/or weeks. Indications of European corn borer larvae will initially be apparent as stem tunneling in petioles and the main stems of cotton plants. This easily visible damage (wilting of the plant above the tunneling) is not typically a cause of significant economic damage, but may signal upcoming serious damage to bolls of various sizes. A moderate to heavy European corn borer larval population may sometimes justify protection with pyrethroid insecticides further into the bollworm season, particularly in later, ranker cotton. European corn borers have not been a significant pest of cotton for a number of years in North Carolina, but, as the case with plant bugs, this year seems to be a potential exception.
The moth counts in the Washington and Chowan counties rose sharply during the end of last week. Tables 2 and 3 provided below show trap catches in Washington and Chowan counties (10 traps per county). This is the beginning of what I believe will be a rather long bollworm moth flight. Overall, there is a rather high infestation of corn earworms (cotton bollworms) in corn and we are finding a wide spectrum of development situations in corn ears. Early planted cornfields show only old damage with a few caterpillars; most have left the ear to pupate in the soil. These bollworms are likely the ones that began emerging last week. Later planted corn fields show a mixed population of mostly moderate size larvae, with a lower proportion of larger and small larvae, remaining in the ear. These caterpillars will pupate and the moths will emerge to extend the moth flight.
Growers and consultants should be alerted of the need to begin bollworm and budworm scouting (see Cotton Information book and Cotton Insect Corner web page for scouting and threshold information). Both conventional cotton and Bollgard cotton can be seriously damaged by bollworms. In fact, last year (a bad bollworm year) we showed significant losses to Bollgard cotton (see Table 1).
Table 1. Response of BGII, BG, and conventional cottons to insecticide treatment for bollworm. Average of four tests in Edgecombe, Martin, and Washington Counties in 2002.
******************************************************************** Variety Insecticide* Seed Cotton/Acre Percent Difference** ******************************************************************** DP50BGII Yes 3,048 100 DP50BGII No 2,913 96 DP50BG Yes 2,580 85 DP50 Yes 2,329 76 DP50BG Yes 2,235 73 DP50 No 391 13 ******************************************************************** * Treated plots were sprayed twice with a tank-mix containing the maximum labeled rates of Tracer and Karate. ** Differences based upon the highest yielder.
Reported in Tables 2 and 3 are pheromone trap catches of bollworm (Helicoverpa zea) moths at five differing crop interfaces. These data are collected as part of a study to determine the relative concentration of bollworm moths moving to and from alternate host crops in relation to movement to and from Bollgard cotton. Findings will be important in determining the relative effect of non-Bt cotton as a resistance management refuge as compared to contributions of alternate crop hosts. We are also measuring the concentration of living bollworm larvae in each crop (not shown).
Table 2. Bollworm moths caught per two large Hartstack style pheromone traps per crop interface for approximately one week. Washington Co., NC 2003.
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Corn/ Peanut/ Soybean/ Cotton/ Bollgard/
Date Bollgard Bollgard Bollgard Bollgard Bollgard
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July 16 81.0 16.0 20.5 93.5 17.0
July 23 13.5 12.5 9.5 33.0 2.0
July 28 162.0 273.0 201.0 585.0 383.0
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Total 256.5 301.5 231.0 711.5 402.0
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County total to date = 1,902.5
Table 3. Bollworm moths caught per two large Hartstack style pheromone traps per crop interface for approximately one week. Chowan Co., NC 2003.
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Corn/ Peanut/ Soybean/ Cotton/ Bollgard/
Date Bollgard Bollgard Bollgard Bollgard Bollgard
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July 16 72.5 5.5 3.5 4.5 5.5
July 23 59.0 27.0 10.0 34.5 19.5
July 28 309.0 481.0 187.0 61.0 317.0
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Total 440.5 513.5 200.5 100.0 342.0
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County total to date = 1,596.5
Grand total to date = 3,499
It is time for the caterpillars to be present in peanuts. I saw a lot of moths flying in some fields this past week, so that means egg laying is taking place there. Keep a watch on the crop over the next several weeks. In most cases the threat from spider mites is low, so the flare up of mites is not near as much of a concern as last year. Good scouting can lead to timely applications of foliar insecticides for worms and may allow you to tank mix with a leafspot spray. The threshold is 4 worms per row foot during August. Foliar insecticides for worms will generally reduce leafhoppper populations as well.
Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Last modified on August 4, 2003 by Stephen J. Toth, Jr.