Thrips a problem in Carolina cotton

Entomologists typically show far more skill in analyzing insect pest problems after the fact than they demonstrate in predicting problems in a way that might be helpful — that is, before they occur.

With the power of 20-20 hindsight, the causes of most of last year's insect headaches (or lack of thereof) make sense. For many of North Carolina's cotton producers, drought dominated the insect scene in 2007, with high thrips damage due to dry weather and poor insecticide uptake, and low bollworm and stink bug levels due to sorry, unattractive plants.

Other items remain a mystery.

With the combination of high acephate (Orthene in most cases) use for thrips and dry hot weather, most of us had expected a spider mite tsunami. Oddly, it never happened.

Essentially all the factors that influence the severity of our upcoming 2008 “pest year” — such as pest survival during the winter, the abundance and quality of nearby insect crop and weed hosts during the spring and early summer months, and the development of the cotton crop itself — are related to weather patterns. Unfortunately, weather forecasting on a farm or county-wide basis is often unreliable even just a few days in advance. And predicting weather patterns that might impact insect levels weeks or months in advance are virtually worthless, especially in the Southeast.

Additionally, all of our major insect pests — thrips, bollworms, stink bugs, cotton aphids, spider mites, and others — undergo a number of generations on other crops and wild hosts before moving into cotton, making early predictions several generations down the road even less reliable.

Despite the above limitations, a few observations are offered:

  • Thrips have caused major headaches for North Carolina and Virginia cotton producers for the past three years, 2005-2007. Unfortunately, our region leads the Southeast and other parts of the Cotton Belt in high thrips levels and damage.

    Our slower seedling grow off conditions and high amount of surrounding thrips host vegetation that serve to funnel thrips adults into small cotton fields often result in a rough start for cotton seedlings.

    Unfortunately, this situation is more the rule than the exception in this area.

    Behind the seed treatments Gaucho Grande, Cruiser, Avicta, and Aeris, plan on a foliar spray targeted at the first true leaf stage or at 3 weeks after planting (whichever comes first) — unless cotton is planted after about May 20.

    We often observe higher levels of cotton aphids and spider mites following seed treatments than following Temik.

    With Temik 15K at the 5 pound rate per acre, a foliar spray can sometimes be avoided with adequate soil moisture and close scouting.

  • At our latitude, plant bugs are usually kind to producers during the pre-bloom period (we have averaged approximately 3 percent to 8 percent treated acreage for plant bugs over the past 7 years), so the odds favor this trend holding for 2008.

    Weekly square retention counts should define most potential problem fields up to about a week or two beyond bloom initiation here.

    In North Carolina, it's not unusual to observe 90-plus upper square retention 4-5 weeks into the bloom period. However, these pests can be an occasional headache in blooming cotton, particularly in our far-eastern counties.

    “Dirty bloom” assessments, though not the sampling method of choice in the Mid-South where plant bugs are a more serious pest, are quick, easy to do, and can tell a scout or consultant if more intensive scouting is needed, particularly if dirty bloom counts are in the 5 percent to 10 percent range, or less.

    If dirty bloom levels are this low, more intensive sampling for plants bugs such as sweep net or ground cloth sampling is probably not necessary.

  • The potential for stink bug appears to have a strong correlation with moisture, with dry years often resulting in low stink bug damage to bolls. Dry weather negatively influences early wild and cultivated stink bug hosts resulting in fewer stink bugs, as well as makes cotton plants less attractive and prone to early cutout.

    Scouting wheat both before and after the big 2007 “Easter Freeze” confirmed the huge reduction in stink bug levels. Between this freeze and the drought that gripped much of North Carolina, stink bugs never recovered.

    Most North Carolina cotton producers would probably trade the potential for higher stink bug damage in 2008 if it meant we could also count on good moisture levels and high yields.

    With our ever-higher adoption of Bt cotton — more than 97 percent in 2007 — we can probably count on the bug complex to continue to account for most of our late season boll damage on Bt cotton.

    No matter what 2008 has in store, we need to be paying much more attention to the bug complex in our Bt cotton.

    Additionally, as we approach mandatory planting of Bollgard II and perhaps WideStrike varieties (Bollgard varieties will be phased out in the fall of 2009), our expected lack of treatment for caterpillars in all but a few circumstances will likely result in an even greater potential buildups of bug pests.

    Adequate sample sizes, lots of interior boll examinations, and green vs. brown adult stink bug ratios are a must in Bt cotton fields, especially during weeks 3 to 6 of the bloom period.

  • Bollworm moth levels have seesawed up and down for the past 8 years here until 2003, when both 2002 and 2003 were rough bollworm years; 2004 showed only moderate bollworm levels, and in 2005 the flight was both very late and exceptionally light, while 2006 and 2007 were about average.

    Although bollworm damage to Bollgard cotton fields has averaged approximately 1 percent during the 1996 to 2007 period, replicated tests show that a foliar application for stink bugs with either Orthene or Bidrin just prior to or during the initial 10 days or so of the moth flight can increase boll damage by bollworms by approximately 3-fold, with proportional losses in yields.

    This will not likely be as strongly the case with Bollgard II cotton, as late season bollworm damage to these varieties has average less than 0.2 percent in 2005-2007 boll damage surveys of growers' cotton fields.

    Widestrike lines typically provide intermediate bollworm control between Bollgard and Bollgard II varieties.

  • In recent years, our cotton producers have had only minimal damage from other caterpillars, such as fall and beet armyworms, European corn borers, and loopers. Unlike conventional and Bollgard cotton, Bollgard II and Widestrike varieties show high resistance to both armyworm species and loopers.

Weather patterns during the upcoming crop year will essentially determine the timing and intensity of our potential 2008 insect outbreaks. Most producers would gladly trade higher moisture levels for potentially higher boll damage from insects like stink bugs.

Whereas we can't control the weather in dry years, we certainly can minimize insect damage in wet years. Although meteorologists have difficulty in predicting weather patterns more than about a week in advance, on the plus side, sound insect and plant monitoring and well-timed sprays can play a major role in making the best of what nature has in store for us in 2008.

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