Cotton insects difficult to predict

In 30 years at North Carolina State University as a cotton entomologist, each year has brought it’s share of upcoming unexpected insect surprises — some good; some not, but all difficult to predict.

We entomologists are often far more skillful in analyzing insect pest problems after the fact than 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) conveniently make sense after the fact.

For many of North Carolina’s cotton producers, moderate to droughty weather patterns dominated most of the insect scene in 2007 and 2008, with high thrips damage due to dry weather and poor insecticide uptake, and low bollworm and stink bug levels due to unattractive plants.

Looking back on the last two years, the status of other pests is still a mystery. For example, with the high use of foliar treatments for thrips (83 percent of our cotton acreage treated with a foliar application following Temik and 98 percent following a seed treatment), most of us had expected a spider mite tsunami. Oddly, it never happened.

Essentially all the factors that influence the severity of our upcoming 2009 “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 countywide 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 is 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’s cotton producers for the past three years, 2005-2008, particularly on early planted cotton. The North Carolina/Virginia region leads the Southeast and other parts of the Cotton Belt in high thrips levels and damage.

Our slow seedling grow off conditions (extending the period of seedling susceptibility to thrips damage) 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 almost always observe higher levels of cotton aphids and spider mites following seed treatments than following Temik. With Temik 15G at the 5 pound rate per acre, a foliar spray can sometimes be avoided with adequate soil moisture and close scouting, especially on cotton planted after May 10.

• At our latitude, plant bugs are usually surprisingly kind to producers during the pre-bloom period (we have averaged approximately 3 percent-8 percent treated acreage for plant bugs over the past 8 years), so the odds favor this trend continuing into 2009. 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 percent 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 bugs appears to have a strong correlation with moisture, with dry years (2006 to 2008) 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 reaching damaged levels.

Most North Carolina cotton producers would probably trade stink bug damage in 2009 if it meant that we could also count on good moisture levels and high yields.

With our ever-higher adoption of Bt cotton — more than 98 percent in 2008 — 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 2009 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 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 decade; 2004 showed only moderate bollworm levels, in 2005 the flight was both very late and exceptionally light, 2006 and 2007 were about average, and 2008 was probably the lowest flight in the past 20 years.

Although bollworm damage to Bollgard cotton fields has averaged approximately 1 percent during the 1996 to 2008 period, replicated tests show 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 is not as strongly the case with Bollgard II cotton and Widestrike cotton, as late-season bollworm damage to these varieties has averaged less than 0.5 percent in 2005-2008 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.

Upcoming weather patterns during the crop year will essentially determine the timing and intensity of our potential 2009 insect outbreaks. Most producers would gladly trade higher moisture levels for potentially higher bolls damage from insects like stink bugs.

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 2009.

TAGS: Cotton
Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.