Cotton insects difficult to predict

You’d think that with just over 30 years’ worth of experience looking into the rear view mirror of cotton insect pest management, past experience would be handy in providing insights into the upcoming year’s pest problems.

As most producers can attest, however, each year brings its share of insect surprises — some good; some not, but all difficult to predict prior to planting. (Also see

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. However, if 2010 sets up like 2007 and 2008, droughty weather patterns could result in high thrips damage due to dry weather and poor insecticide uptake and as well as low bollworm and stink bug levels due to unattractive plants.

Looking back on the past three years, the status of other pests is still a mystery. For example, with the high use of foliar treatments for thrips in 2007 and 2008 (83 percent of our cotton acreage treated with a foliar application following Temik and 96 percent following a seed treatment), most of us had expected a spider mite tsunami. Oddly, it never happened.

All the factors that influence the severity of our upcoming 2010 “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 which are often difficult to predict on a farm or county level basis even just a few days in advance.

Additionally, predicting weather patterns that might impact insect levels weeks or months in advance are virtually worthless, especially in the Southeast, although a degree day weather model developed at NCSU shows some potential in predicting our major thrips peak.

Finally, 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 unreliable. Despite the above limitations, a few observations are offered:

(1.) Thrips have caused major headaches for North Carolina’s and Virginia’s cotton producers for the four of the past five 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 potential 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 relatively small cotton fields, often result in a rough start for cotton seedlings.

Even though hot, dry weather may result in quicker grow-off conditions and a narrower window of thrips vulnerability for cotton seedlings, the rapid drying of alternative thrips hosts (weeds, wheat, etc.) help create high levels of migrating adult thrips. Unfortunately, thrips headaches are more the rule than the exception for most North Carolina cotton producers.

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 15G. With Temik 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.

(2.) At our latitude and throughout most of the Southeast, plant bug damage is usually surprisingly light to producers during the pre-bloom period and beyond (we have averaged approximately 3-8 percent treated acreage for plant bugs over the past 9 years), so the odds favor this trend continuing into 2010.

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 3-4 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. If dirty bloom counts are in the 10 percent to 15 percent range or less, more intensive sampling for plants bugs such as sweep net or ground cloth sampling is probably not necessary.

(3.) The potential for stink bug damage appears to have a strong correlation with moisture throughout the bloom period, 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.

As a general rule, higher moisture levels are correlated with higher stink bug levels, but also higher cotton yields. Most North Carolina cotton producers would probably trade higher stink bug damage in 2010 if it meant we could also count on adequate moisture levels and high yields, as was the case for many in 2009.

With our ever-higher adoption of Bt cotton — more than 98 percent in 2009 — we can probably count on the bug complex to continue to account for most of our late season boll damage on Bt cotton.

A plastic, pocket-sized stink bug scouting field guide has been developed by our Southeast Cotton Entomology Working Group for 2010 distribution in early June. This device should increase the adoption, efficiency and accuracy of scouting for stink bugs in the Southeast.

(4.) Bollworm moth levels have seesawed up and down for 30 years or more. Although bollworm damage to Bollgard cotton fields has averaged approximately 1 percent during the 1996 to 2009 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 to 10-fold, with proportional losses in yields.

This is not as strongly the case with Bollgard II and WideStrike cotton lines, as late season bollworm damage to these varieties has averaged less than 0.5 percent in 2005-2009 as evidenced by boll damage surveys of more than 700 producer-managed cotton fields.

However, on occasion we have been able to create a perfect storm of bollworm damage (approximately a 200 pound yield loss) to both of these 2-gene technologies by over-spraying cotton fields with a high rate of an organophosphate at the beginning of a large bollworm moth flight, so bollworm scouting is still recommended.

(5.) 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 2010 will determine the timing and intensity of our potential insect outbreaks. Most producers would gladly trade higher moisture levels for potentially higher boll 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 2010.

TAGS: Cotton
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