Entomologists are skilled at explaining insect pest outbreaks — during and at the end of the growing season. In hindsight, the causes of last year’s insect problems make sense.
Although many interrelated factors impact the severity of upcoming “pest years” — such as pest survival during the winter, the abundance and quality of nearby crops and weed hosts during the spring and early summer months, and the development of the cotton crop — weather appears to tie these factors together.
And with weather predictions on a farm or countywide basis often unreliable just a few days in advance, forecasting 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 several generations on other hosts before moving into cotton, making early predictions even less reliable.
Despite the above disclaimer, let’s take a stab at what we might expect in 2006.
(1.) Thrips levels are often higher and more damaging in the Carolinas and Virginia than elsewhere in the Cotton Belt. This is in part due to our slower seedling grow off conditions and high ratio of surrounding weed and small grains crops.
Even with last year’s generally moderate thrips levels, unprotected April 27 planted cotton lost over 300 pounds of lint per acre compared to the better treatments in replicated tests. Unfortunately this situation is more the rule than the exception in this area.
Behind the seed treatments Gaucho Grande and Cruiser, plan on a foliar spray targeted at the first true leaf stage — unless cotton is planted after about May 20.
With Temik at the 5 pound rate per acre, a foliar spray can often be avoided with adequate soil moisture.
(2.) Plant bugs are often kind to producers during the pre-bloom period (we have averaged approximately 3-8 percent treated acreage for plant bugs over the past 6 years), but they can be a scattered headache in post-bloom Bt cotton lines, particularly in our far-eastern counties.
Both 2003 and 2004 witnessed moderate to high plant bug levels during the early boll set period in many areas.
Weekly square retention counts should define most potential problem fields up to about a week or two beyond bloom initiation. Plus, they’re easy to do. The crushing or cutting of quarter-sized bolls is probably best correlated with treatment need in the post bloom period for both plant bugs and stink bugs, though monitoring ‘dirty blooms’, visual observations for adult/nymph plant bug ratios, and sweepings are also helpful.
(3.) Stink bug damage in 2004, in both conventional and on Bollgard cotton, was far and away our earliest and highest on record (I go 30 years at NCSU). Stink bugs were also no picnic in 2005 in many areas of the state.
With our ever-higher adoption of Bt cotton — more than 90 percent in 2005 — we can probably count on the bug complex to continue to account for most of our late season boll damage.
The harder to control brown stink bugs also appear to have been more common in recent years. No matter what 2006 has in store, we need to be paying much more attention to the bug complex in our Bt cotton.
Additionally, as Bollgard II varieties become more widely planted in 2006 and beyond, 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.
Fortunately, the 3 or so applications typically sprayed on conventional cotton usually keeps stink bug and plant damage to bolls low, although this was not always the case in 2004 and 2005.
On the plus side, it appears that, on average, a single stink bug damaged boll only accounts for about one third as much yield loss as a bollworm damaged boll in North Carolina. Unfortunately, cotton fields with final year-end boll damage of 20 to 30 percent were fairly common the past two years. That’s a pretty big hit.
Pyrethroids are a best choice if green stink bugs predominate. If browns make up a significant part of the stink bug mix, consider Bidrin — with or without a pyrethroid in the tank.
(4.) Bollworm moth levels have seesawed up and down for the past 7 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.
Although bollworm damage to Bollgard cotton fields has averaged less than 1 percent during the 1996 to 2005 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 the case with Bollgard II cotton.
Widestrike lines typically provide intermediate bollworm control between Bollgard and Bollgard II varieties.
(5.) Other caterpillars, such as fall and beet armyworms, European corn borers, and loopers continue to cause only minimal damage, even in conventional varieties, although fall armyworm damage to bolls was moderate in some eastern North Carolina counties in 2004 and was also found in scattered cotton fields in 2005.
Unlike their Bollgard predecessor, Bollgard II and Widestrike varieties show high resistance to both armyworm species and loopers.
Upcoming weather patterns during the upcoming crop year will essentially determine the timing and intensity of our potential 2006 insect outbreaks. As a general rule of thumb, North Carolina’s cotton producers fair worse with both insects and yields during droughty years.
As of this late March writing, we have a moisture deficit of over 6 inches throughout much of our cotton production region. 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 where needed play a major role in making the best of what nature has in store for us in 2006.