We entomologists are skilled at explaining insect pest outbreaks — that is, during and at the end of the growing season. In hindsight, the causes of last year's insect problems usually 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 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 several generations on other hosts before moving into cotton, making early predictions even less reliable.
However, a few observations may be in order.
Thrips levels are almost always 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 thrips host vegetation to our small 14-acre average field size.
Unfortunately, both 2005 and 2006 were rough thrips years, primarily due to moderate to high thrips levels and poor seedling grow off conditions. 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.
With Temik at the 5 pound rate per acre, a foliar spray can often be avoided with adequate soil moisture. Also, behind seed treatments we often see higher levels of cotton aphids and spider mites than following Temik use.
Plant bugs are often 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 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. These tiny bugs were not as bad in 2005 and in 2006.
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 during the boll set period, as this sampling serves for both plant bugs and stink bugs. Monitoring ‘dirty blooms’, visual observations for adult/nymph plant bug ratios, sweepings and drop cloth sampling are also helpful.
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. Thankfully, stink bug damage in general across the state was low in 2006.
With our ever-higher adoption of Bt cotton — more than 95 percent in 2006 — we can probably count on the bug complex to continue to account for most of our late season boll damage on Bt cotton.
Being hard to effectively control with pyrethroids, brown stink bugs can complicate insecticide choices. No matter what 2007 has in store, we need to be paying much more attention to the bug complex in our Bt cotton.
Additionally, as Bollgard II and perhaps WideStrike varieties become more widely planted in 2007 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 through 2006.
On the plus side, it appears that, on the 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 percent to 30 percent were fairly common in 2004 and in 2005. 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.
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, while 2006 was about average.
Greatly increased corn acreage across most of the state will likely translate into more bollworm (corn earworm) moths.
Although bollworm damage to Bollgard cotton fields has averaged approximately 1 percent during the 1996 to 2006 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 as strongly be the case with Bollgard II cotton. Widestrike lines typically provide intermediate bollworm control between Bollgard and Bollgard II varieties.
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 counties in 2004 and was also found in scattered cotton fields in 2005 and in 2006.
Unlike their Bollgard predecessor, Bollgard II and Widestrike varieties show high resistance to both armyworm species and loopers.
Weather patterns during upcoming crop year will essentially determine the timing and intensity of our potential 2007 insect outbreaks.
As a general rule of thumb, North Carolina's cotton crops fare worse with both insects and yields during droughty years.