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Thrips No. 1 cotton pest in North Carolina, Virginia

Unfortunately, as North Carolina and Virginia cotton producers know from experience, and as tests confirm, this region has the dubious distinction of having the highest levels of thrips and potential damage in the Cotton Belt.

Under heavy population levels and with the help of a microscope, we sometimes count as many as 200 to 400 immature thrips per 5 seedlings! That’s a pile of thrips under any circumstance, especially if seedlings are unprotected. It’s probably no coincidence that our states probably also have the highest ratio of surrounding host vegetation to small average cotton field size.

To make matters worse, our often cool spring conditions limit plant growth, leaving the tender seedlings in the very susceptible cotyledon to two or three true leaf stage for extended periods. Thrips damage at this growth stage can result in significant yield losses and maturity delays.

Unfortunately, our often cool wet seedling grow off conditions, coupled with the very high ratio of thrips host acreage to small average cotton field size (approximately 15 acres) and resulting high thrips levels, often seem to create a ‘perfect storm’ of thrips headaches, especially on early planed cotton.

In a series of 70 or so replicated tests conducted here and in Virginia during the past decade, untreated cotton lost an average of approximately 300 pounds of lint compared to the best at-planting treatments. That’s a lot of cotton lost from such a tiny insect.

For starters, North Carolina producers should plan on using either the 5 pound rate of Temik 15G, or one of the seed treatments (Cruiser, Gaucho Grande, Avicta, or Aeris) and also plan on a foliar application for thrips following a seed treatment.

In the case of Temik, this application, if needed, should be based on scouting for the tiny immatures on the new leaves or in the bud area with the aid of a hand lens.

With seed treatments, the foliar application is not a case of if, but when. Following treated seed, a foliar application for thrips is best timed at either three weeks after planting or at the first true leaf stage, whichever comes first.

Any subsequent applications for thrips should also be based on inspections of the bud area for live immature thrips. In this case, don’t be fooled by crinkled older leaves — they’re not going to straighten out.

With cotton planted after May 20-25, we’ve had pretty good luck with a seed treatment or the 3-4 pound rate of Temik without a follow-up foliar treatment for thrips, though scouting is still highly recommended.

Large scale consultants’ surveys revealed that approximately 6 percent of our cotton producers used both the 5 pound rate of Temik/seed treatment combination in 2008. Although on the expensive side, with this approach producers needed a follow-up foliar application on approximately one third of their acreage versus 83 percent and 98.5 percent following Temik alone or a seed treatment plus Orthene, respectively.

We are just beginning to evaluate these seed treatments plus full Temik rate combinations — perhaps not a bad idea for April planted cotton that always seems to get hammered by thrips, especially in northeastern North Carolina.

Seed treatments are hard to beat for convenience and safety, and this technology now is deployed on about half of our cotton acreage. However, growers should be aware that the odds of having to treat for cotton aphids or spider mites increases dramatically following seed treatments compared to Temik.

In a series of large scale surveys of our independent crop consultants in 2004-2005 and in 2007 and 2008, foliar applications were 2.5 to 10 times higher for cotton aphids and 9.5 to 12 times higher for spider mites following seed treatments than behind Temik. Fortunately, even following seed treatments, the amount cotton acreage sprayed for either pest was here still on the low side — at least for now.

But if you were one of the producers who had to deal with treatable levels of one or both of these pests, they can be a headache. If the status of these pests increases in North Carolina, the impact of increased spraying could well be a factor in choosing one’s at-planting insecticide approach.

In some years, especially following extended hot dry weather, western flower thrips occur at high enough levels to result in significant control problems. Although tobacco thrips are far and away our most common species on cotton and can be very damaging in their own right, in most cases they can be reasonably well controlled.

Cotton producer-supported research conducted here the past two years has shown:

• Control of western flower thrips with a “normal” rate of acephate (most often Orthene in the past at 0.25 pound active per acre) is at best very limited and often no better than the check plots. Producer and consultant experience has suggested that very high rates of acephate or Monitor (one half to one pound active per acre) provides some control of “westerns”, and that other organophosphates and pyrethroids fare worse.

• In a 2006 test here in which thrips adults were identified to species following either Temik 15G at 5 pounds product per acre or a seed treatment followed by an Orthene foliar spray at 0.5 pound active per acre at three weeks after planting, at week four after planting Temik had provided approximately 63 percent control of western flower thrips compared to 33 percent control for the seed treatment followed by Orthene.

Control of adult tobacco thrips at this time with Temik or the seed treatment plus Orthene exceeded 96 percent. Fortunately, in most cases tobacco thrips are far and away the predominate species.

An old myth about “late” planted cotton has persisted in North Carolina for many years. That is, cotton planted after about the first week in May was thought to lose approximately 10 pounds of lint per acre on average for each day past this date. This “finding” was largely based on drawing a straight line on a graph from high yields in early May to low yields in June planed cotton.

More recent information by North Carolina Cotton Specialist Keith Edmisten has revealed this relationship was quadratic, not linear (the straight line). This finding translated into essentially no yield loss for cotton planted here in late May.

We have long known that cotton planted later in May is far less subject to thrips damage than cotton planted in late April to early May, primarily due to the shorter “window” of seedling vulnerability under warmer grow-off temperatures.

In 2006, North Carolina producers planted 860,000 acres of cotton. For the past two years, our cotton acreage has dropped by approximately 50 percent, significantly shortening the time to plant. This may be a good time for cotton producers here to consider planting dates in the May 10 to May 20 period.

Planting dates in this range could well present a combination of good yield prospects coupled with far fewer headaches.

In essentially all our research trials, when the cotton plants have an average of approximately 5-6 true leaves with adequate moisture levels and reasonably warm weather, thrips control was no longer needed. Let’s keep our fingers crossed for warm, moist conditions conducive to rapid seedling growoff and few thrips headaches in 2009 — and perhaps more mid-May cotton planting dates. We’re due a break.

For an earlier article by Jack Bacheler on the upper Southeast cotton insect situation click here. For information on integrating nematode and thrips control to save money click here.

TAGS: Cotton
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