Thrips a problem in upper Southeast

As is often the case with many other damaging insect pests of cotton, one might assume that cotton in the Mid-South, Texas, or even Arizona would take the crown for the highest thrips levels, right? Wrong.

As North Carolina and Virginia cotton producers know from experience, and 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 300 to 500 immature thrips per 5 seedlings! That's a pile of thrips under any circumstance, let alone if seedlings are unprotected.

It's probably no coincidence that our states probably also have the highest ratio of surrounding host vegetation to 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 true leaf stage for extended periods. Although not recommended as a “thrips damage avoidance strategy”, cotton planted here after about May 20-25 often avoids these cool conditions, and the quick seedling growoff greatly reduces the thrips damage window.

Additionally, approximately three weeks after these late plantings, levels of adult migrating thrips are often declining.

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.

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.

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

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.

In the case of 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 three to four 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 5 percent of our cotton producers used both the 5 pound rate of Temik, plus a seed treatment in 2007.

We are just beginning to evaluate these combinations with full rates of both — perhaps not a bad idea for April planted cotton, especially in northeastern North Carolina.

Seed treatments are hard to beat for convenience and safety, and this technology now represents 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, 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 of cotton acreage sprayed for either pest was 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 an 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 (0.5 to 1.0 pound active per acre) provide some control of “westerns”, and that other organophosphates and pyrethroids fare worse.

  • In another test in which thrips adults were identified to species following either Temik 15G at 5 pounds of 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 both Temik and the seed treatment plus Orthene exceeded 96 percent.

In essentially all of 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 2008. We're due a break.

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