Thrips tough pest in upper Southeast

As many cotton producers know from experience, and as tests confirm, the upper Southeast could well be designated the “Thrips Capitol of the Cotton Belt.”

Our region has the dubious distinction of having the highest levels of thrips and greatest potential damage to seedling cotton of anywhere in the U.S. Under heavy population levels and with the help of a microscope, we sometimes count as many as 200 to 500 immature thrips per 5 seedlings That’s a bunch of thrips under any circumstance, especially if seedlings are unprotected. It’s probably no coincidence that our states 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 (approx. 15-20 acres) and resulting high thrips levels, often seems to create a ‘perfect storm’ of thrips headaches, especially on early planed cotton.

Extended hot, dry weather is no picnic either in dealing with thrips. In a series of 80 or so replicated tests conducted here and in Virginia during the past decade, untreated cotton lost an average of approximately 250 pounds of lint compared to the best at-planting treatments. That’s a lot of cotton lost to 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). In most cases, plan on a foliar application for thrips following a seed treatment.

With Temik, a foliar application for thrips, 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. Treat if you find an average of more than one immature per true leaf.

With seed treatments, a foliar application is most often not a matter of if, but when to treat. Following a seed treatment, 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 or treat on the basis of crinkled older leaves — they’re not going to straighten out.

A large scale consultants’ survey revealed that approximately 10 percent of their cotton producers used both the 5 pound rate of Temik/seed treatment combination in 2009. Although on the expensive side, with this approach producers needed a follow-up foliar application on approximately one third of their acreage vs. 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 increase dramatically following seed treatments compared to Temik.

In a series of large scale surveys of our independent crop consultants in 2004-2005 and in 2007-2009, subsequent foliar applications for spider mites and cotton aphids were 2 to 10-fold more likely 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 here — at least for now. But if you were one of the producers who had to deal with treatable levels 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 for mites and aphids 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 in the upper Southeast 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 to 0.3 pound active ingredient 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 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 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 Temik or the seed treatment plus Orthene exceeded 96 percent. Fortunately, in most cases tobacco thrips are far and away the predominate species. The best remedy for “westerns” is an inch or two of rain.

Although producers up this way typically prefer to plant their cotton during the first reasonable extended planting window (often during the last week in April to the first week to 10 days in May), new varieties have performed very well here in mid-to late-May plantings.

Fortunately, the potential for damage from thrips damage drops off significantly with mid- to late-May plantings, and can result in substantially lower insecticide inputs. With cotton planted during the May 18-25 window, we’ve had pretty good luck with a seed treatment only or the 3 pound rate of Temik without a follow-up foliar treatment for thrips, though scouting is still highly recommended.

Whereas we are not encouraging our producers to bail on often-preferred earlier plantings, cotton planted in mid- to late-May often gets a substantial break from potential thrips damage as a result of quicker grow-off and waning adult thrips populations moving into cotton.

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 2010 — and perhaps more mid-May cotton planting dates.

TAGS: Cotton
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