Despite an increase in tools to fight them, thrips continue to be a formidable foe for cotton growers in the upper Southeast. And an increase in western flower thrips populations could create even more problems in managing these tiny pests in 2008.
In 2007, growers in the Carolinas and Virginia routinely sprayed twice for thrips, despite using one of several new seed treatments. Even growers using Temik frequently had to come back and spray once or twice for thrips. Though no one knows for sure, entomologists in Virginia and North Carolina attribute much of this loss of control to increasing numbers of western flower thrips.
“As we got dryer and dryer in 2007 we kept getting higher thrips counts,” says veteran North Carolina State Entomologist Jack Bacheler. “In general, I saw more thrips on Gaucho Grande and Arista than I saw on Cruiser and Avicta in North Carolina. However, in Virginia Ames Herbert (Virginia IPM Leader and Virginia Tech entomologist) saw the opposite. In terms of yields, we saw very similar results from all these materials,” Bacheler says.
The ‘perfect storm’ for thrips is a cool, damp planting season, followed by extended periods of drought. For many cotton growers in the upper Southeast 2007 provided these perfect conditions for high thrips populations. Unfortunately long-term weather forecasts call for similar, if not more serious drought conditions for 2008.
In North Carolina there appeared to be more western flower thrips in 2007. And, worse news is that western flower thrips seem to be able to tolerate insecticide-treated seed better than other thrips species in the area.
Knowing which thrips to treat could influence which treatment to use, but identifying these tiny insects is all but impossible for growers.
Western flower thrips, once considered a pest only in the Southwest and West Coast Cotton Belts, are now common pests throughout the entire Cotton Belt. In recent years western flower thrips have seemed to thrive in the cooler growing areas, including North Carolina and Virginia. Though only one of five or six species of thrips that attack cotton plants in the upper Southeast, western flower thrips are far more difficult to manage than the other species.
Female western flower thrips are small, slender, winged insects which are amber or yellowish-brown to dark brown in color. Females are about one-sixteenth-inch long. Males are similar to females but smaller and always light yellow.
Western flower thrips eggs are delicate, cylindrical, slightly kidney shaped, smooth and translucent white.
Tobacco thrips are typically more common in the upper Southeast and are different enough from western flower thrips to affect control. Female tobacco thrips are dark brown or black, slender, and about 1 mm long. Each 8-segmented antenna has several yellowish middle segments; the rest of the segments are much darker. Males are usually yellow.
Regardless of the species of thrips, growers who plant early, typically in mid- to late-April in the upper Southeast, tend to have more problems with thrips. Bacheler says there is no good answer for control, other than for growers to be diligent in scouting and treating fields where high thrips counts are found.
The cost for not being diligent and not managing thrips can be high for cotton growers.
In a series of 50 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 — more often than not either Temik 15G at 5 pounds per acre or a seed treatment followed by a first true leaf foliar spray, according to Bacheler.
Though all the new insecticide-treated seed work to some degree to control thrips, especially tobacco thrips, Bacheler says growers should not expect control beyond about three weeks post planting.
To extend this short residual activity, foliar application following a seed treatment — the closer to the first true leaf stage the better — and no more than 3 to 3.5 weeks after planting.
Although it seems on the early side, a cotyledon stage spray is probably far better timed for thrips than a second or third true leaf stage application. In most cases a single application at the first true leaf stage provides cotton seedlings with enough thrips protection time to get the plants “over the hump”, thus reducing further thrips vulnerability and often extending into a period of fewer migrating thrips, Bacheler explains.
Western flower thrips seem to be increasing in the North Carolina-Virginia Cotton Belt, but thankfully aren't a problem every year and rarely a problem in some parts of the two states. In areas where this species is a frequent problem, there seems to be a correlation between hot, dry weather and build-ups in western flower thrips populations.
For growers who can identify western thrips as a problem, Orthene at 0.5 pounds per acre (active ingredient) is about as good as it gets. Other insecticides, including Vydate, Bidrin, dimethoate and a number of pyrethroid insecticides appear to be less effective in controlling western flower thrips than other thrips species in cotton.