The unrelenting rains of 2013 made for less-than-ideal conditions when it came to conducting certain on-farm research in the Southeast, but trials in North Carolina and Georgia confirmed what has been observed in earlier work on cotton thrips injury.
According to a survey conducted this past year of independent crop consultants, thrips have the potential to be the most damaging insect pest of cotton, with average control estimated at $24.50 per acre – $15 to $18 for the seed treatment and $8 for the foliar spray.
In research presented at this year’s Beltwide Cotton Conferences in New Orleans, Dominic Reisig, North Carolina State University Extension entomologist said the objective of his experiment in 2013 was to determine if cotton can compensate for early season thrips loss later in the season by manipulating moisture through tillage and irrigation.
It was a very bad year, he said, to conduct an irrigation study in North Carolina, where heavy rainfall routinely washed out fields in 2013. Still, some earlier observations about thrips were confirmed.
“The injury we see from thrips is from the insect piercing the leaf as it’s emerging out, with the leaf often being misshapen. Sometimes you won’t actually see this in terms of a reduction in the plant weight itself. The plant may maintain some of its mass, but it’s just not getting the adequate leaf surface area to get the photosynthesis required to grow properly,” he says.And here is the div code to render this in the ârelated linkâ format:
Research from the University of Georgia’s has shown, he says, that a seed treatment of Cruiser and a foliar overspray of Orthene will increase the cotton plant’s dry weight compared to when there is no treatment. “That’s essentially a one-to-one relationship – what you’re seeing above ground is mirrored below ground. When your fruits are hammered above the ground, there’s less root mass,” says Reisig.
Other work in Georgia has shown a relationship between the amount of cover and the density of thrips, he adds. “So basically, when you start to till and reduce the amount of cover, you tend to have more thrips density.”
The question of his research, says Reisig, was whether or a factor such as moisture later in the season could compensate for thrips loss.
“So even though we’re having this reduction of root mass, can we make it up by providing the plant with moisture later in the season? Factors we looked at in this study were irrigation – we looked at irrigated and non-irrigated. We also had three different types of tillage – conventional, strip-till and reduced till. We also had treatments with an insecticide and with no insecticide. My insecticide plots had a base seed treatment and a foliar overspray, with the foliar overspray being made at the second true-leaf stage.”
Insecticide treatment delayed
A cover was in place, he says, when it came time for tilling and planting. “We planted early, on May 3. Typically, when we plant early in the planting window, it’s a good period for thrips to be out there, temperatures are a bit cooler, and cotton doesn’t always grow very well. Generally, we put out our insecticide at the first true leaf, but we were delayed so by the moisture this past year. Also, we put it out the insecticides just before a rain, which isn’t a good idea.”
Reisig looked at thrips abundance and injury ratings, along with plant height and number of leaves, starting at three, four and five weeks after planting. Then, six weeks after planting, he weighed the dry weight above and below the ground. He pulled up the plants when the soil was moist, cutting the plant at the soil line, and took yield from the two middle rows.
“North Carolina really doesn’t have a sophisticated method of irrigating cotton, with less than 2 percent of the acres being irrigated. We used the University of Georgia checkbook method, which is based on crop growth stage. We had a deficit moisture situation on the third week of July, when we put out 1 ½ inch of irrigation. It looked like a deficit moisture situation during the first week of August, but I put out ½ inch which was followed by 3 inches of rain. So it wasn’t the best year to do an irrigation study.”
Conventional untreated plots had some thrips injury, he says. “It looked as though we had smaller plants and a little more injury where we had strip-till, which is contrary what we’ve seen. As we start to increase the cover on the ground, we tend to decrease thrips populations. But also, as we start to move towards reduced-tillage situations, the plants tend to grow a little slower.
Once we got into the reproductive stages and past the thrips-susceptible time, it literally was a wash. We received so much rain, and the field was poorly drained, giving us poor cotton as a result.”
A curious interaction was observed with the irrigation factor, he says.
“Four weeks after planting, we had not put on an irrigation, so it was curious why there would be a significant interaction for irrigation. We used drip tape, and it was apparent in the field, so maybe it was a mulch effect that repelled thrips.
“We had lower insect density where we treated with insecticides and had no irrigation, but we didn’t see the same thing where we did have irrigation and treated, so something’s going on with the drip tape. In general, we had fewer thrips where we treated with insecticides.”
Taller plants were observed where the plots were tilled and treated with insecticides, he says.
“Where we looked at below-ground biomass, we saw a doubling of the amount of biomass where we treated with insecticides compared where we didn’t treat. Whatever goes on above ground is also seen below ground, as has been shown in earlier research from the University of Georgia.”
Tillage type didn’t significantly influence thrips density in 2013, says Reisig. “We had plants that were taller where we used conventional tillage, and below-ground biomass was significantly influenced by the insecticide regime. These results are very preliminary, and it was a very bad year to conduct an irrigation study.”