Plant bugs have become “a nightmare to control” in much of the Delta, says Jeff Gore, and to contend with the problem growers need to rotate chemistries, shorten application intervals, plant as early as conditions allow, and consider planting hairy leaf cotton varieties.
“Last year, the best control we were able to get with two applications of any of the insecticides commonly used was 64 percent,” he said at the annual meeting of the Mississippi Agricultural Consultants Association. “With huge numbers of insects present, in a lot of cases 64 percent control didn’t even get us below the threshold level.”
Gore, who is Mississippi State University associate Extension/research professor at the Delta Research and Extension Center at Stoneville, says “in our tests, two applications of Orthene were always better than one, and a 4-day to 5-day interval between applications gave much better control than 7 days, which gave very little control. If you wait 7 days and let plant bugs recover from the first application, a second application isn’t going to do much good.”
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Rotating chemistry is important, he says. “It’s best not to make a second application with the same product you used for the first application. Where we sprayed twice with Orthene, 9 days after the second application the plant bug population moved back above the threshold, but where we used Centric for the second application, the population stayed below the threshold.”
For the past several years, Gore says, “We’ve been studying all the components of crop management to develop an overall plant bug management program. Scott Graham, who’s in a master’s degree program, is working on combining all these approaches into one best management program.
“Among the things we’ve looked at is how cotton variety affects plant bug damage. In our trials, we’ve seen that hairy leaf varieties can make a big difference in the impact plant bugs have on yield. These varieties could have a benefit in the Delta, where plant bugs are really bad. It’s certainly advisable to stay away from true smooth leaf varieties, which can cost you one to two applications compared to a hairy variety.”
Planting early can help
A former master’s degree student, Brian Adams, studied the impact of planting date and varietal maturity on plant bugs, Gore says. “His work indicates the earlier we plant cotton, the less impact plant bugs have, and the fewer sprays are needed.
“If we can plant by at least the first week in May, we tend to save one to two applications over planting beyond the first week in May. In our studies, earlier-maturing varieties tended to have less yield loss from plant bugs — they stayed in the fruiting/flowering period for a shorter time, with less opportunity for plant bug damage.”
Another element in plant bug control is the use of Diamond, he says. “We’ve done a lot of research on this product for plant bug management, with different application times for one 6 oz. application.
“When we applied Diamond, regardless of timing, we saw a 123 lb. lint increase. If we made the application at first flower, one application gave us an additional 74 lbs. of lint beyond the 123 lbs. If we backed it to early/late squaring period, we saw an additional 69 lbs. Overall, by applying Diamond early, we saw a 266 lb. lint increase from just one application — and that definitely pays for itself.”
Gore emphasizes that Diamond won’t kill adult plant bugs. “But what we’ve seen is that it sterilizes them to some degree; the females lay a lot fewer eggs, and a much lower percentage of those eggs hatch. A first instar nymph is 10 times more sensitive to Diamond than a second instar nymph. So by having Diamond out there when eggs hatch, we’re getting a lot higher level of mortality than if the insects are allowed to get to the second instar stage.
“Any product we apply after that one Diamond application seems to work better for about two or three applications. For several weeks into the season, a lot of other products seem to do better when Diamond is in the field.”
Another graduate student, Chase Samples, looked at the impact of nitrogen rate. Gore says in the Delta, in a high population plant bug environment, yield is maximized at 80 lbs. of nitrogen. “Above that rate, he saw no appreciable yield increase. If we start pushing N rate, it just makes the plants a lot more rank, making it harder to find plant bugs.
“When we have big, lush plants, it’s just that much more difficult to control plant bugs with insecticides. And with the 80 lb. N rate, we sprayed one to 1.5 fewer times than where we had higher N rates. So, we saved money with less N and made fewer plant bug applications.
Reduced applications by half
“In the Delta, where we used all these best management practices — planted early, used hairy varieties, sprayed Diamond early, reduced N — we made three plant bug applications compared to six applications with our standard practice. We cut the number of applications in half just from the way we managed the crop.
“Using best management programs, in the Delta, yield averaged 1,589 lbs., compared to 624 lbs. for Delta standard practices. In the hills, BMP average was 892 lbs. versus 734 lbs. for standard practices.”
In other insect work, Gore says, studies confirm that Bt cotton varieties benefit from foliar overspray for bollworm control when populations are high.
“In my 2014 tests at Stoneville, we had very high bollworm pressure, and non-Bt cotton that was untreated sustained 60 percent to 70 percent yield loss. But with all the technologies — Bollgard II, Widestrike, Widestrike III, and new TwinLink from Bayer — two applications of Prevathon provided greater yields than for unsprayed plots. When we applied defoliant on sprayed varieties, the leaves came off much easier, with less damage than in the unsprayed plots.
“None of the Bt technologies will provide absolute control of bollworms. Regardless of the Bt technology, yield where we sprayed was better than unsprayed. Even with the new three gene products, we still saw benefit from foliar overspray for bollworm control under very high pressure.”
An area of concern, Gore says, is that “we seem to be seeing more damage in Widestrike and Bollgard II, with one-day to three-day-old worms feeding in the terminals and on small squares, working their way down the plant. Until about two or three years ago, we never saw much square damage from small worms with any of these technologies. This can be somewhat alarming, but foliar applications of the appropriate insecticides can take care of this.”
All Bt technologies do a good job of controlling tobacco budworms, Gore says.
For thrips, he says, “We’ve pretty much shown they are resistant to thiamethoxam, and last year we began recommending imidacloprid. For the time being, thrips are still susceptible to imadacloprid, and it’s doing a good job.
“But we don’t know how long that will last. Six or seven years ago, we wouldn’t start seeing immatures in Gaucho-treated cotton until two, three, or four weeks after planting. Now, in some of my tests, we’re seeing them as soon as cotton comes up and puts on its first true leaf.
“It seems something is changing with imidacloprid, but we haven’t been able to measure it yet in the lab. Just keep an eye on the thrips situation and spray behind any of the seed treatments when you need to.”