Plant bugs and stink bugs are perhaps the most well-equipped of cotton pests when it comes to feeding on cotton bolls, thanks to their long beaks.
But their messy eating habits can often transmit nasty pathogens into the boll and cause extensive boll rot.
“They aren’t elegant feeders,” said University of Tennessee Extension entomologist Scott Stewart, author of a paper on the relationship between insect boll feeding and boll rot. “The plant bug and stink bug inject enzymes into the boll to dissolve things, then suck it back up. Then they move onto the next boll. So there is certainly a lot of opportunity for exchange of pathogens.”
The tarnished plant bug is the No. 1 pest in the Mid-South today, but once a boll accumulates between 225 and 250 DD-60s, it’s virtually safe from tarnished plant bug damage, Stewart says. “Most of the time, it will feed on squares and blooms unless it’s forced not to.”
In the central part of the Mid-South, which is considered the hot spot for tarnished plant bug, cotton producers can make 10 to 12 applications a year for the pest. “It’s not that way across the entire Mid-South. In Tennessee, it would be typical to make two to three applications a year,” Stewart said.
West Tennessee is more of a center for clouded plant bug, “which tends to show up where you’re not doing a lot of spraying, and it’s relatively easy to kill with insecticides,” Stewart said. “It’s a little bigger than the tarnished plant bug, and is much more inclined to feed on bolls. They can wear a boll out, which obviously creates an opportunity for pathogen introduction.”
The stink bug complex, primarily green, southern green and brown stink bugs, “feeds a little differently than tarnished and clouded plant bugs,” Stewart said. “They specifically target seeds. The damage can vary quite a bit and considerably, a lot of which is related to whether or not you get secondary rots.”
Minor stink bug injury might include loss of seed and/or lint staining. Major injury might start with a small amount of feeding, but secondary rot may have a significant effect on the boll.
Stewart says looks can be deceiving with stink bug damage. “You can see the characteristic pock marking on a boll. Sometimes, you pop the boll open and there is very little damage from the insect. Other times, it’s on its way to being completely lost.”
A test in Jackson, Tenn., in 2009, showed that when high infestations of stink bugs were not treated, over 90 percent damage occurred with a loss of 500 pounds of lint compared to better treatments. “Most of this was from the direct feeding of the stink bug, but there clearly was some relationship between insect feeding and hard locks,” Stewart said.
Caterpillar pests can also play a role in boll rots, according to Stewart. “With Bt cotton, we’re primarily talking about bollworm and fall armyworm. What we’re seeing with the new Bt technologies is that even when the Bt technology kills the worm, a lot of times the worm will try to feed on the boll. Depending on the caterpillar and the expression of Bt toxin, they may penetrate the carpel wall or they may not.”
A heavy infestation can result in a large number of bolls exhibiting this type of feeding, but Stewart says entomologists are not sure of the extent to which this causes problems for cotton producers.
Another west Tennessee study shows interesting results on the impact of fungicides and insecticides on boll rot. Researchers planted a non-Bt cotton, Phytogen 315, and applied insecticides three times beginning July 20, “when we first broke threshold for tarnished plant bug. Our treatment was a mixture of Karate and Bidrin,” Stewart said.
One fungicide application of Quadris was made on Aug. 10, about the third week of bloom when cotton was again at threshold for clouded and tarnished plant bug, and there were populations of stink bug as well. The study was complicated due to the excessive rain that fell in the fall of 2009, which caused a significant amount of weather-caused hard lock.
The study showed a very clear statistical relationship between the percentage of hard lock and insecticide spraying. “We reduced hard lock by spraying insecticide,” Stewart said. “There was not such a clear relationship with fungicide. There was a small benefit, but it didn’t show up statistically.”
Stewart says the continued development of Bt cottons “is going to further reduce the impact of our caterpillar pests directly on the crop or on incidental rots. Bugs are going to continue to rise in importance.”
Georgia research backs up Stewart’s observations on stink bugs. Georgia entomologist Michael Toews says, “It’s clear that stink bugs are transmitting boll rot pathogens into developing cotton bolls. It’s highly variable across years and across fields. We’ve developed very tight sampling thresholds that work very well. But much of that data was developed over the last three years and we haven’t had a lot of pathogen data during that time.”
A Georgia study indicates that spraying for stink bugs weekly at bloom increased lint yields by 67 pounds over the untreated check. But according to Toews, “By the time you make 6.5 sprays, you’re losing money.”
Researchers sprayed two times using a 10 percent damage threshold, which showed a slight increase in yield but still produced a negative net return.
Toews noted that well-timed applications at 20 percent damage showed a positive net return. “The most exciting result is the dynamic approach, where we’re more aggressive during weeks of bloom when we know we’re making the bulk of our crop. We were less aggressive early during bloom and less aggressive late in the bloom cycle.”