Nature may hate vacuums, but tarnished plant bugs love them.
They have rushed into the void left by the removal of worms and weevils from cotton fields with the advent of Bt cotton and boll weevil eradication.
The result has been a steady increase in foliar insect control costs for Mississippi cotton producers over the last five years, according to Don Cook, research entomologist at the Delta Research and Extension Center (DREC).
Cook, speaking at the Mississippi Crop College, in Stoneville, Miss., noted that plant bugs are adapting well to their new role as a primary pest.
“We’re now dealing with a pest that infests cotton throughout the entire season. In the last two to three years, the intensity of these infestations has gone up tremendously. We’re also seeing the evolution of insecticide resistance. Back in 1993, we were using lower rates of a quarter to a third of a pound of acephate and Bidrin per acre and getting good control. Now we’re bumping close to the maximum we can put out with these products, and control is less than adequate.”
Coordinated research is being conducted by cotton entomologists in Arkansas, Louisiana, Mississippi, Missouri, and Tennessee to address the plant bug problems. This includes several studies to re-evaluate thresholds, determine the impact of maturity and planting date on plant bug damage and identify insecticidal options for control.
One study indicates that control of tarnished plant bugs is critical between the third and sixth weeks of flowering. Delaying treatment for tarnished plant bugs past the third week of flowering will reduce cotton yield, as will terminating treatments before the sixth week of flowering.
Researchers believe that later in the season, thresholds based on dirty squares can do a better job of picking up infestations than a drop cloth. “When we have really large cotton, it’s hard to sample with a drop cloth. Dirty squares are easier to sample, especially in this bigger cotton.”
Studies showed no yield differences between research plots using a 5 percent and 10 percent dirty square threshold versus plots sampled using a drop cloth, Cook said. However, the 5 percent dirty square threshold did require additional applications.
Research also indicates that the maturity of cotton varieties can impact plant bug infestations. For example, in DP 444 BG/RR, an early variety, plant bug numbers stayed relatively low all season. In a full-season variety, DP 555 BG/RR, plant bug numbers increased significantly just before cutout. The later-maturing variety required two additional insecticide applications.
A similar result can be accomplished with planting dates, according to Cook. “Studies indicate that the later you plant, the more yield loss from plant bugs. The plant bug pressure gets more and more intense later in the season. It may be a function of running out of season, but it could also be from these insects getting more difficult to control later in the season.”
Cotton adjacent to corn will usually be negatively impacted by plant bug movement, according to Cook. But growers can use this knowledge to their advantage. “Over the last two years, we’ve been working on a regional project funded by Cotton Incorporated applying Temik at around pinhead square to the first 32 rows adjacent to corn. In 2008, we had a positive economic benefit from treating out to 24 rows past the corn.”
Cook noted that foliar insecticides directed at plant bugs are not performing as well as they have in the past. But shorter application intervals can help, according to Cook.
This is backed up by a study in which a heavily infested field was treated with Orthene, which reduced the infestation to eight plant bug nymphs per drop cloth. Researchers then divided the field into plots and applied Orthene at various different intervals. “At four and five days after the initial treatment, 100 percent control of plant bugs was achieved. When the second application was delayed to 6 days, it dropped to 70 percent, and when it was delayed to seven days, it dropped to 20 percent.”
Rotating chemistries will also help. For example, a research plot in which Orthene was followed by Orthene exceeded threshold nine days after a second application. However, where Orthene was followed by Centric, plant bugs did not exceed threshold.
The life stage of the plant bug is a big factor in its susceptibility to insecticide. Data from USDA research entomologist Clint Allen indicates that first instar plant bugs are much easier to kill than adults, while the fourth and fifth instars are actually much more tolerant than the adults.
“This is an advantage of us going to the black drop cloth,” Cook said. “We can pick up the smaller nymphs a lot easier, and if we can target these, we can get much better control with the same insecticides we’re using now.”
Scientists say Diamond, an insect growth regulator, can be effective on plant bugs, although it tends to work slowly. “A study conducted by Jeff Gore (research entomologist) at DREC indicated that Diamond either by itself or with Orthene or Centric resulted in more yield than with either Orthene or Centric alone.”
Researchers are also looking at piperonyl butoxide (PBO), an insecticide synergist that ties up some enzymes in insects that break insecticides down. “We tank-mixed it with either acephate or Brigade and compared that to the insecticide alone. There was some improvement with acephate when mixed with PBO and a significant improvement when mixed with a pyrethroid.”
Cook is concerned about the increasing use of neonicitinoid insecticides against plants bugs. “We’re making multiple applications in cotton, prior to bloom for aphids and plant bugs and later in the season after bloom for plant bugs. The products are used as seed treatment on many crops including cotton, soybeans, corn, and rice. We also have foliar labels in other crops. We could very possibly use this chemistry up in the near future.”
With standard insecticides not working as well as they used to, Cook says growers will have to go to more tank mixes and pre mixes to control the tarnished plant bug.
Growers should also keep in mind the impact of crop maturity on plant bug control, consider plant-based thresholds based on dirty or damaged squares, and integrate as many different tactics and chemical insecticides as possible to help manage plant bugs.
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