Since Cotton leafroll dwarf virus (CLRDV) is vectored by cotton aphids, researchers have searched for effective insecticides to eliminate aphids. While no insecticide has been shown to eliminate cotton aphids, research shows which insecticides are more effective at controlling aphids in cotton acres.

During the online Beltwide Cotton Conferences in January, Phillip Roberts, Extension entomologist in cotton for the University of Georgia, spoke on research on cotton aphid management.

"This project, funded by Cotton Incorporated and which the Southeast Row Crop Entomology Working Group participated in, relates to CLRDV," Roberts said. "For the last two years, we have taken a more in-depth look at cotton aphid insecticide efficacy.

"We're looking at this because Cotton leafroll dwarf virus is vectored by cotton aphids, so questions were raised from our growers and other stakeholders, 'Should we try to control this vector?' From the work done, we have not been able to reduce the incidence of CLRDV infection by aggressively treating aphids in a cotton field."

Cotton aphid Ecology

Cotton aphids have many different host plants and a very high reproductive capacity. In addition, aphids are prone to develop insecticide resistance.

"If you look at cotton aphids as a pest in the state of Georgia where I'm located, we're going to have a hundred percent of our plants infested at some point during the year," Roberts said. "These infestations can start in the seedling stage and can be found on cotton until defoliation."

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Cotton aphids vary from field to field and year to year. Populations generally build to high numbers in June and July before being suppressed by a naturally occurring fungus. However, if you look, you can generally find some level of aphids on a plant almost any day of the year.

"Cotton aphid is a sucking pest and feeds on plant sap, which creates stress on the plant, especially when we have high infestations," he said. "In our state, very high infestations may result in reduced plant growth, yellowing of terminals, and honeydew developing in the lower canopy of the plant.

"However, from all of our research we've done to date in Georgia, we cannot demonstrate a consistent yield response from controlling for aphids."

Controlling aphids

Effective insecticides are available which can reduce plant stress from aphid feeding. The researchers looked at 14 different insecticides across trials in Florida, Alabama, Georgia, South Carolina, North Carolina, and Virginia.

"For most of the insecticides, we looked at the highest labeled rate to see what treatments would provide the greatest reduction of cotton aphids in a field," Roberts said. "These were standard, small plot trials, randomized complete blocks with four reapplications. Our objective was to make three counts, and we looked at the fourth main stem leaf below the terminal. We counted aphids around three to four, seven, and then 14 days after treatment."

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In total, there were 12 trials, which included one or more of these select groups of insecticides but not all insecticides were in every trial.

"We cannot make direct comparisons between all of the insecticides since not all insecticides were in each trial, but we can get a general feel of how these products are performing," he said. "Transform, Sivanto, and Assail all had greater than 80% control three to five days after treatment."

Most of the treatments used got above 50% control, which in terms of reducing plant stress is acceptable in the short term.

"If we look at our second rating date, which is seven to nine days after treatment, we start to see a break in how these products are performing," Roberts said. "The treatments Transform, Sivanto, Assail, Carbine, Bidrin, PQZ, and Sefina appears to hold up better than some of the others tested, such as Venom, Exirel, Centric, Movento, Fulfill, Admire Pro, and Brigade.

"Again, we're just looking for trends. We can't make direct comparisons because not all of these insecticides are found in each of the trials. If we look after 14 days, we don't see a lot of differences there, but still, we're seeing 40% to 50% reduction in aphids compared to our untreated even at 14 days."

What worked best

Data which included insecticides recommended for aphid control in the Southeast were analyzed using trial locations as replicates. They found that Transform, Assail, Carbine, and Bidrin provided significantly greater control than Centric and Admire Pro three to five days after treatment.

"Seven to nine days after treatment, the same four treatments were still providing significantly greater control than Admire Pro or Centric. If we go out to 14 days, we still see the same general trend," Roberts said. "The point here is there are differences in how these insecticides perform. This is likely due to some level of resistance from these products since Admire Pro and Centric have been used for many years."

In another trial conducted in Georgia in 2019, the cotton was planted on May 14, and weekly sprays of Assail insecticide were initiated two weeks after planting. The cotton was sprayed for five consecutive weeks.

"With this particular trial, we came in and counted aphids on July 2," he said. "We were expecting the fungus, Neozygites fresenii, to come in and crash our populations, so we counted aphids on July 2.

"One point I want to make is we cannot eliminate aphids from a cotton field even with weekly applications of an effective insecticide like Assail. We still achieved greater than 90% control, but it's important to understand that we cannot eliminate aphids or the risk of transmission of CLRDV by cotton aphid. The data looks similar for 2019 as well as 2020."

While aphids cannot be eliminated, yield data taken in a few locations show no significant difference in yield when comparing untreated versus treated checks.

"Another point to note when you make applications in the case of re-infestation, this can influence your perceived control," Roberts said. "If you look at the insecticides we evaluated, we received nearly 90% control with some of our better treatments at three to five days after treatment, above 70% a week out. At 14 days, we didn't see a lot of difference, but we have insecticides capable of reducing plant stress associated with aphid feeding."