Zero tolerance for worms
Tart cherry growers struggle to make reduced-risk insecticides work.
Insecticide trials conducted by Michigan State University's Nikki Rothwell has found that the widely used practice of spraying alternate row middles may not work as effectively when reduced-risk pesticides are used. Photo courtesy of MSU.
Nikki Rothwell, MSU
As pressure increases on growers to eliminate organophosphate insecticides from their spray programs, Michigan’s tart cherry growers are not yet sure they can control pests—especially plum curculio.
There is zero tolerance for this beetle’s larvae in processed fruit, and nothing is more visible than a white worm that floats to the top of a pack of pitted red cherries.
“We’re really nervous about it,” said Dr. Nikki Rothwell, a Michigan State University research entomologist and coordinator of the Northwest Michigan Horticultural Station near Traverse City. “We’re seeing huge increases in pest populations in orchards where we use the reduced-risk insecticides.”
In experiments last year, Rothwell found that the new insecticides may increase growers’ costs even more than first anticipated. In her tests, the widely used practice of spraying alternate row middles (ARM) did not work as effectively.
“Our preliminary data suggest that ARM may not be an effective strategy with the new reduced risk insecticides due to their modes of action, which are different than the traditional contact insecticides,” she said.
She compared the effectiveness of two insecticides—the organophosphate insecticide Imidan (phosmet), which kills on contact, and the reduced-risk insecticide Avaunt (indoxacarb), which requires ingestion. She compared two kinds of sprayers, an airblast type and the Curtec, which more gently swirls spray into the trees.
She applied the sprays to one side of a row of trees and measured chemical deposition and insecticide performance on the front and back sides of trees in that row and the two rows next to it.
With an airblast sprayer, she found excess residue deposition on row 1 that dropped off dramatically at row 2. The first row got 2-1/2 times the amount of phosmet needed to do the job and 1-3/4 times the amount of indoxacarb needed to provide control.
With the Curtec, spray was deposited more evenly to all the rows, but the amount deposited was lower. Because phosmet is a contact insecticide, it was able to give 100 percent clean fruit in both rows 1 and 2 and 74 percent in row 3. Indoxacarb gave only 73 percent clean fruit in the first row, 60 percent in row 2 and 46 percent in row 3. This data suggests growers using the reduced-risk insecticides will need to spray both sides of all trees to get the coverage and the control they need.
Growers in the Eastern U.S. have used ARM for many years, and it works well when insecticides kill on contact and mobile insects like plum curculio and cherry fruit fly move and bring themselves into contact.
“Growers use alternate row middles because it saves them time, fuel, and money,” Rothwell said. “Evidence suggests the strategy may not work with the new insecticides. Every-row spraying will increase costs of materials, time, and fuel and dramatically changes growers’ bottom line. Ultimately, they must maintain the zero-tolerance standard.”
Rothwell plans more research this year, repeating the trials with fungicides and insecticides in demonstrations that more closely mimic alternate row middle sprays than her initial work last season. Last year, data was collected after spraying one row on one side, with no return pass that would have increased deposition levels.