Fruit growers in Michigan can use existing stocks of the insecticide azinphos-methyl (Guthion) one more time before it is phased out at the end of September this year. When should they use it?

Michigan State University tree fruit entomologist Dr. Larry Gut suggests they target flies—the apple maggot in the case of apple growers, the cherry fruit fly if they grow tart cherries.

For several years, Gut has been working with growers helping them shift to newer “reduced risk” insecticides as the U.S. Environmental Protection Agency put Guthion on a phaseout program that gradually reduced the number of applications they could make and the amount of active material they could use in a season.

The phaseout was supposed to end with the 2012 season, but in Michigan, both the apple and cherry crops were decimated by spring freezes. The EPA was persuaded to allow one more season of use—so growers could use up stocks they had on hand.

So growers can use up to three-fourths of a pound of active material—about one spray—this year.

In the announcement last August 29, the EPA made it clear it has  not changed its views about the 2008 cancellation order that was to phase out azinphos-methyl over six years because of “high worker and ecological risks.”

“As a result of this year’s abnormal weather events, however, growers have been left with unused stocks that cannot be used unless EPA modifies the existing cancellation order to permit use of the existing stocks in the growers’ possession,” the announcement said.

The EPA said it was appropriate to allow growers to use their remaining stocks for one more season—through September 30, 2013. Though the decision would stretch out use for another year, it would not result in any greater use of the product, and it would avoid the need for costly ­disposal arrangements for stocks in ­growers’ hands.

Michigan’s entomologists, and growers, fought hard to keep azinphos-methyl—and are still concerned they might not be able to adequately control some pests with the “reduced risk” replacement products.

The new materials, Gut said, need to be ingested by pests; they do not kill by contact as Guthion does. Pests such as cherry fruit fly and apple maggot adults are not heavy feeders, making it more difficult to get them to consume a lethal dose.

Growers have about ten days to target the pest after these flies enter the orchards, during which time they mate and prepare to lay eggs. The newer insecticides have their effect during the times when adults ­consume small amounts of food or the brief time after eggs hatch and larvae dig their way into fruit.

Once inside, they are protected and can’t be killed by the new insecticides. Guthion could do that, too.

“EPA is phasing out our most effective material,” Gut said at the Northwest Michigan Orchard and Vineyard Show in Traverse City in January. “Cherry fruit fly is our most important pest.”

He suggested that growers set traps, fairly high in the tree, to catch the first flies and set the time frame for spraying. Using new materials, they must spray within the first five days to assure they kill flies before egg laying begins.

This timing of the Guthion spray will also hit plum curculio, another pest that can put a white worm in a red cherry. There is zero tolerance for plum curculio or cherry fruit fly larvae in cherries for processing.

In working with the new pesticides, Gut studied emergence patterns as well as the numbers of insects and found an increasing number of cherry fruit fly adults in tart cherry orchards as the early July harvest approached.

He believes this is because cherry fruit flies are infesting fruit left on the trees after shaker harvest. He now recommends that growers make one application of an effective insecticide within a week after harvest to kill these flies and reduce the number of larvae that will contribute to an overwintering population to infect the next crop.

Tart cherry growers need to do more spraying after harvest than they once did. They need to keep healthy leaves on the trees longer to assure good tree health and fruit quality in future years, so they now spray more postharvest fungicides to reduce the effects of cherry leaf spot. Now, insecticides should join the tank mix, Gut said.