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Damage by apple flea weevil.

Damage by apple flea weevil.

Matt Grieshop­­­

Organic apple growers in the Midwest appear to have a relatively simple solution to their problems with apple flea weevil, which appeared suddenly as a problem in Michigan two years ago and took out 90 percent of the fruit in some orchards.

The best solution, entomologists say, is to spray Entrust (spinosyn) very early in the season, at green tip to pink. While pyrethrum materials such as Pyganic do kill the weevil, as does M-Pede, neither of these provide residual activity after they dry.  The apple flea weevil adult does its most serious damage by feeding on and destroying fruit buds before they have a chance to emerge in the spring. In on-farm field trials, lime sulfur, Mycotrol ­(Beauvaria bassiana), and neem oil did not control the weevils as well as Entrust.

John Pote, a graduate student at Michigan State ­University working with Dr. Matt Grieshop and Dr. Anne Nielsen in the organic pest management laboratory, gave the good news to organic growers during a session at the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids, Michigan, in December.

Pote noted that the weevil emerged as a problem for organic growers, who normally do not apply insecticides that early in the season, since few insects become active so early. The apple flea weevil is quite active at low temperatures. “No other insects are out there that early,” Pote said.

In conventional apple management programs, the weevil is likely incidentally ­controlled by insecticide applications made to kill other insects either in April—when overwintering weevils emerge—or in early July—when the summer generation emerges from leaf mines. At low populations, the weevil likely goes unnoticed, Grieshop said, but at high ­populations, it is capable of causing total crop loss.

Organic practices do not encourage preventative insecticide applications, so growers need to be vigilant early in the season, Grieshop said. “Management of this pest will depend on preventing adults from feeding on young foliage and buds.”

Early spring

The apple flea weevil emerges in the early spring between the green tip and pink stages of apple bud development, Pote said. Adults feed on leaves and buds, leaving distinctive shot-hole damage. Adults lay eggs in the leaves, and larvae develop as leafminers, typically feeding from the middle of the leaf to the edge. After two to three weeks of larval development, larvae pupate within a chamber formed between the two leaf surfaces and emerge in June and July. Newly emerged adults feed on leaf tissue before entering diapause in late August or ­September.

There appears to be only one generation per year, Pote said.

Feeding by adults prior to fruit set causes buds and blossoms to abort, while larval feeding causes brown patches on leaf margins that resemble sulfur burn damage, Grieshop said. Adult damage further mimics frost damage because beetles begin feeding at the base of trees and work their way up to the top.

While damage so far appears mostly in organic orchards, it has shown up in conventional orchards as well, he said. As conventional growers shift to newer, more targeted insecticides with the phaseout of ­azinphos-methyl (Guthion), this insect could become more of a problem for them.

Scouting for this pest when it first appears can be ­difficult; damage later is easier to find. Adult beetles are typically found on the underside of leaves, after leaves appear. “Adults are fairly shy and will typically hop or drop off foliage when approached or disturbed,” Grieshop said.