The western cherry fruit fly has totally destroyed the romantic notion of plucking and eating cherries directly off backyard trees, says a Utah State University horticulturist.
"For those who may have been with Rip Van Winkle for the past 10 years and have never heard of this pest, rest assured it is here to stay," said Larry A. Sagers, USU Extension specialist. "The most often asked horticulture question at my USU Extension office has been, `What do I do about the worms in my cherries?' "Sagers said the pest did not exist in Utah until some 10 or 11 years ago when it was first discovered in the Holladay area. It has now spread to all areas in the northern half of the state and into many southern counties.
The small worms or maggots in the cherries are the larvae of a picture wing fly. This fly is slightly smaller than a common house fly and is dark brown or black with transparent, banded wings, he said.
"Picture wing flies are some of the most difficult to control pests that affect our fruit," he said.
The adult flies lay their eggs through the skin of the fruit, so pesticides applied after egg laying are not effective, he said. The eggs hatch into maggots that feed on the interior of the fruit. As the maggots mature, they burrow through the skin and drop to the ground.
"Those fruits with holes in them are probably better to eat than those without the holes," he said. "Holes mean the maggot is not still inside the fruit."
Cherry fruit fly control programs are directed at the adult flies. Sagers said those unwilling to spray their cherry trees should probably have them removed, "as they serve as an infestation for all other cherry trees in the neighborhood."
Controlling the pest is not an easy proposition, even by spraying, he said. Regular and thorough coverage of the trees is important to prevent the adults from laying their eggs.
"To prevent flies from migrating from tree to tree, encourage everyone in the neighborhood to spray at the same time," he said. "Spray timing is critical because it is based on trapping insects in selected locations."
This year's flies are two to three weeks later than last year. If you live in an area where traps are not placed, spray the cherries when they turn from a bright green to a straw yellow color. Pie cherries require additional sprays because they are harvested later in the season, he said.
Products registered for cherry fruit fly control include Diazinon, Methoxychlor and Malathion.
"It is important to follow the interval between the last application and the time of harvest," he said.
Diazinon requires a 10-day harvest interval; Methoxychlor, seven days; and Malathion, three days. Since sprays are applied every seven days, gardeners need to switch to Malathion for the last sprays prior to harvest time, he said.
Sagers said organic controls are difficult and limited in their effectiveness.
"The tremendous reproductive capacity of the fly interferes with even carefully planned integrated control programs," he said.
If this pest is not controlled, Sagers said 90 percent to 95 percent of the fruit is infested. "There is no practical way to determine which cherries are infected and which are not."