The Western cherry fruit fly invaded Utah about 15 years ago and spread quickly. All local cherry-growing areas are now affected.
Like many of you, pleasant childhood memories include walking into the back yard, pulling a few cherries off the tree and popping them into my mouth. Cherries were almost pest-free and did not require spraying to keep them that way. Unfortunately, that situation has changed along the Wasatch Front. The Western cherry fruit fly invaded about 15 years ago and spread as fast as any pest introduced in recent times. All local cherry-growing areas are now affected, and almost 100 percent of the fruits become infested unless they are sprayed.This small, elusive fly is one of the picture wing flies. These include some damaging and difficult-to-control pests, including apple maggot, Mediterranean fruit fly and the currant fruit fly. The adults lay their eggs underneath the skin of the fruit, so pesticides applied after egg laying are not effective. The eggs hatch into maggots that feed throughout the interior of the fruit. As the maggots mature, they burrow through the skins and drop to the ground. When selecting cherries to eat, you are probably better off choosing those with holes because the worm has already emerged. The maggot is still inside those without holes.
Cherry fruit fly control is difficult because the controls are directed at the adult flies. Lacking a giant fly swatter that will kill all flies in the tree, spraying is the only effective choice. If cherry fruit fly is such a nuisance that you no longer want to bother controlling it, consider removing the trees, because they serve as a breeding and infestation site for the rest of the neighborhood.
Controlling the pest is not easy, even with sprays. Regular, thorough coverage of the tree is important to prevent the adults from laying eggs. Often the pests simply migrate from tree to tree as sprays are applied. Encourage all neighbors to spray at approximately the same time to get better control of this pest.
The first spray should be applied from the 10th to the 12th of June in Salt Lake. If you are unsure about the timing in your area, start spraying when the cherries start turning from a bright green to a straw-yellow. Prior to this color change, the cherries are too hard for the fly to penetrate, so sprays are not needed.
Homeowners can use diazinon or malathion. Diazinon requires applications every seven days, and malathion needs to be applied every three days. Most gardeners need to use a combination of the two products because diazinon requires 10 days from the time of the last spray until the cherries can be harvested while malathion requires three days. This means that diazinon can be used for the first sprays of the season, but the last spray will need to be malathion. Pie cherries may require additional sprays because they are harvested later in the season.
Organic compounds are not very effective for cherry fruit fly. This fly has such a tremendous reproductive capability that it infests almost 100 percent of the fruits. Baits, sticky boards and other kinds of organic controls are not usually effective.
Some gardeners have sprayed their trees in the past and have not been satisfied with the results. Results may vary because of the following reasons:
- Diazinon that is several years old is probably not going to be as effective as diazinon that has been recently manufactured.
- Large cherry trees are difficult for homeowners to spray. Thorough coverage is the key, and if your sprayer does not reach the top of the tree, you may protect a few cherries on the lower levels, but the flies continue to breed and reinfest the trees from the upper level.
- Timing is critical. You must start before the cherries are infested and continue to protect the fruit at the proper intervals until harvest. Intervals depend on which product you are using.
- Neighborhood control programs, mentioned previously, are much more effective than single tree applications.
The Western cherry fruit fly provides at least one more "good old days" story of "Remember in the `good old days' when we didn't have to spray the cherries?" Other than that, I can't think of a single positive aspect of this pest. If you want to enjoy your cherries, start an effective and thorough control program. Finding worms in the fruit you've been eating is not a pleasant surprise and one I would just as soon be spared.
Larry A. Sagers is a horticulturist with the Utah State University Extension Service.