1 - Western cherry fruit flies are a problem in cherries in some regions of New Mexico.

Yard and Garden - November 6, 2010

Q.

We live in Los Lunas and have a sour cherry tree. The tree is healthy and produces a lot of cherries. Our problem is that the cherries have small white worms. We are hoping you have some ideas how to prevent them.

- D. and J. Dreis

A.

You are describing the Western cherry fruit fly. This is not an uncommon problem in our area. They infest both sweet cherries and sour cherries. The small adult flies are distinctive because their wings are clear with black markings across the wings. They deposit their eggs in the cherry fruit as they are just beginning to ripen. The larvae then burrow through the fruit toward the pit. As the fruit ripens and falls to the ground, the larvae emerge and burrow into the soil under the cherry tree. It is here that they pupate and overwinter. In the spring the adult fruit flies emerge from below the trees to deposit their eggs in the newly forming fruit, beginning the cycle again.

There are insecticides labeled for controlling the fruit flies if you want to use them, but be certain that you choose a product labeled for this purpose, and that you use the product exactly as instructed on the label. Since the adult flies do not emerge to lay their eggs until the cherries are nearly ripe, you must use a product that can be used just before harvest (short residual life), and you should wash the fruit well after harvesting. Knowing that the fruit fly adults emerge from the soil directly below the tree, it may be possible to apply the chemical to the soil there and kill the adults before they deposit their eggs.

A non-chemical treatment is to place a sheet of plastic on the soil under the tree to intercept the adult flies as they emerge from the soil. Clear plastic will capture solar energy and heat the area under the plastic, killing the flies as they emerge. Even if there is not sufficient heat to kill all the flies, this plastic may trap them long enough to prevent infestation of the fruit. An alternative practice is to place the plastic under the tree to intercept any fruit that falls, or larvae that emerge from the fruit and fall to the ground. You can them remove the infested fruit or larvae from under the tree and reduce the infestation next year. If your neighbors also have cherry trees (sweet or sour cherries), you should encourage them to choose one of these treatments as well. If there are unmanaged infestations nearby, the adult flies will fly to your tree and infest your fruit.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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