Fruit flies emerge from the soil for about eight weeks, attacking cherry trees. They are most active around harvest time. Nearly 20 per cent of the population becomes active each week during the peak emergence period. One female fruit fly can lay 100 to 300 eggs each month. The only way to get rid of fruit flies on cherry trees is with an insecticide.
Evaluate the presence of fruit flies. Place yellow sticky traps on the cherry trees by the middle of May or when the cherries are still a light green colour. Put them in a sunny, exposed location that is at eye-level.
Check the traps daily until the first fruit fly is caught. A spraying routine should being one week after the first fruit fly is trapped.
Spray a spinosad insecticide, which is derived from a bacterial fermentation. It's the least harmful to animals. The solution will stay on the trees, treating the fruit flies, for five to seven days when sprayed at the highest rate. Apply it every seven days during the pest's active period.
Consider carbaryl-based insecticides. While the chemical treats fruit flies, it can cause spider mites to attack the tree over time. Colorado State University Cooperative Extension's website does not recommend repeated use but it's effective if rotated with other insecticides.
Target the pests in the week before harvest with a product containing malathion. This is the only time these insecticides are recommended. They only have a few days' residual, so reapply after about four days.
Kill fruit flies with a pyrethroid insecticides containing permethrin or esfenvalerate. Only a few of these products are meant for use on cherry trees, so check the label first. The solutions will give one week of residual activity. Spray the chemicals once a week while the cherries are expanding.
Climb a ladder to spray the entire cherry tree. To be effective, spray residual needs to cover the entire cherry. The concentration must be strong enough to kill adult insects before they lay eggs. Increase the amount of insecticide applied as the cherries get larger. The fruit tends to outgrow the chemical.