Spraying Apple Trees for Maggot Control

Have you ever sliced a homegrown apple and found narrow brown streaks or lines winding through the flesh? The damage is caused by the apple maggot, the most common apple insect in the Upper Midwest.

The culprit is rarely seen, because by harvest the small maggots have usually exited the fruit. Although damaged fruit are safe to eat and commonly used for cider and sauce, the apples are unappetizing for fresh eating.

The apple maggot is the larval stage of a small black fly, one-fourth inch long, that pierces the skin of developing apple fruit to deposit eggs in late June or early July. The eggs hatch into little worm-like larvae, maggots, that tunnel internally in apple fruit, feeding through the summer. The discolored, winding steaks have earned the insect the nickname “railroad worm.” By late summer the larvae exit the fruit and drop to the ground where they overwinter, emerging the following spring as adult flies to start the cycle over again.

To control apple maggots, begin a spray program about June 20 to 27 , just before female flies are ready to lay eggs in small apple fruits. (Emergence of the adult flies is dependent on the season’s weather to date.)  Repeat sprays every seven to ten days, or as directed on product label, until August when flies stop laying eggs. The relatively new insecticide spinosad is increasingly recommended by university Extension Services because it’s effective, and approved for organic uses. Malathion and Carbaryl (Sevin) are additional choices, and it’s wise to rotate sprays between the three to avoid the insects becoming resistant. Always follow label instructions.

When spraying trees with insecticides for apple maggot control, it is important to thoroughly wet all leaves and developing fruit with the product. 

Adult fly activity can be monitored by hanging red apple-looking balls in the tree, covered with sticky substances like Tanglefoot. Hanging 6 or more per tree can help reduce populations, but won’t likely eliminate the problem entirely. Such traps can be ordered online or found in some garden centers.

A very novel idea tested at the University of Minnesota involves enclosing each developing apple in a plastic sandwich bag to exclude the egg-laying flies.

Sanitation helps greatly. Promptly remove all fruit that drops from the tree to prevent larvae from entering the soil through fallen fruit and living through winter to re-infest the apple tree next year.