Why didn't the two worms go into Noah's Ark inside an apple? Because Noah told them they had to go in pairs.
Joking aside, worm damage to apples is no laughing matter. Have you ever sliced a homegrown apple and found brown streaks or lines winding through the flesh? The damage is caused by the wormlike apple maggot, the most common apple insect in the Upper Midwest.
Besides affecting the flesh, the apple’s exterior often becomes bumpy or pockmarked. Apple maggot damage also shortens the fruit’s storage life.
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, the apples are unappetizing for fresh eating.
The apple maggot is the larval stage of a 1/4-inch-long black fly that pierces the skin of developing apple fruit to deposit eggs in late June or early July. The eggs hatch into little wormlike larvae 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 spend the fall and winter, emerging the following summer as adult flies to start the cycle over again. Control measures for apple maggots must be in place before the adult flies emerge from the ground, which varies each year, depending on how warm the spring and early summer have been. On average, adult flies appear around July 1.
We can monitor whether the flies have become active by hanging traps, which are red apple-looking balls, in the tree. The round traps are available at some garden centers and online, or they can be made from red-painted balls, coated with sticky material like Tanglefoot. Or simply hang actual red apples from the tree, coated with the material. Average-sized trees should have five or six traps for best monitoring.
As soon as the described small flies begin appearing on the traps, begin control. To control apple maggots with insecticidal spray, begin a program between June 20 and July 1, before female flies begin laying eggs in small apple fruits.
Effective insecticides are carbaryl (original Sevin) and spinosad, which is also registered for organic use. Follow the product label and repeat as directed, which means you might need to spray for apple maggots three or four times during the summer. The number of apple maggot adults reduces as the season progresses, and you should be able to stop spraying sometime in August.
A novel idea for maggot control, tested by the University of Minnesota, excludes the egg-laying flies by enclosing each apple in a plastic sandwich bag, either a zipper closure bag or a plain bag closed with staples. Snip the bottom corners off each bag with a pair of scissors to leave a small opening for water to run out.
Bagging is easy to do if you have a small to medium-sized tree that can be managed from the ground or a short ladder. If you have a tall tree, you may choose to bag only the fruits that are easy to reach, and let the apple maggots have the fruit growing higher up.
Sanitation helps for future infestations. Promptly remove any fruit that drops from the tree to prevent larvae from entering the soil through fallen fruit.
Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at firstname.lastname@example.org or call 701-241-5707.