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Growing Together: A healthy apple harvest starts with June care

Spraying to prevent damage from apple maggots, the region's most serious insect pest, should begin about June 20.Michael Vosburg / Forum Photo Editor1 / 4
Thin apple clusters to one or two fruits, spaced four to six inches apart. University of Minnesota Extension Service2 / 4
Don Kinzler, gardening columnist 3 / 4
Apple maggot damage. University of Minnesota Extension Service4 / 4

Many of us have heard the apple and the worm joke before, but old humor bears repeating for enjoyment by the next generation. What's worse than biting into an apple and seeing a worm? Biting into an apple and seeing half a worm.

Instead of leaving apple trees to their own devices until harvest, only to find worm-damaged fruit, we can mitigate problems starting in June. If you've experienced apple trees so laden with fruit you're afraid the branches will snap, the problem can also be remedied in June.

Apple maggot

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, 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 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, just before female flies are ready to lay eggs in small apple fruits. Repeat sprays every seven to 10 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, yet less toxic to humans and beneficial insects. Sevin can also be used, but can cause fruit drop if applied too early, and it's more dangerous to pollinators.

Adult fly activity can be monitored by hanging red apple-looking balls in the tree, covered with sticky substances like Tanglefoot. 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 any fruit that drops from the tree to prevent larvae from entering the soil through fallen fruit.

Thinning fruit

Apple trees can produce such heavy crops that branches are in danger of snapping. Mother Nature has given trees an instinctive remedy called "June drop" in which some excess fruit are shed in mid or late June.

Because nature's workings aren't always predictable, we can give a hand by thinning fruit ourselves, about the time of the natural June drop. When apples are dime-size, hand pick fruits from crowded clusters so there's only one or two apples per cluster, with an ideal spacing of four to six inches apart. This might seem wasteful, but the remaining apples, properly spaced, will be larger, sweeter, more flavorful and less likely to break the tree apart.

Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler's Greenhouse in Fargo. Readers can reach him at

He also blogs at " target="_blank">