Jeffrey Hahn is an Extension entomologist and has some great information regarding apple maggot control. The apple maggot is the most destructive pest of apples grown in home gardens in Minnesota. Also known as the railroad worm, this insect is a type of fly. Commercial growers are usually successful in managing apple maggots, but the general public faces a more difficult task when attempting to protect only one or a few trees.
Apple maggots spend the winter in the ground as nonfeeding pupae. They begin to emerge from the soil as small (about 1/4 inch long) flies starting in July. You can identify an apple maggot fly from the characteristic black and white banding pattern on its wings and a conspicuous white spot on the body (figure 1). These flies do not all appear at the same time, but emerge at different times from early July until September. Peak emergence usually occurs from late July through early August.
Apple maggot flies tend to emerge from the soil soon after a moderate rainfall. Once they emerge, apple maggots stay in the general area. They fly from tree to tree, although they usually do not move more than 200 or 300 yards at a time. They can be found on apple tree foliage, fruit, or bark as well as nearby trees, shrubs, and weeds.
About seven to ten days after emergence, adult females start to lay eggs. They possess a sharp ovipositor (egg laying apparatus) that pierces the skin of the apple and allows eggs to be inserted into the apple flesh.
One egg is laid per site, although more than one egg is often laid in a single apple. In about five to 10 days, egg hatch into cream colored legless maggots that feed and tunnel in the flesh of the fruit, leaving brown "tracks" or trails.
The larvae live for three to four weeks. They usually finish their development once the apples fall to the ground. After the maggots mature, they exit the fruit and burrow several inches into the soil to pupate where they remain until the following summer. There is one generation a year, although a few pupae may remain in the soil for two winters.
Apple maggots cause two types of injury. The first injury is caused by oviposition that damages the area around the site where the eggs are laid. The flesh stops growing, resulting in a sunken, misshapen, dimpled area.
The second injury, which is the more severe of the two, occurs as the maggots tunnel through the flesh. As a result, the pulp breaks down, discolors, and starts to rot.
Sanitation is important in reducing apple maggot numbers. Pick up and dispose of apples properly within a few days after they have fallen to the ground. This reduces the number of overwintering pupae on your property. Bury the apples in the ground at least one foot deep. You can also use these apples for cooking or cider.
Although results may not be as good as insecticide sprays, you can manage adult apple maggots by capturing them with sticky red sphere traps (figure 4). These traps are readily available from gardening stores or gardening catalogs.
You can also make your own traps. Use a plastic or wooden ball about three inches in diameter (a little bigger than a real apple) and colored red or black. Attach a wire to the traps (e.g., using an eye screw). Coat the traps with a sticky substance such as Tangle Trap. These materials are available in one of three formulas: paste, brush on, or aerosol. You can purchase a feeding attractant to increase the number of apples maggots that go to the traps (if you buy a kit, many include the lure with it).
Hang one trap for approximately every 100 apples in your tree. That's about five traps for an average standard tree in Minnesota. Traps should be hung around July 1. When an insect lands on the sticky sphere, it becomes trapped and dies.
Apple maggot flies can be prevented from laying eggs in apples by placing a plastic bag over each apple (see figure 5). Many types of plastic bags will work. Place the bag over the apple before the apple maggots emerge, no later than July 1. Tie or staple the bag loosely around the stem. Cut off the bottom corners of the bag to allow moisture to drain out.
When properly timed and applied, insecticide sprays help manage apple maggots before the flies are able to lay eggs.
Once eggs are laid, there is no effective management. Sprays need to thoroughly cover all surfaces of fruit and leaves to be effective. Do not spray before July 1.
It is more difficult for home gardeners to find insecticides labeled for treating apple maggots. Diazinon and dimethoate are no longer available. Home gardeners can still buy carbaryl but it is available in fewer products. Also look for products containing esfenvalerate. However be careful of potential restrictions.
For more information regarding gardening, you can reach me at Stephanie@starpoint.net