Fig. 15. Apple maggot adult
University of Minnesota.
Fig. 16. Apple maggot external injury
Photo: Dept. of Entomology,
University of Minnesota.
Fig. 17. Apple maggot internal injury
Photo: Dept. of Entomology,
University of Minnesota.
Fig. 18. Bagging apples to protect against apple maggot
Photo: Larry Zilliox.
The most important insect pest of Minnesota-grown apples is the apple maggot, sometimes called the "railroad worm.” It is very common, infesting apples in all parts of the state. Heavily infested fruit is inedible and is suitable only for cider or animal feed. It is very important to manage this pest to protect your apples.
Fruit infested with apple maggot may be pitted and misshapen due to egg-laying. Each "sting” or hole created by the female fly as she lays an egg, forms a tiny spot or dimple. Inside the apple, small white larvae (maggots) feed on apple flesh, creating small tunnels which turn brown and rot. The adult fly is ¼ Inch long, smaller than a common housefly, with characteristic dark markings on the clear wings, a conspicuous white spot where the thorax joins the abdomen, and three (male) or four (female) white stripes on the abdomen.
Adult apple maggots begin to emerge from the soil starting about July 1, continuing through most of the summer. After emergence, apple maggot adults often leave and feed outside the orchard, in wooded or brushy areas, then return to lay eggs. They lay eggs just under the skin of apples with each female fly laying hundreds of eggs. Once eggs hatch, larvae feed for three to four weeks. When apples drop to the ground, the larvae enter the soil to pupate. Pupae overwinter underground, emerging as adults the following summer.
Sanitation can help reduce apple maggot populations. Frequently pick up and remove any apples that fall during the growing season and after harvest. Place these apples in the trash or send them to a municipal composting site. Do not compost them in your yard.
However, apple maggot flies can enter the planting from outside and you cannot rely on sanitation alone to prevent apple maggot infestations. There are three methods of managing apple maggots in Minnesota home plantings: bagging the fruit, trapping out, and monitoring and spraying.
This method was developed in western Minnesota. After thinning the fruit, in early to mid-June, enclose each apple in a plastic sandwich bag, either a zipper closure bag or a plain bag closed with staples. Using a pair of scissors, snip the bottom corners off each bag, leaving a small opening for water to run out. At harvest, remove the bag. Although bagging fruit may take a few hours, the apples are protected from apple maggots for the rest of the season.
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. Or, 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 higher fruits.
The trap-out method uses sticky traps to capture apple maggot females that attempt to lay eggs on the fruit. Apple maggot traps are red spheres coated with tanglefoot, a sticky substance that adheres to almost any surface and permanently holds insects. The flies are attracted to the red color of the spheres, land on them, and are stuck.
You can use wooden or plastic spheres painted red, red plastic spheres designed and sold specifically for this purpose, or even store-bought large, red, fresh apples. Whichever type of trap you choose, the spheres or apples should be at least 3 inches in diameter and bright red.
Wooden spheres can be hung using an eye screw and a wire hook, while fresh apples may be skewered with a piece of stiff wire such as a coat hanger, and the excess wire bent into a hook. Although wooden balls and red plastic spheres sold as traps are convenient to hang, many people dislike cleaning them for storage, yet don’t like the idea of throwing them away and purchasing new ones each year. Store-bought apples can be coated with tanglefoot and composted at the end of the season.
A variant of the red sphere is the Ladd trap, a red sphere/yellow rectangle combination that may be more effective in catching apple maggot flies. It may be worthwhile to use the Ladd trap, although they are more difficult to clean. You may also choose to purchase scent lures to hang with your traps to increase their attractiveness to flies. The lures contain volatile chemicals that apple maggot flies perceive as the scent of apple fruit.
Hang one trap per 100 fruit (after thinning) in each tree. In small trees this will mean only one or two spheres. Make sure one trap is on the side of the tree facing any wooded or brushy area. A second trap should hang on the south side of the tree. In larger trees, you could have five or more traps distributed around the outside of the tree. If there are apple or hawthorn trees in wooded or landscaped areas nearby, you may want to hang traps in these trees as well, to further reduce apple maggot pressure.
Hang traps in the trees by the end of June, to catch the apple maggot flies as they first attempt to lay eggs. Remove any leaves or fruit touching the traps. Check all the traps weekly and, as needed, clean off the tanglefoot-coated insects, and/or apply more tanglefoot.
Monitor and spray
You can use a sticky red sphere trap as a way of monitoring apple maggot presence in your orchard, so that you can spray an insecticide at the right time to control the flies. In a small planting, you should hang one trap at eye level on the outside of the tree canopy, facing any wooded area nearby, or facing south. Many other insects will also get stuck, some of them small flies with similar wing markings, so learning to positively identify the apple maggot fly is very important.
Check the trap frequently. If you do not use a scent lure, spray for apple maggots as soon as you catch a single adult. If you hang a trap with a lure, wait until you have caught a total of five flies, whether they are all caught at once or one at a time.
Effective insecticides available for apple maggot control are esfenvalerate, carbaryl, and spinosad.
Check the pesticide label for the spray interval, typically given as a range of time, such as "7 to 10 days.” Do not spray any more often than this. At the end of that interval, clean any insects off the trap, reapply tanglefoot if needed, and begin checking the traps every day or two as before. When the one-fly threshold (without lure) or five-fly threshold (with lure) is reached, spray again, and repeat the process of cleaning and checking traps. You will probably spray for apple maggot three or four times per season.
It’s important to observe the pre-harvest interval, or the "days to harvest,” i.e. the interval between pesticide application and harvest. Any fruit picked sooner than this should not be eaten and should be discarded. In some cases this interval could be as long as three weeks. Read the label of the particular insecticide you are using for this information. Fortunately, apple maggot adults are found in smaller numbers as the season progresses, and you should be able to stop spraying sometime in August.
Another option: kaolin clay
Kaolin clay, sold as "Surround at Home,” can discourage apple maggot flies and other insects from laying eggs in apple plantings. It is applied in a visible layer to all surfaces of the tree, leaves, and fruit, and acts as a visual and physical repellant to insects. Apple maggot females are attracted to the red color of ripening apples, so a grayish-white apple is not attractive to them. Further, the sensation or chemical signal of the layer of clay particles is repellant to many insects, so that although they may land on the fruit, they immediately leave.
However, it’s difficult to achieve and maintain the excellent coverage necessary in a climate like Minnesota’s, where heavy rains can occur throughout the growing season. The clay washes off easily even in moderate rain. As fruits expand, coverage must be renewed frequently. A very well maintained coating of Surround can protect 90% of fruit from apple maggots, although effectiveness may be only as high as 30% of the apples. In addition, many home apple plantings are part of an ornamental landscape, and trees covered with the white clay are aesthetically unappealing.
See Appendix: sources of IPM supplies, for information about where to buy trapping materials.
The information given in this publication is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by University of Minnesota Extension is implied.