Apple maggot fly

The apple maggot fly (Rhagoletis pomonella) is known for laying its egg inside immature apples, but occasionally infests crabapple, pear and plum and saskatoon. It is considered a nuisance in hawthorn, mountain ash and cotoneaster. The egg hatches into a small, cream-coloured larvae which creates irregular brown tunnels inside the fruit. Sometimes the skin of the apple has small depressions or dimples. Infected fruit may fall from the tree prematurely.

Apple maggot fly looks similar to a house fly but is slightly smaller at 6 – 9 mm. It has a black body with a cream or white bands on its abdomen. Its wings have irregular black and clear markings. There is one generation per year. 

Life cycle:

The life cycle begins in spring when the fly emerges in late May or early June. The fly lays an egg under the skin of immature apples. The egg hatches and the larva eat into the apple causing the brown tunnels. When the apples fall, the larva exit the fruit and burrow into the soil beneath the tree to pupate. The pupa remains in the soil all winter, emerging as a fly in spring to begin the cycle again. 

Control:

These flies are very mobile and can infest trees in an entire neighbourhood, however, there are things you can do to break the cycle and reduce the damage.  It’s best to use more than one technique to control the flies.

Bagging immature apples (highly effective, some labour required)

• Bagging apples is a technique developed in western Minnesota and is growing in popularity by commercial apple producers. This is highly effective provided the apples are bagged at the correct time.
• Bag fruits when they are about 1.5 – 2 cm in diameter, usually a week or two after the petals fall. Thin fruits to one fruit per cluster.
• Simply place a small bag around immature fruit and secure the opening. It does not take long to bag fruit once you get the hang of it.
  • Typically, a plastic sandwich bag with a zipped closure is used. Before you start bagging, snip a small hole in the bottom corner of each bag to allow excess moisture to escape.
  • Other options: small nylon gauze bags (from the dollar store), small paper bags, wax paper bags or commercially available Japanese apple bags.
• Remove the bags about 3 weeks before harvest to allow the fruit to colour properly.
• Bagging is more practical for smaller apple trees. If your apple tree is large, consider bagging a portion of the tree to ensure at least a portion of the crop is unaffected.

 Insect tree covers (highly effective)

• Specially designed covers are available in a range of sizes that completely cover the tree with insect netting. These should be applied as soon as the tree has finished flowering – any earlier and pollinators such as bees will not have access to the flowers. If it is properly fitted, the net will exclude all of the flies.

Tarping the ground to prevent pupae from overwintering in soil (highly effective to prevent pupa from over-wintering)

• As fruit starts to ripen, place a tarp beneath the apple tree when the fruit is ripening to catch all of the falling apples to prevent the larvae from burrowing into the soil to overwinter. Keep an eye out for premature fruit drop in mid to late summer and collect the fruit as soon as possible. Put spoiled fruit in the garbage, not your composter.

 Sticky traps (somewhat effective, good for scouting for the fly)

• Yellow sticky traps are available in a large size for this purpose. Hang traps just after the tree has finished flowering and the fruit starts to develop. Use 3 to 6 traps (depending on the size of the tree) throughout the tree near the edge of the canopy. The University of Saskatchewan research plots found that yellow sticky traps were more effective than red ball traps in trapping the flies.
• Red ball traps resembling apples to catch flies. The balls are coated with TanglefootÓ, a very sticky substance that traps the flies. Re-apply TanglefootÓ, as needed. Use 3 to 10 traps (depending on the size of the tree) throughout the tree near the edge of the canopy.
• Traps in general will not completely eliminate the problem but are useful for scouting for the flies.

 Kaolin clay (moderately effective but labour intensive)

• Kaolin clay is not a true pesticide. It is natural mineral clay ingredient used in toothpaste. When this powder has been mixed with water and sprayed on fruit, it creates a barrier that deters flies from laying eggs in the fruit. The entire fruit must be coated to be most effective and must be re-applied regularly (every 7 – 10 days), and after every rain event. Surround WPÓ is the recommended product as it is very finely ground clay, made for this purpose. However, it is difficult to source in Canada in smaller quantities suitable for the home gardener.

 Disposing of infested fruit

• Do not compost infested fruit as the larvae can move into the soil near the composter to over-winter as larvae. You can, however, crush the infested fruit and place it in a black garbage bag. Leave the bag in hot sun for a week to kill the larvae. The mash can then be placed in the compost or buried provided that all the larvae are dead.

Fruit that has been infested with the larvae is very unappetizing and not suitable for fresh eating or pies. However, gardeners report that they use the spoiled fruit (cut up with the larvae gone) for making cooked apple products or cider. If you do, ensure that there is no rot or secondary bacterial infection inside the fruit.

Sources:

Williams, S.; Skinner, H. (2011). Gardening, naturally: A chemical-free handbook for the Prairies. Regina, Saskatchewan: Coteau Books.

Bush, M. R., Klaus, M., Antonelli, A., Daniels, C. (2002). Protecting backyard apple trees from apple maggot. Washington State University, Extension Bulletin #1928.

Apple maggots in home orchards. (2018). Retrieved March 27, 2021, from https://extension.umn.edu/yard-and-garden-insects/apple-maggot