A reader wrote to us about some white worms he discovered living inside the trunks of some of his red delicious apple trees. He explained that the trees were not leafing out, and suspected that the worms might be the problem. The reader also observed that the worms appeared to start at or near the top of the tree and work their way down. The reader hoped we could help him figure out which type of worm this is, and how to protect his other apple trees.
First, we must clarify that the pests attacking this man’s apple trees are not technically “worms”. Although these creatures look like worms, they are actually caterpillars – which are the larval form of moths and butterflies – or a different type of larva.
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There are two common pests that infect apple trees. One is known as the apple maggot. The apple maggot larvae is what actually causes harm to apples, and it is a white or yellow color and about a quarter of an inch in length. The larvae are deposited into the apple by an adult female fly, and then live off the fruit for 3-4 weeks. If an apple has been damaged by apple maggots it will appear dented and sunken, and tunnels might be visible through the skin of the apple.
The other pest that is known to damage apples is the codling moth. Their larvae are pinkish-white and can be up to 1 inch in length. Like apple maggots, codling moth larvae feed on apples by tunneling through. However, adult codling moths often deposit their larvae on the leaves or twigs of the trees, not directly into the fruit. For this reason, a tree that has been infected with codling moths might be easier to identify by the crevices in the bark resulting from the moths tunneling through the trunk.
Since the reader didn’t provide a picture or detailed description of the pests he found, it is difficult to rule out either of these creatures. Since he found worms inside the trunk of the tree, and not just inside the apples, we believe he is dealing with codling moths. However, without further information it is impossible to completely rule out the apple maggot.
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A great option for controlling codling moths is to plant butterfly weed (Ascepias tuberosa), which attracts the parasites of codling moths, near the apple trees.
Another product that will work on both apple maggots and codling moths is liquid kaolin clay. You can spray it on the fruit to annoy and repel the pests, and can easily be washed off after harvest.
To sum up, two of the most common pests that affect apple trees are apple maggots and codling moths, so it is very likely that this is what our reader is dealing with.