Apple trees are a favourite with home gardeners. Desirable for their attractive, creamy-white spring blossoms and crisp, tasty fruit, apple trees are rewarding to grow but can suffer from a number of diseases and insect pests. Home gardeners must take steps to prevent insect pests from taking up residence on their apple trees. Be vigilant in recognising the signs of insect pest infestations so that they can be treated immediately.
Apple trees infested with insect pests can exhibit a wide variety of symptoms, from chewed-upon or curled-up leaves, to clusters of eggs on the leaves. One common symptom is large amounts of honeydew, a sticky substance excreted by sucking insects such as aphids. Often, the honeydew attracts ants, so a large amount of ants climbing up and on your apple tree can be another symptom of an infestation. Brown or black spots that appear on the flowers, leaves and developing apples are another obvious sign of an insect problem, although this can also signify a fungal disease.
Serious Insect Pests
The coddling moth is the most serious insect pest of apple trees, according to W.S. Cranshaw, a horticulturist with Colorado State University. This moth emerges in early spring to lay eggs on the leaves. The eggs hatch later in the summer, and the larvae feed on the leaves. The larvae will then burrow into the young apples, feeding on the core. Once they have matured, they exit the apples and crawl down the trunk. A second generation of moths lay the eggs directly on the fruit in late summer, allowing the newly hatched larvae to enter the apple and begin consuming it immediately. In these cases, most of the fruit on the tree will be rendered inedible.
Apple maggots are also serious pests, as they too burrow into the fruit. They are not nearly as widespread as the coddling moth, however, and there is only one generation of these pests each year.
Slightly Injurous Insect Pests
Most insect pests that infest apple trees are not seriously threatening to the tree. The majority suck the sap of the wood, such as scale, or the plant juices of the leaves, such as aphids. Look for scale to cluster in the angles of branches, and aphids to hide under the tree's leaves. The redbanded leafroller is a moth that feeds only on the leaves of the tree, causing them to curl up at the edges. Spider mites are serious insect pests in apple orchards, but rarely affect backyard apple trees, according to Cranshaw.
One of the best ways to prevent insect pests from harming your tree is to make sure you plant sturdy, disease-resistant stock and provide environmental conditions that will ensure the development of a vigorous, healthy tree. Moist, well-draining and fertile soil will encourage strong growth, as does plenty of space around the tree for air to circulate. Very healthy trees will resist insect damage and tolerate insecticides much better than apple trees weakened by fungal disease or poor environmental conditions.
Insecticides are the most effective way to treat insect infestations, according to Cranshaw. In some cases, such as with the coddling moth, applications of the insecticide must continue throughout the growing season, save when the tree is blooming. Apply the first dose of insecticide to the tree after petal drop and repeat it every other week until fall for coddling moth control. Horticultural oil sprays, applied to the apple tree while it is still dormant in the spring, are effective for controlling minor insect pests.