There's only one thing worse than biting into an apple and finding a little green worm, and that's finding half a worm. Green worms can frequently be found on apple trees feeding on leaves. Some types of green worms will also eat the fruit, boring into the flesh of an apple. They are a pest and, in sufficient numbers, can produce significant damage to the tree itself and its resulting crop of apples.
The 'worms' that live on fruit trees, including apple trees, are not worms at all. In fact, they are the larval stage of different species of moths or flies. Females will lay their eggs on an apple tree as a source of food for their offspring. When the eggs hatch, the worms will feed on the leaves, and some species will also eat the fruit of the plant until the time comes for them to change into adult insects.
Many types of green worms affect apple trees. These include several species of moth larvae called leafrollers. Another type of moth that produces a green worm that infests apples is the green fruitworm. At least 10 different species of green fruitworm have been identified. Many species of Geometrid moths also produce green larvae that are called inchworms. They have a characteristic 'inching' movement and enjoy feeding on apple trees. Saw flies also produce green worms that will feed on apple trees. The California pear saw fly and the Dock saw fly are examples.
Leafrollers feed on the leaves of the tree, causing the leaves to roll up from the damage. This rolled leaf will often be used as a home for the worm, held together with silk. Geometrid inchworms also feed on the leaves of apple trees. Green fruitworms attack both the leaves and fruit of apple trees, and can cause damage to crops. Saw fly green worms will feed on the leaves and fruit of apple trees, often boring several different holes into the same piece of fruit.
Insecticidal oils can be sprayed on trees to control soft-bodied insects, including green worms. These should be applied to the trees during dormancy, just before new buds open. In severe infestations, insecticides, such as carbaryl (Sevin) and diazanon, can be used to control green worms. This should be a last resort tactic, however, as these treatments are non-selective and will also kill many beneficial insects and may effect other wildlife.
Keeping weeds under control to limit the overwintering food sources and breeding areas for insects will reduce the risk of infestation. If necessary, early fruit harvesting may help to control further damage. Beneficial predatory insects, such as Trichogramma flies, lacewings and ladybugs, can also help to reduce green worm numbers. Spraying with the bacteria Bacillus thuringiensis, which infects the worms and kills them, can also be used as a biological control, particularly where organic growing practices are in place.