A pest is different to a disease! Apple tree pests are living insects, whereas disease of apple trees are normally fungus types. At one time - in the UK - it was possible to use a winter tar wash on trees, and this killed the overwintering apple tree pests for the following growing season. Tar oil wash has now been banned, though it is still possible to get other types of winter wash treatment for apple tree pests.
It is absolutely essential that you identify firstly whether it is a pest or disease causing problems, and then apply the correct treatment.
Generally, there are only two or three pests that cause problems when growing apple or other top fruit trees - mainly worms or maggots in the apple fruit!
The main thing, is to get the adult pest, before it lays its eggs, which then become maggots!
Codling Moth Larvae is probably the worst offender, but can be - and should be - controlled by several different means. The Codling Moth larvae - maggot - is the normal 'worm' found inside apple fruits. Treatment of Codling Moths. The main problem with Codling Moth, is that it can have three generations in a year! Each generation needs treatment!
Apple Maggot Flies are not a problem in the UK. The problem visualises with ripe fruit, where the maggot is found inside. Typical identification, is the 'railway track' tunnels it leaves in the fruit on its way to the centre of the apple.
The wasp in the image is having a meal with fallen fruit. Generally wasps are only attracted to ripe fruit as far as apples are concerned. Plums are another problem!
Fruit Slugworms. These are little slug-like pests of apple
trees and can be dealt with
Spider Mites. Not too much of a problem, and is normally identified with white speck damage on the upper surface of the apple leaf. The Red Spider mite is under the leaf, sucking away and causing the damage that you first see. Sometimes, you will also see some of the leaves being deformed with a fine web. Treatment here.
Scale Insects are an occasional pest in home grown apple trees. Slow to take hold, but difficult to see unless actually looking for it! It is a small brown, scale-like insect on the bark of the apple tree. Usually on younger growth. Treat as here
Aphids - Greenfly and Blackfly - also Woolly Aphid! Quick to take hold, but relatively easy to deal with. Woolly Aphids in particular need special attention, for they can cause Apple canker by damaging the stem tissue. Blackfly and Greenfly Control
Apple Maggot - Also known as the railroad worm because of the tracks it leaves in your apples, the Apple Maggot causes pitting and dimpling in your fruit. Trap adult flies on red sticky balls by hanging them in your tree after petal fall. Another way to reduce apple maggots is to promptly remove any fallen fruit.