Biological pest control means using mother nature to put pressure on pests. Most biological controls will not eradicate the pest, but will play a part in reducing the pest. Timing is very important in choosing which biological control to use. Different organisms will attack pests at different stages of the life cycle. So you will want to monitor your trees for the pest you want to control. Let's take codling moth for example. Use monitoring traps to know when the first flight of adult moths emerge (biofix). Now you can calculate when they will mate and lay eggs. At that point you can time release of predators or parasites of moth eggs. At about 250 degree days from biofix, the eggs will start hatching and you can use biologicals that are toxic to the larvae. When the larvae leave the apple to pupate, you can use cardboard bands to remove the pupae. None of the biological controls will completely eradicate the codling moth, and in fact, most of these controls would need to be used together with other sprays to get good control of this pest.
Lacewing nymphs are voracious eaters. They will eat aphids and eggs of moths, etc. While you won't be able to get a lot of biological control of codling moths with these creatures, at least they will be helping you with other pests as well. In the early spring you can release them and even release another batch a little later. If your apple leaves are curled, or if you see ants marching up the tree, you undoubtedly have aphids, so the lacewings will be busy with that. Later they might eat some codling moth eggs for desert. It is helpful to have some nectar plants on the ground around your apple tree to give the adults something to eat. That would be plants like alyssum, queen ann's lace, etc. Think tiny flowers.
These little guys lay their eggs in codling moth eggs. Then a wasp hatches instead of the codling moth. The wasps are extremely tiny. They, too need a nectar source. If you don't have a nice bunch of tiny flowers below your tree, then you can buy a food source to spray on your tree. You can buy your wasps just after biofix and they will infect some of the eggs the moths lay.
Nematodes - specifically Steinernema feltiae and Steinernema Carpocapsae be sprayed on the ground, the tree bark and around any other cracks and crevices where moth larvae might hide in the summer and fall to attack the overwintering larvae. This can be done up to October if the weather is still warm, as these nematodes are sensitive to colder temperatures. It is a good idea to give the apple tree a fine mist of water for several hours after applying the nematodes to improve viability of the nematode. They can significantly reduce the number of moths to appear in the following spring.
Bacillus thuringiensis (Bt) is a bacterium that can infect moth larvae when they eat it. Here's the trick. It is hard to get the larvae to eat the bacteria and the window of time to get them infected is quite small, since the larvae are making a bee line to get inside the apple. It might help to add some molasses when you spray on the Bt. And you would probably need to spray the tree many times to make sure the Bt is there when the moth larvae first hatch out.
This virus infects codling moth larvae also. As with Bt, it has to be eaten in order to be infective. It needs to be sprayed on the trees just prior to moth larvae hatching, as the larvae are only outside of the apple for a few days. This spray needs to be applied frequently during the hatching period, as it does not last very long after application. Fewer particles of virus need to be ingested to cause the larva to die, so it will be a more effective biological control than Bt.
There are many insect-eating birds that could be an asset to your apple orchard. Bluebirds, tree swallows, house wrens, chickadees are great. I keep nest boxes for swallows. There are quite a few different birds that could be encouraged to eat codling moths. It is great to encourage insect-eating birds with nest boxes. Just make sure you don't invite birds that also eat apples.
Bats fly at night, and that is also when the codling moth adults are flying to find a mate. One night I was out in the hot tub on a moonlit night, I could see the bats darting all over my apple trees. It is a good bet that they were making moths their main meal. Bats can be encouraged by putting bat houses up high and in a southeast exposure to give them plenty of warmth during the day when they are sleeping.
Biological control of apple tree pests will not take care of the problem completely, but will definitely add their share of help against many pests. You should think about using many of the different ones together rather than relying on one only. Go from Biological Controls back to the home page.