Aphids can be controlled with sprays of Orthene, or malathion. Aphids are not always obvious. Sometimes the insects are not seen but the distorted growth, honeydew on the leaves, and sooty mold growing on the honeydew are obvious.
Borers attacks may be prevented if the trees are kept in good vigor with regular fertilization.
Leaf miners symptoms are brown blotches on the leaves. Use sprays of Diazinon or Orthene applied when the leaves are just fully expanded or when the blisters first appear.
Lace bugs can be a serious, though occasional, problem. The insect feeding on the undersides of the leaves causes chlorotic flecks on the upper leaf surfaces. The lower sides of the leaves are covered with brown sticky flecks. Use sprays of rotenone or Orthene.
The pear slug skeletonizes hawthorn leaves and these sawfly larvae have a slimy appearance. Use Sevin when the insects are first seen. A few insects can be washed off with a garden hose.
Tent caterpillar nests can be pruned out while still small. Sprays of Bacillus thuringiensis may be used as can Orthene, malathion, or rotenone. Do not burn nests while the nests are in the tree. The injury from the fire may exceed that caused by the insects.
Overwintering scales may be controlled with dormant oil sprays. Control crawlers with Diazinon, malathion, Sevin, or Orthene. To time crawler sprays, identify the specific scale insect.
Spider mites are so small they can cause much foliage discoloration before being detected.