Aphids are not always 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.