The most common borers infesting ash are ash borer, lilac borer and carpenterworm. Ash borer bores into the trunk at or near the soil line causing tree dieback. Ash borer, bark beetles, and other wood boring beetles can be controlled with labeled pesticides applied to the trunk and branches in early to mid May or as the buds begin to break. Avoid spraying the leaves. Lilac borer causes swellings on the trunk and limbs where the insect enters the tree. The carpenterworm larvae bore into the heartwood but come to the outside of the tree to push out frass and sawdust. Heavily infested trees can be severely weakened. When few carpenterworms are encountered put fingernail polish remover containing ethyl acetate into the holes. Use one to two drops per hole then plug the hole with mud or putty. Keep trees as healthy as possible by fertilizing regularly and watering during dry weather.
Aphids can usually be controlled with sprays of Orthene.
In late summer, fall webworm covers branches with webbing. The nests in branches close to the ground can be pruned out when first noticed. Bacillus thuringiensis will control fall webworm.
The ash flower-gall looks like a disease but is actually a mite problem. The mites feed on the flowers causing abnormal growth. The galls dry out and persist on the tree into winter. Apply dormant oil sprays before bud break.
Scales can be controlled with sprays of dormant oil applied in the spring. Sevin, Orthene, or malathion may be used when crawlers are active. Identification of the scale is needed to control crawler sprays properly.