Answer: It sounds suspiciously like ash tree borers. A serious infestation can kill the tree. Borers within trees cannot be successfully treated with insecticides because the insects are in a protected site. Insecticidal control is best achieved if sprays are made during periods of adult activity and egg laying.
Trunk sprays can kill the adult borers during egg laying and also may be effective against newly emerged larvae before they enter the trunk. Canopy spraying also may increase control of longhorned beetles and metallic wood borers. Adults often feed on twigs and foliage before laying eggs.
Timing of trunk sprays varies with different borer species. The general time of adult emergence, associated with egg laying, is summarized in Table 1. insecticide applications typically persist for about one to two months and may need to be reapplied if egg laying occurs over a long period.
Without knowing for sure if the culprits are ash tree borers, you may need to find and capture an adult and take it into the cooperative extension office for positive identification. Helpful folks there can recommend control treatments.
Good luck with your ash tree!
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