During this period the caterpillars eject a great deal of coarse frass from the burrows which may cling ribbon-like on the outside of the trunk or accumulate like sawdust around the base of the trees. The caterpillars also return at intervals to the outer sapwood to feed. Often they emerge from the burrows to crawl about on the bark, and then re-enter the burrows. in May of the fourth or fifth year, depending on the length of time required for larval development, the caterpillars retreat to the upper parts of the burrows where, in specially prepared chambers, they transform into pupae. The period of pupal transtormation is relatively short, as the adults begin to appear in June. Before changing to the adult form, the pupae propel themselves downward by means of the backward-projecting spines on their bodies to the burrow openings so that the moths upon emerging from their pupal skins escape directly to the outside.
Woodpeckers dig out and feed on larvae in the wood.
Moths are killed, newly-hatched caterpillars are eaten or lost before they can enter the trees and older caterpillars which emerge from the burrows and crawl about on the bark are destroyed. Parasites, disease, weather and the vigor of the host trees during the long period of caterpillar development also have controlling effects. These natural factors cause the fluctuations in borer abundance which occur in the absence of applied control. Despite the ameliorating effects of natural influences, applied control of the ash borer and the carpenterworm is usually desirable because of the importance of the trees they infest and the destructive capacities of the pests. The age, size and number of infested trees, the severity and extent of infestation, the value of the plantings and the availability of equipment and labour will determine the means used.