speculate about why the spruce budworm hasn been eliminated after 40 years of spraying
There are several factors that have allowed the spruce budworm to not only persist, but even to thrive in some areas, despite spray campaigns. One such factor is the budworm's great ability to disseminate. The spruce budworm can travel great distances as a winged adult, but the larvae also can travel, by spinning silken threads and catching the wind. Hence spraying is not effective unless very large areas are sprayed simultaneously, which is difficult to coordinate.
In the last century, logging practices and forest fire suppression have changed the distribution of spruces. In the primeval forests of North America, spruce was mainly limited to cooler, damper areas, and pine and hemlock occupied the areas between spruce stands, which naturally limited outbreaks of budworm. Currently spruce are growing up in areas formerly limited to other species, creating larger contiguous areas of spruce which allow the budworm to get a good foothold.
Current research is looking at integrated pest management techniques which will help to limit the need for spraying, which has negative impacts on entire ecosystems when widely used.