Jack pine budworm on red pine!
This summer, DNR staff discovered foliar damage in a 147-acre red pine plantation of the Sand Dunes State Forest in Sherburne County. The plantation is more than 40 years old and has been thinned. When the site was inspected this summer, both red pine shoot moth and jack pine budworm (JPBW) were found damaging the stand. JPBW had defoliated 79 acres, some of which was already showing top kill. There are only a few scattered jack pines in the area and, for the most part, they looked better than the red pine.
In the first year of defoliation, damage ranged from 30-90% in the core area. It is fairly unusual to see extensive defoliation of red pine by JPBW, particularly when there is little if any jack pine in the area. (The nearest noticeable JPBW defoliation of jack pine this year was in northern Todd Co.) So how this infestation became established is not known. It is also not known what factors contributed to the heavy defoliation.
The only other records of JPBW on red pine in Minnesota that we could find were in 1956 and 1957 near Cloquet, Pequot Lake and Bemidji. Top kill occurred in 1957 after 80% defoliation. However, in those events, defoliation occurred over two years and the adjacent jack pine was also heavily damaged.
In the Sand Dune's case, the JPBW population is relatively isolated and apparently exploded in one year's time. That leaves a number of management questions open to discussion. Can JPBW thrive on red pine alone and then survive the winter? Given the level of defoliation already, will there be enough foliage to support another year of defoliation on the same trees? What is the impact of the red pine shoot moth? How did JPBW get here and can they spread to the adjacent red pine stands, of which there many. Has there stress been sufficient to initiate a bark beetle attack?
At this point, the most reasonable options include harvesting the red pines to avoid a bark beetle infestation and to avoid further spread of the JPBW; spraying the stand with Bt to control JPBW and hoping the stand recovers before bark beetles move in; or taking a wait and see approach, hoping the JPBW doesn't survive the winter, which is a relatively common occurrence in jack pine. JPBW overwinters in the 2nd instar larval stage. When they first emerge in the spring, they begin to feed on the new needles or on the staminate cones. If the young larvae were less than vigorous going into the winter due to improper nutrition, or if there isn?t enough cones or young needles to feed on in the spring, the larvae are not able to survive to complete their life cycle. If either occurs, a second year of defoliation is unlikely. But there isn?t any way to know that until next spring and then there isn?t enough time to act before July moth flight.
To help determine the likelihood of defoliation next year, an eggmass survey will take place this month. The extent of the population and the likelihood of it moving to adjacent stands will also be assessed.