Jack pine budworm
Jack pines near Bemidji and in most of Beltrami County are beginning to show signs of budworm infestation. Unfortunately, most of the affected stands are more than 80 years old, are suffering the ill effects of a recent drought and are low vigor as evidenced by having only two years of needles on their shoots. This means that negative impacts from defoliation, topkill and mortality, are likely to occur in the next couple of years.
Jack pine forests occupy about 300,000 acres in northwestern Minnesota and are subject to periodic outbreaks of jack pine budworm, Choristoneura pinus, a native defoliating insect. Historically, outbreaks vary widely in size and severity but occur at ten to twelve year intervals and last two to four years. The damage produced by these outbreaks can range from moderate defoliation producing minor growth loss to severe, multi-year defoliation producing topkill and mortality.
Forest managers are again confronted with the practice of keeping mature and over-mature jack pine ?on the stump? in the hope of having the stand survive unscathed into extreme old age. First of all, population dynamics shows that only a few individuals will ever make and sustain old age. This requires a lot of foliage to produce enough energy to feed an increasingly large tree/biomass. Defoliators quickly take their toll on a tree?s energy reserve. At some point, there?s not enough energy to support the living biomass. Secondly, low vigor trees, no matter what their age, are insect and disease magnets. In jack pine, it?s part of the natural order; if fire doesn?t get them, budworm will.
Silvicultural goals for jack pine management involve reducing stand susceptibility to budworm defoliation and reducing stand vulnerability to budworm-caused mortality. Briefly, here are some concepts to help you think about managing jack pine in the western counties for the next few years. (Expect to hear more about this in the future.)
Susceptibility to defoliation can be reduced by minimizing pollen cone production since young caterpillars need pollen cones to get a good start on life.
Silvicultural management activities include:
- Remove old, wolfy jack pines from the stand.
- Eliminate two-storied stands (jack pine over jack pine).
- Eliminate stands that are less than 50 sq ft of BA, open and scrubby jack pine.
- Minimize edge and openings.
- Manage large blocks.
- Promote within-stand tree species diversity.
Once defoliated, jack pines are susceptible to topkill and mortality. These negative impacts can be prevented by maintaining vigor in stands reaching maturity.
- Control stocking. Maintain stocking at 70 to 110 sq ft of BA from age 25 until harvest.
- If stocking is more than 120 sq ft, thin down to 80 sq ft.
- Shorten rotation age. A good rule of thumb is to take the Site Index value, subtract 5 to give the rotation age.
What can be done in the midst of an outbreak?
- Do nothing and hope for the best.
- Spray insecticides on stands 25 to 35 years old to keep their foliage green until the outbreak passes.
- Salvage and pre-salvage older stands to take the best economic returns and recycle the jack pine stands.