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Spruce budworm: low egg mass counts

In Region 1, spruce budworm egg mass surveys, in white spruce stands primarily 25-35 years old, were completed on August 27th. Defoliation was determined to be more severe in 1998 than it was in 1997. At least nine sites are continually sampled for early larval and egg mass surveys each year. Although defoliation was higher in 1998, egg mass counts were considerably less this year. The exception was one white spruce plantation near Cass Lake in S 9 -T145- R31 which produced 2.3 EM per branch sample indicating it could have heavy to severe defoliation in 1999. The other eight plots sampled ranged from 0.0 to 1.33 EM per branch with an average of 0.5 EM per branch per plot. This indicates the possibility of light to moderate feeding in 1999.

Summer surveys of spruce budworm egg masses in Region 3 indicated that there will be fewer budworms and less defoliation by this forest insect in l999. At ten different sites where egg mass surveys had been completed in l997 and in l998, there were less egg masses in l998. Even where there had been heavy defoliation this spring, there were less egg masses, indicating that most budworms had not developed into fertile moths. There is some evidence in the literature to suggest that budworms find it difficult to feed on "hardened off" new needles and thus fewer caterpillars make it to the adult stage. Since our early spring and warm weather of l998 probably allowed spruce needles to reach this "hardened off" condition at earlier dates, fewer budworms may have been able to reach their fertile moth stages. Spring larval surveys in l999 may support this hypothesis.

Ideally, spruce stands having mortality and top kill should be harvested while the wood has some economic value instead of stagnating on the stump producing mostly branch growth.

Spruce Budworm (w/tree)

Spruce budworm induces epicormics

Nature is fascinating! Did you know that all trees have "back-up" systems for replacing lost foliage and branches? Epicormic shoots originate from preformed, suppressed buds that are carried along as the tree grows and, once stimulated by tree stress or changing tree conditions, are induced to grow and develop. Here's where spruce budworm comes in for balsam fir and white spruce trees. Spruce budworm stresses a host tree by causing defoliation and that induces epicormic buds to break and epicormic shoots to elongate in order to replace the lost foliage. Replacing foliage consumed by budworm is a high priority for the survival of the host trees.

What makes this story really interesting is the fact that balsam firs and white spruces have very different strategies for dealing with defoliation by altering the timing of epicormic shoot formation. And, it is based on whether needles alone or needles and buds are consumed by the budworms.

First, the host trees' similarities: When budworms are at high densities, they consume needles and buds on both host species. Both balsam firs and white spruces compensate for the loss of this foliage with prolific epicormic shoot production. This makes a single twig tip look tufted because as many as twelve epicormic shoots are present. After a few years of severe defoliation, the branches have had multiple episodes of epicormic proliferation and you can find hundreds and hundreds of epicormic shoots on them.

Now for their differences: When budworms are at low densities, defoliation is much less and does not include bud destruction; only needles are consumed. The response of the two host species differs dramatically. When only needles are consumed, white spruces form epicormic branches and balsam firs do not. So at low budworm densities, white spruces produce epicormics to compensate for the lost foliage and are able to generate and store energy. White spruces are extremely sensitive to needle loss. In fact, if about 50% of the current year's needles are lost, there is a 40% increase in the amount of epicormics the next year.

This is what we've observed in Minnesota over the decade. Spruce budworm populations have spread into white spruce plantations as far south as the Twin Cities and as far west as Park Rapids. After just a few years of defoliation, white spruce branches begin to have prolific epicormic shoot production. In balsam fire, our experience has shown that excessive tufting of branches is the beginning of the end for those trees. So we were anticipating that the infested white spruces sound not live long. Much to our surprise, they lived and are still living.

Here's Piene's theory on how most white spruce and balsam fir trees react to spruce budworm outbreaks.

White spruces form lots of epicormics at low budworm densities. Since foliage on epicormic shoots is very palatable and highly nutritious to budworms, this are stimulates the budworm population to increase. At first this is not a problem for the white spruces because the epicormic shoots are also producing lots of food and energy for the trees. Later, as budworm densities build, spruces are weakened both by the loss of foliage caused by budworm feeding and by the loss of energy caused by creating epicormic shoots that were immediately consumed. So white spruces only loose energy during the later half of an outbreak and this allows them to live a little longer. Early epicormic shoot formation is advantageous for white spruce. When the spruces run low on energy, they start dying. A few large, mature white spruces in native stands do survive, though, because budworm outbreaks generally collapse before white spruces have run out of energy. These surviving spruces are the "seed-sources" for the future forest.

In contrast, balsam fir trees loose energy during the whole outbreak. In the early stages of the outbreak, the firs continually loose needles and store little or no energy each year. Balsam firs are only stimulated to form epicormic shoots when needles and buds are destroyed by high budworm densities. Foliage on balsam firs' epicormic shoots are also very palatable and nutritious. These epicormics are quickly consumed and boost the already high budworm population. With fewer needles on the twigs producing food and energy and the energy used up in epicormic shoot production, balsam firs start dying off rapidly. Large, mature balsam firs in native stands generally do not survive because they use up their energy and die before the budworm outbreak collapses. Fortunately, balsam firs are prolific seeders during the early phases of the outbreak, seedlings establish well and aren't terribly bothered by budworms. These young balsams are the future forest.

Although white spruce and balsam fir have different strategies, both species do survive spruce budworm outbreaks. And, because these trees persist in the forest, they will provide a ready food source for the next spruce budworm outbreak.

For more information, read "Spruce budworm defoliation-foliage production: differences between white spruce and balsam fir" by Harald Piene, 1998. In USFS General Technical Report NE-247.