Forest Insect and Disease Newsletter

Impact of drought and bronze birch borer on yard trees.

image: Birch TreesIt's mid-April here in the far north of Minnesota and I'm already seeing patches of green grass rising above the brown turf where just a few days ago snow and ice had covered the ground. This is the first time since I can remember that my sump-pump hasn't been running steady while snow is melting and exiting as fast as a disgruntled camper from a crowded park.

The weather forecasters last October said "just wait until November, we'll be getting buried in snow, and tempered with dreadful col". I thought at least we'll be getting enough snow to help the drought situation next spring. This year illustrates just how fickle the weather can be and how hard it is to predict the weather these days, especially in the Lake States. I remember one forecast last fall predicting that we wouldn't see much winter until February. Most meteorologists didn't buy it, but it's history now and the on-going affect of this drought will start showing up and may do so for years to come.

image: Birch tree showing signs of bronze birch borerBronze birch borer (BBB) is a regular and unwanted guest to my birch trees, except the ones directly fed by the down-spouts of the eaves. When birch trees are stressed by drought, they become attractive to an insect called the bronze birch borer. The adult beetle lays eggs on stressed trees usually starting on branches in the upper crown. The eggs hatch and the larvae bore into the tree and girdle the branches and eventually the stem of the tree killing it. Stressed birch trees also are attacked a soil fungus called Armillaria. This attacks and kills the roots of stressed trees. The original and real problem is the drought stress of the trees. The insects and fungi are just doing their job of killing (finishing off) the stressed trees.

Soon after the trees leaf out this spring, check the crowns carefully for parts or areas of the crowns with branches lacking leaves (which would have been flagged last August). This indicates that the trees were invaded last summer but haven't been killed down to the ground by BBB yet. They have larvae and pupae and adults present which will emerge and re-invade the tree further down the trunk, or invade other trees.

I have so many birches that I use them for firewood so I really don't mind that much if I have to cut remove some of them. However, the key to managing the impact on your trees is how you go about the removal process. Just removing trees after the die off doesn't get rid of the problem. You may lose more trees than you want to lose or are able to handle. If your trees died in 2010 or earlier, they are not an insect problem. If they died last year or this spring, those trees have to be taken care of immediately. If they are currently infested, pruning and the use of insecticides might be beneficial. If they are still healthy, you can help them by watering during droughty weather and avoiding root damage.

Management recommendations include:

  1. Sanitation – Cut and remove infested dead birches with at least 50% dieback. Remove infested trees before May 1st and dispose of the trees, and branches down to at least 2 inch diameter. Prune infested tops and branches out of trees to be left standing. Debark, burn, bury, landfill, or tarp the wood and branches. Bark needs to be removed or wood treated (burned, chipped, buried) if trees are cut after mid-July. If tarping, cover with a heavy plastic tarp with the edges buried with soil. Keep the pile covered now until July 15th of next year.

However, if you are in a neighborhood or forested area with lots of infested trees, sanitation is unlikely to reduce the overall population of BBB and unlikely to protect your trees from attack.

  1. Avoid bring fresh birch firewood into your yard unless it has been debarked or can be kept covered with a tarp. Bringing more infested birch firewood into your yard will only increase the problem.
  2. Chemical insecticides are ONLY useful when used in combination with sanitation. 137 insecticides are listed on the MDA website. Please read and follow all label directions. The label is the law.
  3. Protect natural enemies of BBB. Natural controls, such as woodpeckers, parasitic wasps or predatory beetles, are generally sufficient during normal periods, but cannot ramp up their populations to meet that of the bronze birch borer during periods of drought. So avoid insecticide applications that hurt natural enemies.
  4. Water the trees you want to keep. They should get at least 1 inch of water each week during the growing season either from rain or watering. Laying an open hose directly on the ground is best. Water each group of trees for 2 to 3 hours. Water from early spring until mid-October.
  5. Avoid damaging trees or roots. Practices that damage roots include trenching, compacting soil by driving and parking on root systems, smothering roots by adding soil or changing the grade or level of the site and especially, severing roots. All of these increase the chances of BBB damage.
  6. Protect trees from defoliation, especially the second (or more) year of birch leaf miner or FTC defoliation.

image: Symptoms of bronze

Symptoms: galleries under the bark and
D-shaped exit holes.


image: Tarped and untarpped wood piles

Tarped and untarped wood piles.


For more information, see