Pine bark beetles

Pine bark beetle

photo: Pine Bark BeetleIf you live anywhere on the Anoka Sand Plain, you've seen the red (Norway) pines change color over winter and, in some cases, get worse this spring. Normally, winter injury symptoms disappear as the new, green shoots and needles grow. Some of the red pines have followed the typical scenario and are fine, but others are struggling. A number of pines have already died. On the dead and dying pines, you'll see orange/brown needles on most of the branches and no new shoots or needles developing in the bud tips.

Tree root systems were left high and dry and this puts all trees under stress. For the last two years, red pines have been indicating their level of drought stress in the form of winter injury. You can see it in pine plantations, windbreaks, roadside plantings, and backyard trees. Even the small understory pines in pine plantations are dying off due to lack of soil moisture.

With the deepening drought stress, red pines became vulnerable to insects that took advantage of their weakened state. Pine bark beetles attacked the upper crowns of many of these pines. An aerial survey this week found nearly 50 red pine plantations with discolored, dead, and dying crowns in Sherburne and Isanti counties. Most of the affected trees are located in plantation interiors, indicating a serious drought stress and likely bark beetle buildup. Pines in backyards, along roadsides, and in windbreaks were also affected. In some cases, 100 percent of the pines were orange colored. Back on the ground, a few discolored trees were felled and all had bark beetles developing in them.

A few pine plantations did not show any discoloration. Trees growing in these plantations apparently have plenty of rooting space and an adequate supply of water because they have been recently thinned by harvesting.

How can I tell if my pines are infested with bark beetles?

You'll notice several large branches or the top half of the tree with discolored needles. These branches are dead and do not have new growth. Pine bark beetle activity begins in the upper crown and progressively moves down the stem. It's easiest if you can cut a symptomatic tree down to check it for bark beetle galleries and exit holes. Galleries occur in the thin layer of inner bark between the outer bark and the wood. In the active galleries, you can find eggs, grubs, or immature or mature beetles. Exit holes are created as the new generation of bark beetles chews its way out of the bark and, when hundreds occur in a group, look like holes from a shotgun blast. This is positive evidence that bark beetles are active and could threaten nearby pines (within 1/4 mile).

What can I do?

For dead and dying pines in backyards, along roadsides and in windbreaks:

1. Cut down the pines as soon as possible. To keep bark beetles from spreading to adjacent trees, you then must get rid of the tree tops and branches by chipping, burning, burying, or otherwise destroying them. The logs should be debarked, chipped, burned, or removed from the property. Do this within three weeks of cutting the trees. If you don't, you've made it worse for the remaining pines because you've just raised a huge crop of hungry bark beetles.

2. Water, water, water. Where possible, use a soaker hose to irrigate the root systems of the remaining pine trees for the rest of the summer. This is especially important if rainfall dips below average.

3. Watch your trees carefully. Are more trees changing color? Are bark beetles still a problem? Repeat step 1 and, later, step 3.

For dead and dying pines in plantations:

1. Doing nothing is always an option. If there are trees with discolored crowns in your plantation, there will likely be bark beetle-caused mortality in your plantation this year. And, if the drought is truly over, mortality that occurs this summer will be the only mortality that the bark beetles will cause.

2. Use a management method called the "trap tree technique" to limit mortality, thin the stand, and make a little money. In a nutshell, some trees are girdled and harvested and all products and slash are removed or destroyed within three weeks of being cut. Hint: have a contract with a logger in hand before you begin girdling the trees! That way you won't create a bigger bark beetle population and extensive tree mortality.

3. If your plantation is heavily damaged, you may want to thin or clear cut harvest the stand this fall or winter. Make sure bark beetles are the cause of the damage first, then contact a forester for further advice.

4. A long-term goal is to improve tree vigor by thinning the plantation. There's a double benefit; your trees will be better able to withstand drought stress, and you'll make some money. But wait for a year or two so the lingering effects of drought are over, and only cut trees during the fall and winter to avoid bark beetle buildup and tree mortality.

More information on managing pine bark beetles: