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Birch trees and bird vetch prompt slew of questions in Interior Alaska

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Posted: Tuesday, August 21, 2012 12:33 am | Updated: 10:34 am, Mon Jan 21, 2013.

FAIRBANKS — As the new Tanana District agriculture and horticulture extension agent, one of the most interesting aspects of my job is the variety of questions I receive. However, there are two questions I have been asked repeatedly: “What is going on with the birch trees?” and “How do I get rid of bird vetch?” So, I thought I would answer both of these questions here.

First, let’s start with birch trees. There has been an outbreak in Fairbanks of the amber-marked birch leaf miner (Profenusa thomsoni), an insect that came to North America in the early 1900s and arrived in Fairbanks by about 2002. Leaf miners overwinter in the ground as pupae and emerge as adults in the spring. The adults lay eggs at the tips of young birch leaves. The eggs hatch into larvae (small caterpillars) that eat the insides of the leaves, leaving yellow areas scattered with worm nuggets. After a few weeks, the larvae fall to the ground and pupate. The leaves that were mined turn brown. Birch trees can tolerate a lot of leaf loss, and healthy, well-watered trees can tolerate more leaf loss than stressed birch trees. 

The U.S. Forest Service has an excellent publication on this insect. Go to www.fs.usda.gov/detail/r10/forest-grasslandhealth/?cid=fsbdev2_038906 to download a copy.

At present, the best way to deal with this outbreak is to keep your birch trees watered and perhaps apply some fertilizer in the spring. There is an insecticide that can be injected into the trees to provide protection. In 2011, the USFS, with the universities of Alberta and Massachusetts, introduced a small parasitic wasp to Fairbanks that attacks the larvae inside the leaves. If the wasps are successful, perhaps they will keep the birch leaf miners from becoming a more serious problem.

The second question concerns bird vetch (Vicia cracca), a plant named by Carolus Linnaeus, the father of taxonomy. This is the plant with the purple-blue flowers that is growing up fences, small trees, undisturbed forest sites and slow-moving cars all around town. The plant species was brought to Alaska years ago by scientists, just as it was brought to Canada a hundred years before as a potential feed for cattle.

Everyone wants to know how to get rid of it. Soon, there will be a Cooperative Extension Service publication that you will be able to pick up at your local office, but until then, here is how to manage it:

Its weak point is that the seeds only live for about five years. Knowing that, the key to getting rid of bird vetch is to not let it produce seed for at least six years. This can be done by mowing (have you noticed the Alaska Department of Transportation mowing roadsides earlier in the summer?), pulling and trimming before it produces seed. These plants die back to the roots every winter, and it is impossible to pull the roots out of the ground (they break off very easily).

Depending on the weather, repeat this pulling and cutting every few weeks. I do it in my lawn whenever I see that flower. After six years of steady pulling have elapsed, the seeds should be gone.

To kill the roots, products like Weed-B-Gone will work if applied before flowering (about mid-June here). The plants come up at various times, so one treatment will not be enough; you must keep after it and get those that were missed. Spraying after flowering is wasting time and money. Remember, read the herbicide label carefully and follow the directions.

If only we had kept the amber-marked leaf miner and bird vetch out of North America in the first place. Prevention is so much easier than control.

Steven Seefeldt, Ph.D., is the Tanana District agriculture and horticulture agent for the Cooperative Extension Service, a part of the University of Alaska Fairbanks, working in cooperation with the U.S. Department of Agriculture.

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