This is the VOA Special English Agriculture Report.

A beetle invasion in the United States has killed at least twenty million ash trees. The invasion of the emerald ash borer was first discovered near Detroit, Michigan, in two thousand two. Experts believe the small green insects arrived in the nineteen nineties in shipments of goods from China.

The emerald ash borer has destroyed trees in the Midwest and as far east in the United States as Maryland. The insects have also spread as far north as Ontario, Canada.

Ash trees are popular. They grow well in heavy clay soils, and they can survive ice storms well. They produce many leaves, so they provide shade protection from the sun. And in the fall the leaves turn a beautiful gold and purple.

Ash trees can resist many diseases. But they cannot resist the emerald ash borer. It lays eggs on the bark. Then the young larvae drill into and feed on the inner bark. This harms the ability of the tree to transport water and nutrients.

The insect is attacking tree farms and can also spread when logs and firewood are transported.

The United States Department of Agriculture is working to save the ash tree. So are agriculture departments and university extensions in a number of states.

In some places, farmers are using "detection trees." These have an area where bark has been cut away. The area circles the tree and is called a girdle. The girdling process weakens the trees. It makes them easier targets for borers, and shows if the insects are nearby.

Efforts to stop the spread of the emerald ash borer include cutting down affected trees. A tree farmer in Maryland, for example, recently faced the loss of hundreds of trees.

There are worries that the ash tree might disappear unless the invasion is controlled. To prepare for such a possibility, a government laboratory is collecting seeds from ash trees.

David Burgdorf works in East Lansing, Michigan, for the Natural Resources Conservation Service; the service is part of the United States Department of Agriculture. He is asking people to send in ash seeds. The laboratory examines and x-rays the seeds to make sure there are no living borer embryos.

The best seeds are then sent for storage in a seed bank in Fort Collins, Colorado. There, they are dried and frozen at the National Center for Genetic Resources Preservation. Should the seeds ever be needed, the hope is that scientists might someday develop an ash tree that could resist the little green attackers.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. I'm Steve Ember.