A Billion-Dollar Insect
This is a tale of a big tree, a little insect, and a fungus you can't even see. The big tree is the American elm. Once upon a time, mature American elms were common in the United States from the Missouri River to the Eastern Seaboard. Majestic elms lined the streets of cities and towns, sometimes forming living tunnels with their arching branches. But no more.
Where have the elms gone? Most have been killed by a fungal disease, Ceratocystis ulmi. Early symptoms of infected trees are wilting leaves on some branches. These leaves turn brown prematurely and drop from the tree. Larger limbs produce numerous trunk suckers. Finally, in two to three years the tree dies.
The bark of dead elms killed by the fungus soon sloughs off, leaving the white wood of the tree exposed. These skeleton trees stand for many years in woodlots and on creek banks in stark testimony to days gone by, when American elms dominated the treescape.
An investigation of the dead wood or sloughed bark of dead trees reveals a series of tunnels in the sapwood. Each group consists of a main tunnel with a number of secondary tunnels running at right angles. These are caused by the feeding activity of an insect, one of the bark beetles that get their name from the habit of feeding just under the bark of the tree.
The particular bark beetle associated with the death of the elm trees is known as the smaller European elm bark beetle. Feeding by the bark beetle will not directly cause the death of the trees, but as the insect moves from tree to tree it transports the fungus that does kill them.
Ceratocystis ulmi also is known as Dutch elm disease. And for good reason. It was first identified in the Netherlands in 1921. Since then, it has spread throughout Europe and is now found in parts of Asia and North America. Dutch elm disease was first identified in the United States in 1930. Both the fungus and the smaller European elm bark beetle came to the states in elm logs to be used for furniture veneer.
The beetles overwinter in the tunnels, where they spend their life as larvae, and emerge through small round holes in the bark in spring. At that time, they fly to the growing points of the newly leafed trees to feed and, as a result, transmit the fungus to another tree. Beetles do not fly great distances — usually not over 200 yards — but they have been known to hitchhike great distances in vehicles.
While the beetles are the primary way that uninfected trees come down with the disease, it is not the only way. Trees that grow close together also can transmit the fungus when roots grow into each other. That is why large trees growing within 20 feet of a diseased tree also become diseased even if beetles aren't present.
Just think what a little beetle can do, at least when it transmits a disease organism. In this case, losses have exceeded $1 billion in the United States!