Elm Bark Beetles
There are two types of the elm bark beetle in North America: the Native Elm Bark Beetle and the European Elm Bark Beetle. Both are exclusive carries of Dutch elm disease and spread the disease by moving from an infected tree to a healthy tree. The European elm bark beetle is more aggressive of the two at colonizing weak elms while the native elm bark beetle is more successful in the northeastern states where they can survive the lower temperatures. Both of these bark beetles attack weakened, stressed or dying trees as well as completely healthy trees.
The Native elm bark beetle consists of two breeding groups. One group over winters as larvae, while the other overwinter as adults. Both remain in tunnels formed in the bark at the base of elm trees. Adults appear in May and lay eggs in galleries in the bark that extend across the grain of the wood. The egg gallery consists of two branches diverging from the point the adults enter the bark. The native elm bark beetle feeds on the bark of trunks and small to medium size branches often two to four inches in diameter. They bore into the bark creating their tunnel by eating the bark along the way.
Adult native elm bark beetles are brownish black in color and are 1/8 inch long. Their bodies are covered in short yellow hairs. Larvae are c-shaped and legless with a yellowish-brown head.
The European elm bark beetle passes the winter as larvae within the folds of the bark. As the weather turns warm they complete their growth and transform to pupae and to adult beetles. The European elm bark beetle emerges in the middle of May through holes made in the bark and feed in the crotches of living elm twigs. They also bore through the bark of dead or dying elm trees or recently cut elm logs. Adults lay about 140 eggs in galleries, or tunnels, parallel to the grain of the wood. The Larvae hatch and feed on the inner bark and surface of the wood creating grooves that extend at an angle away from the main tunnel.
Adult European elm bark beetles are about ¼ inch long, shiny and dark reddish-brown. Their bodies are curved with their spine near the rear of their body. Larvae are c-shaped, have a reddish brown head capsule, and are legless and white.
Dutch Elm Disease is a fungus that infects the watering system of elm trees. This fungus is carried on the bodies of both the European elm bark beetle and the native elm bark beetle as sticky spores. It is deposited in the egg galleries and tree wounds and spreads when the larvae begin feeding deeper into the healthy tree. Once it is introduced in the tree, it prevents water from getting to the top of the tree causing it to brown, wilt and die usually months before the normal time for the tree to turn colors. Symptoms usually begin in late spring. Infected branches and stems develop dark discoloration streaks in the sapwood. Dutch elm disease is often confused with elm yellow disease and bacterial leaf scorch. The difference is that yellow disease turns the leaves a yellowish color causing them to drop prematurely, while bacterial leaf scorch is caused by bacterial causing the leaves to have a yellow border between the healthy leaf and the scorched tissue.
See picture of Dutch Elm Diseased Tree
Dutch elm disease was first found in North America in 1928 in a shipment of logs traveling from the Netherlands. Since then it has spread across the country killing off elm trees. Depending on the time of infection, the diseased tree may survive for one year.