The native elm bark beetle consists of two separate breeding groups. One group overwinters as larvae in the breeding tunnels, while the second group overwinters as adults. These
adults emerge from mid-April to mid-May. It is their feeding phase that causes the majority of DED infections. It is believed that the European elm bark beetle overwinters as larvae.
Much of the effort to control the spread of DED has focused on controlling the beetle population with insecticides or trapping. These methods have enjoyed some degree of success but the beetles
remain the single most important factor in the spread of the disease.
Mature elms have a large system of roots. When these roots come into contact with those of another elm, they can graft together to promote the exchange of nutrients. The fungus can spread
through the root grafts, infecting the neighbouring trees.
A tree infected by root graft transmission shows very sudden and devastating symptoms. Treatment is much less effective.
Pruning tools can also transmit Dutch elm disease. All tools should be cleaned before pruning a healthy tree. Some arborists recommend a 10% solution of household bleach.