Dutch elm disease can be transmitted from tree to tree by the elm bark beetle, root grafts and infected tools.

Elm Bark Beetle

The elm bark beetle (not to be confused with the Elm Leaf Beetle) is by far the most important factor in  Dutch Elm Disease.

These tiny insects’ lives revolve around elm trees. The female beetle tunnels into the tree between the bark and the wood and lays its eggs.  When the eggs hatch, the larvae tunnel further into the tree in order to feed before emerging as mature beetles. 

Adults feed in the crown of the tree, moving from tree to tree before breeding again. 

If a beetle breeds or feeds in a DED-infected tree, the sticky spores of the fungus become attached to its back. When the beetle moves to a healthy tree, so too do the spores.

There are two species of the elm bark beetle in North America – the European and the Native elm bark beetles.  The European is more temperature sensitive and lives mainly in southern regions. The Native is dominant in the mid-west.

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. 

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Root Grafts

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.

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Infected Tools

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.   

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