Pests in Gardens and Landscapes: Quick Tips

Tree Borers

Published   1/04

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Bark beetles make tiny holes in tree trunks and branches.

Bark beetles make tiny holes in tree trunks and branches.

Flatheaded borers leave large exit holes in trunks.

Flatheaded borers leave large exit holes in trunks.

Larvae of clearwinged moths bore large holes with lots of sawdust-like frass.

Larvae of clearwing moths bore large holes with lots of sawdust-like frass.

A number of different types of insects may bore into tree trunks and branches in their larval stages, producing sawdust or sap-filled holes and weakening trees. Most borers can successfully attack only trees that have been stressed by under- or over-irrigation, disease, lack of proper care, or injury by mechanical equipment. Usually by the time the tree is infested with borers, there is little you can do to manage them other than improve tree vigor, prune out infested branches, or remove the tree. Insecticides are occasionally used to prevent infestations of bark beetles on high-value trees or to manage certain clearwing moths.

To avoid a borer attack, keep trees healthy:

  • Plant only species adapted to your area.
  • Irrigate trees properly and separately from the lawn.
  • Avoid injuries to trunks and roots.
  • Protect tree trunks and branches from sunburn.
  • Avoid pruning trees when borer adults are flying, usually late winter through late summer.
  • Replace old declining trees.
  • Monitor tree trunks and branches regularly to detect infestations before they become serious.

If borers are in your trees, identify them correctly:

  • Effective management practices vary according to species.
  • Confirmation of species requires finding the insect, although knowing symptoms and host plant species can help.
  • Many tiny holes in tree trunks and branches may indicate bark beetles; larger open tunnels filled with sawdust-like frass indicate clearwing moths; flatheaded or roundheaded borers leave wet spots and dark stains and D-or 0-shaped emergence holes.
  • Call your Cooperative Extension office or California County Agricultural Commissioner for help in identification or refer to the UC Statewide IPM Program Web site.

Nonchemical ways to manage tree borers:

  • Follow the guidelines above for keeping trees healthy.
  • Local infestations of bark beetles and other boring beetles on branches may be pruned out.
  • If the main trunk is extensively bored, remove the tree and focus on protecting neighboring trees of the same species.
  • Clearwing moth larvae may be killed by probing tunnels with a stiff wire.
  • Clearwing moth larvae may also be killed with applications of beneficial nematodes in the genus Steinernema.

Turning to insecticides:

  • Seriously affected trees cannot be saved with insecticide treatments and should be removed.
  • Insecticides must be applied to kill adults as they are laying eggs on trunks and branches of trees before they are seriously infested. Careful timing is essential for success.
  • No insecticides are effective against larvae within trees, including systemic insecticides such as acephate or imidacloprid.
  • If treatment is warranted, use persistent insecticides labeled for bark treatment such as carbaryl and certain pyrethroids. The most effective materials are available only to licensed applicators.

Minimize the use of pesticides that pollute our waterways. Use nonchemical alternatives or less toxic pesticide products whenever possible. Read product labels carefully and follow instructions on proper use, storage, and disposal.

Statewide IPM Program, Agriculture and Natural Resources, University of California
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