Pests in Gardens and Landscapes: Quick Tips

Tree Borers

Published   5/18

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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 produce large exit holes in trunks.

Flatheaded borers produce large exit holes in trunks.

 Clearwing moth larvae bore large holes and leave sawdust-like frass.

Clearwing moth larvae bore large holes and leave sawdust-like frass.

A number of different types of insects can bore into tree trunks and branches as adults or larvae, producing sawdust or sap-filled holes and weakening trees. Most borers can successfully attack only trees that have been stressed by improper irrigation or care, disease, or mechanical injury. However, some invasive insect borers attack healthy trees. Usually by the time a tree is infested with borers, there is little you can do to manage them other than to improve tree vigor, prune out infested branches, or remove the tree. Insecticides can be used to prevent infestations of borers on high-value trees.

To avoid a borer attack, keep trees healthy:

  • Plant tree species adapted to your area.
  • Irrigate trees properly and separately from regular lawn watering.
  • Avoid mechanical 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.

Correct borer identification is essential.

  • Effective management practices vary by insect species.
  • Confirmation of species requires finding the insect, although knowing symptoms and host plant species can help narrow down the possibilities.
  • Many tiny holes in tree trunks and branches may indicate bark beetles; larger open tunnels filled with sawdust-like boring dust (frass) indicate clearwing moths; flatheaded or roundheaded borers leave wet spots and dark stains and D-or 0-shaped emergence holes.

Nonchemical ways to manage tree borers:

  • Follow guidelines for keeping trees healthy.
  • Prune out infestations of bark beetles and other boring beetles on branches.
  • If the main trunk is extensively bored, remove the tree and focus on protecting neighboring trees of the same species.
  • Clearwing moth larvae can be killed by probing tunnels with a stiff wire.
  • Apply beneficial Steinernema nematodes to kill clearwing moth larvae.

What about pesticides?

  • Limit pesticide (insecticide) use (sprayed or systemic) to protecting healthy trees and combine with other nonchemical methods to improve tree defense.
  • Insecticides are most effective when applied before adults land on the tree to lay eggs on trunks or branches or bore in. Careful timing is essential for success.
  • Insecticides won’t save heavily infested trees because they have limited impact on borers already inside the tree.
  • If treatment is warranted, use persistent insecticides labeled for bark treatment such as carbaryl or certain pyrethroids. Effective insecticides for wood borers 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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Agriculture and Natural Resources, University of California

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