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Stormwater Pollution Prevention

Tree Borers

Stormwater Pollution Prevention

Many insects bore into tree trunks and branches in their larval stages, producing sawdust or sap-filled holes and weakening trees. Most borers successfully attack only trees stressed from over-irrigation, disease, mechanical injury, or lack of care. Once trees are infested with borers, management is difficult. Insecticides cannot save seriously injured trees.

Keep trees healthy to avoid borer attack

  • Plant only species adapted to your area. Replace old declining trees. Avoid injuries to trunks and roots. Protect tree trunks and branches from sunburn.
  • Irrigate trees properly and separately from lawns.
  • Prune during the fall and early winter when borer adults are not flying. Monitor tree trunks and branches regularly to detect infestations before they become serious.

Identify borers correctly

  • Management varies according to species; find the insect. Knowing symptoms and host plants can help.
  • Many tiny shot holes in trunks and branches may indicate bark beetles; larger open tunnels filled with sawdust-like material indicate clearwing moths; flatheaded or roundheaded borers leave wet spots and dark stains and D-or O-shaped emergence holes.
  • Ask your UCCE office for help in identification.

Non-chemical management

  • 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 or with applications of beneficial nematodes in the Steinernema genus.


  • Apply insecticides only when adults are laying eggs on trunks and branches.
  • No insecticides work against larvae in trees, including systemic types such as acephate or imidacloprid.
  • If warranted, use persistent insecticides labeled for bark treatment such as carbaryl and certain pyrethroids. Consider using a licensed applicator.

For more specific information on the different types of borers visit the UC Davis website.

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