Skip to content
Pocket Botanist
Get the app

Guides

Stem Borers

Pest

Stem borers are the larval stage of certain beetles or moths that tunnel into the woody tissues of trees and shrubs. Because they feed inside the plant, they disrupt the flow of water and nutrients, which can lead to branch dieback or the death of the entire plant.

Stem Borers on a plant
Photo: Wynand Uys, CC BY

Symptoms by part

Stems

Small, round entry or exit holes on the bark, often accompanied by sawdust like frass or sap oozing from the wounds.

Causes

  • Stressed or weakened plants due to drought, poor soil quality, or physical damage.
  • Presence of adult insects looking for suitable bark to lay their eggs.
  • Lack of natural predators in the immediate environment.

Could be confused with

  • Bark Beetles · Bark beetles typically create smaller, more numerous holes and often leave distinct gallery patterns under the bark, whereas stem borers create larger, deeper tunnels.
  • Mechanical Damage · Physical wounds from tools or storms lack the characteristic frass and internal tunneling associated with borer activity.

Treatment

  1. Identify the specific entry hole where frass is visible.
  2. Insert a thin, flexible wire into the tunnel to physically kill the larva inside.
  3. Prune away branches that show signs of severe infestation or structural weakness, cutting well below the lowest entry hole.
  4. Seal larger pruning wounds with a clean cut to encourage the plant to heal itself.

Without chemicals

  • Prune and destroy infested branches immediately to prevent the larvae from completing their life cycle.
  • Keep the plant hydrated and well nourished to improve its natural defense mechanisms.
  • Remove nearby dead or dying wood that could harbor adult insects.

Chemical options

  • Spinosad, apply as a localized spray to the bark surface during the egg laying period, follow the label.

Preventing it

  • Maintain plant vigor through proper watering and soil management.
  • Avoid wounding the bark with lawn equipment or tools.
  • Inspect trunks and branches periodically for early signs of entry holes.
  • Encourage birds and beneficial insects that prey on adult beetles.

Season

The warm growing season.

Spread risk

Moderate

Severity

High

When to act

Act as soon as you spot frass or entry holes to prevent the infestation from reaching the main trunk. If the infestation has compromised the structural integrity of a large tree, hire a tree surgeon for safe removal.

Think it's Stem Borers?

This guide covers the general picture. Pocket Botanist looks at your actual plant from a photo and tells you what's really going on, how serious it is, and what to try first. Identification and a health check are free.

Download on the App StoreGet it on Google Play

Get the app · Or check it from a photo now, free

Questions people ask

Can a tree recover from stem borers?
If the infestation is caught early and only affects a few branches, the tree can often recover once the larvae are removed and the plant is given proper care.
How do I know if the borer is still inside?
Fresh frass, which looks like sawdust, appearing near a hole is a clear indicator that a larva is currently active and feeding inside the wood.

Plants it affects

Back to all guides

This guide was generated with AI and is general information, not horticultural advice. Read about how we build these guides.