Lilac / Ash Borer
A clearwing borer that kills individual stems on lilac and ash, most active late spring through early summer and most damaging on drought- or winter-stressed plants.
- Type
- Insect
- Affects
- Shrubs, Trees
- Risk
- Stem loss on stressed lilac and ash
What Is It?
Lilac/ash borer is an insect pest active from late spring into early summer. Larvae tunnel in stems near the base of the plant, cutting off the flow of water to everything above.
Damage concentrates on plants already weakened by drought stress, winter stress or trunk injury.
Diagnostic Imagery
In the field
What Does It Affect?
- Common lilac
- Ash
- Privet
What Should I Look For?
- Wilting or dieback of individual stems.
- Sparse foliage or thinning canopy.
- Small round exit holes near the base.
- Sawdust-like frass at the trunk or branches.
What Does It Mean In Colorado?
Lilac is one of the most widely planted shrubs on the Front Range, and borer pressure follows the drought and winter-desiccation cycles common here.
Seasonal Timing
Late April–May
Begin scouting for borer activity; prune immediately after flowering.
Late May–June
The only window in which preventive treatments are effective, and only where there is a history of borer activity.
What Can Be Done?
01
Prune and destroy infested stems
Removing colonized stems removes the population developing inside them.
02
Reduce drought and winter stress
Consistent watering, including winter watering during dry, warm spells, is the most durable defense.
03
Correctly timed preventive sprays
Preventive sprays are only effective when timed correctly in late spring. Off-timing accomplishes nothing.
Prevention
- Avoid trunk and stem injury from mowers and string trimmers.
When Should I Call An Arborist?
- Entire stems are dying back.
- Repeated severe damage across multiple seasons.
- Significant loss of vigor in an established shrub.
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