Western Spruce Budworm

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What It Is

Western spruce budworm (WSB) is a native insect that feeds on Douglas-fir and spruce trees.  The larval stage destroys the new growth as the buds and new needles form.  Trees will sometimes survive a single defoliation but may not survive repeated attacks. 

WSB has one generation per year.  It overwinters as larvae in silken hibernacula found under the bark of trees. In late May, at the time new foliage emerges, the larvae begin to feed on the young tips of the trees, usually near the top of the trees.  As the larvae grow they begin to feed on older needles.  Larvae can be wind dispersed as they hang from silk.  The larvae pupate starting in July.  Adults emerge in July-August.  Both males and female fly. Females die soon after laying their eggs.  

Treatment of high value trees may be beneficial when populations of WSB are increasing.  The timing for treatment needs to coincide with the early feeding stage in May. You should monitor your trees beginning when the buds emerge. 

Forest-wide treatment is not recommended. 

Maintaining a healthy forest is a preventative technique.  Stands with mixed species and good spacing are better equipped to withstand WSB infestations.

WESTERN SPRUCE BUDWORM LIFECYCLE

 

Jan

Feb

Mar

Apr

May

Jun

July

Aug

Sept

Oct

Nov

Dec

Larvae

Overwinter

ACTIVE STAGE

Inactive stage

Pupae

 





Late-June to early Aug






Adults

 






July to 

early August





Egg 

 






Late July to early August





 

 












Treatment

 




Early
 May








Predators

Normally WSB is kept in check by native predators and pathogens. During outbreak situations landowners may chose to treat high value trees when the WSB caterpillars first emerge, usually in early May.

2021

July 2021- Neighborhoods in Pine and Evergreen are reporting WSB activity.  Owners should monitor their trees for WSB larvae in spring 2022 at bud-break to access if treatment is warranted.  

Resources

Jeffco Western Spruce Budworm Fact Sheet (PDF)