Pest

Galls on leaves and twigs

Mites, wasps, adelgids and aphid relatives

Swellings the plant grows around a feeding mite or insect. They look alarming, but most rarely affect the health of the tree. Spruce galls rarely harm the tree; heavy galls on oak twigs can cause branch dieback.

Signs

• Pouches, round bumps or raised, scab-like patches growing out of the leaf.
• Cone-like swellings on new spruce growth (adelgids); round balls on oak twigs (gall wasps).
• Cut one open: a hollow, a chamber or a tiny larva inside.

What favors it

Mites, tiny stingless wasps, adelgids and aphid relatives, each gall maker with its own hosts; the Cooley spruce gall adelgid alternates between spruce and Douglas fir.

Prevention

• Plant spruce away from Douglas fir where Cooley spruce gall adelgid is common; it alternates between them.

Crops it shows up on

Shade & ornamental trees; Evergreens & conifers (pine, spruce, juniper); Roses & ornamentals.

Often confused with

Sources

• Cooley Spruce Gall AdelgidMontana State University Extension
• Rough Bulletgall WaspMontana State University Extension
• Deciduous Tree GallsUW-Madison Extension

Why →