The round, woody swellings that form on aspen twigs are caused by the poplar twig gall fly. The fly lays its eggs in new, tender growth in spring, and the twig forms a gall around the developing larva. The galls are unsightly, but they rarely threaten the life of the tree.
What it does to a tree
A few galls make little difference to a healthy aspen. Heavy infestations year after year can distort and stunt the affected branches, and on young trees and prominent landscape specimens that is usually the reason people choose to treat.
How we treat
We treat with a systemic pesticide in spring, which the tree carries into the new growth where the eggs are laid. Treatment does not always eliminate the galls. It does reduce the number that form, and that is the result to expect.
What to look for
- Round, woody swellings on the current or previous year's twigs
- Small exit holes in older galls where the adult flies emerged
All applications are made under Brookstone Outdoors' Colorado Department of Agriculture commercial pesticide applicator license.