Lumps and Bumps on Aspen by Joyce D'Agostino

Photo Utah State University Extension

Recently as I was doing some spring garden cleanup, I noticed a bare sapling in the corner of my yard that had lumps and bumps on most of the trunk and branches.

I was curious what could cause this and if it was harmful to that tree or possibly other plants in my garden. In doing some research I learned that these swellings on the wood are cause by the “Poplar Twiggall Fly” (Hexomyza shineri). This insect burrows into the wood and as it feeds, which produces a swelling on the limbs and trunk which are called galls. Often the plant has these galls for several seasons and the gardener does not immediately notice them because they can be obscured by leaves. This insect prefers to invade cottonwoods, poplars and especially aspens.

What concerns many gardeners is that these galls can continue to grow and can become quite large over time and can even develop into swollen bands on the trunks and limbs.  Canker type damage can also occur on the wood due to the insects exiting the galls and leaving holes or from birds that peck at the gall to extract the insects.

The good news is that it seems that most trees that have this damage seem to tolerate it and can continue to grow despite the galls. Once the homeowner realizes that the tree will likely survive this insect damage, the following concern is whether they want to keep this tree in their landscape since some may find these galls unsightly.

If you have noticed these galls on your Cottonwoods, Poplars or Aspens and want to know more, including suggestions for control, please refer to these publications:

http://www.ext.colostate.edu/pubs/insect/05579.html

http://www.ext.colostate.edu/ptlk/1437.html