Poplar twiggall fly damage to an Aspen

Poplar twiggall fly damage to an aspen.

Jim Leser

Jim Leser.

Aspens are one of two iconic trees planted by Coloradans. The other is the Colorado blue spruce. But aspens come with a multitude of potential problems from many diseases, insect infestations, and animal damage. If you are planning to plant an aspen tree this year or already have one in your yard, you might look into why aspens might not be your first choice for a dependable, long lived tree for this area.

One of the most visible impacts from an insect pest of aspen would have to be from the poplar twiggall fly which produces galls on newer twigs. While these galls might be unattractive and bother you, unless you have a severe case of these galls I would not be overly concerned.

These galls form early in the season but continue to grow, forming large knots on progressively larger branches. Once formed they never go away. If multiple galls stack up along a twig or small branch this may severely restrict the flow of nutrients and water to the distal parts of the infested branch. This could result in branch death and eventual branch loss.

But as I indicated earlier, these galls rarely cause significant harm to your aspen tree. The poplar twiggall fly has one generation per year, overwintering within the gall as a fully grown maggot. Pupation will usually occur early in the spring, sometimes even before leaf buds begin to open. Pupae then drop to the ground and soon after the adult fly emerges.

Once mated, these adult female flies will then insert their eggs, one or more per site, into the tissue of new twigs. Once these eggs hatch and the maggots start feeding, the tree responds by producing abnormal growth around this site resulting in a swelling or galling.

Removal of “galled” twigs is generally a waste of time and may be more detrimental than the galls themselves. There also is a tiny chalcid wasp that parasitizes this pest, resulting in control around 25%. Pruning off these affected branches later in the year may actually remove this beneficial parasitic wasp.

If you still feel you need to control this insect pest do not try by using foliar sprays of insecticide. You probably won’t time it right. The best insecticidal control I have achieved is with imidacloprid, a systemic insecticide applied as a soil drench around the base of the tree. Since it lasts up to twelve months, timing isn’t so critical. I apply my treatment in the fall when I’m not so busy.

But if some twig galling is your main concern, consider yourself lucky. There are several more serious maladies that can befall your prized aspens. I’ll cover these in subsequent columns.

Happy New Year and good luck with your landscape plans and may you have a bountiful harvest from your vegetable garden or fruit trees. If you need help please call the Colorado Master Gardener’s desk at the Delta County Extension office. We want you to succeed!

Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Certified Colorado Gardener.

Load comments