Garden Alert: Poplar & Aspen Borers

One pest that is causing a lot of grief for those who have poplar trees on their properties is an insect called the Poplar Borer.

It is a native beetle that evolved feeding primarily on native aspens, but has developed a taste for Swedish Columnar aspens that are typically planted in rows along fences for privacy screening. Poplar Borers are rather large, gray beetles with faint, yellow stripes on its body and antennae that are as long as its body.

The problem with these borers is that they not only feed on the green “phloem” that sits just below the bark and moves sugars up and down the tree, but the larva (worms) also tunnel into the wood and leave a labyrinth of trails that weaken the tree, leaving portions of the trunk prone to snapping-off on windy days.

Aspen Borers prefer aspens that have trunks about 10 cm wide or larger and they typically seek trees that are stressed. The adults prefer to lay eggs on the south to southwest side of trees that have lots of exposed bark (extra trunk heat is better for larva growth and development).

The lifecycle of Aspen Borers can take several years to complete in our region, but once they invade trees they are very difficult to control. Given the great benefits of having Swedish Columnar aspens, and the expense of removing these trees, the battle to keep the borers at bay is critical.

Here are some of my observations and a bit of a game plan for Poplar Borer:

  • Aspens growing in landscape fabric with rock around the base are the worst affected, typically
  • Drought stressed aspens growing in poor soil are also preferred by the borers
  • Aspens with branches removed on the south/southwest side of tree are attacked more often

Symptoms of borer attack:

  • Small holes in trunk with brown sap stains on bark
  • Small piles of ‘wood shavings’ at trunk base from borer tunneling

What can be done?

  • Inspect your poplars several times during the growing season and look for any signs of damage
  • Pest control products like ‘Garden Protector’ can be used as a trunk and foliage spray prior to the borers penetrating the wood
  • If the borers enter the wood, control is difficult. A number of gardeners have been quite successful applying Knock Down aerosol insecticidal spray directly into the entry holes on the tree trunks.

Aspen Borers are destructive pests so if you have Swedish Columnar aspens always be vigilant! Being proactive with controlling the beetles is the best strategy!

- Jim