Though most species of insect do not cause harm to trees, some can be quite the pest! It's important to know which insects are causing problems before taking steps to control them.

Image of caterpillars on a tree

How to identify and manage caterpillars and other leaf rollers

There may be a number of different insects attacking aspen trees in the County any given year. The two main ones are typically aspen leaf rollers and speckled green fruit worms. There are also small numbers of large aspen tortrix and linden loopers, among others.

Aspen leaf roller

  • Attacks aspen poplar – rolls the leaf up to be protected through metamorphosis
  • Won’t kill the tree, but the appearance will be affected for the summer
  • There will be a flush of new leaves
  • The caterpillar is the larva of a small moth; the insect is going into the pupa stage so there shouldn’t be any more damage
  • The population increases in years of mild winters and warm wet springs
  • There are no control methods for this insect as it is protected by the leaf
  • Keep desirable trees well watered

Linden looper

  • Linden loopers are dull-to-brightly-yellow-sided
  • They feed on a variety of hardwoods, including poplar, birch and maples
  • Start feeding early in May and pupate by the middle of June
  • Drop to lower branches by silken threads
  • Outbreaks generally last one to two years
  • Adults emerge in the fall; the females do not fly
  • They crawl up the tree trunk to mate and lay their egg
  • Overwinter in the egg stage
  • A sticky product, such as Tanglefoot, placed around the base of the tree would stop the females from getting up the tree

Large aspen tortrix

  • Mature larva is green to black with a black head and anal plate
  • Major defoliator of trembling aspen, but also feeds on balsam poplar, willow and birch
  • Larva start feeding in early May, just before the buds break
  • Larva will roll leaves or pull several leaves together and secure with a silken thread
  • Pupation occurs by mid-June
  • Adults emerge at the end of June to mid-July
  • Their eggs are laid on the surface of the leaf and hatch by the end of July
  • The larva pulls leaves together and feeds on the surface of the leaf until mid-August
  • Damage is insignificant at this stage
  • Overwinter as a larva under bark scales
  • Outbreaks are two to three years in duration

Speckled fruit worm

  • Greyish to green in colour with several narrow white stripes along the top and sides
  • Eggs are laid on twigs and trees
  • Overwinter as pupae in the soil
  • Major defoliator of trembling aspen and willow

Bruce spanworm

  • Bruce spanworm larvae are variable in colour, ranging from vivid green to dark brown, with three thin yellow lines on each side of the body
  • Trembling aspen is the primary food source but they will feed on willows, balsam poplar, Manitoba maple, chokecherry, Saskatoons, currants and wild roses
  • Start feeding early in May and pupate by mid-June
  • Drop to lower branches by silken threads
  • Often occur with outbreaks of other defoliators
  • Outbreaks generally last two years
  • Adults emerge in the fall; the females do not fly
  • They crawl up the tree trunk to mate and lay their eggs
  • Overwinter in the egg stage
  • A sticky product, such as Tanglefoot, placed around the base of the tree would stop the females from getting up the tree

Forest tent caterpillar

  • Forest tent caterpillars have blue stripes and white keyhole shapes down their backs
  • Overwinter in the egg stage
  • Hatching occurs in the spring, about the time the tree buds begin to swell
  • Feeds on a wide variety of hardwoods; trembling aspen is preferred
  • Eggs are laid in masses in a ring around the branch containing 100 to 350 eggs
  • One generation per year

Management tips 

  • One insecticide that can be used on the caterpillars is Bacillus thuringiensis (Bt). It would need to be applied in May while the larvae are still small.
  • Any registered contact insecticide, like Malathion, can be used to help control these caterpillars on ornamental trees.
  • Control of these caterpillars is usually not necessary due to the short duration of the outbreaks as there are a number of natural predators that keep the numbers in check.
  • A large species of ground beetle is also present, commonly called the fiery hunter, that feeds on caterpillars. Avoid insecticides, as they may harm these beneficial beetles.

Poplar borer

Poplar borers live in aspens, cottonwoods, willows and poplars. They are often found in columnar poplars, around the edge of native aspen stands and lone trees. Noticeable signs include large bore holes with sawdust (shredded wood) coming out of the holes and piled at the base of the trunk. Sap leaks down and stains the bark brown attracting other insects. Trees can be weakened structurally when several larvae are present.

Management tips

If a tree is infested, remove it and destroy the wood by chipping, burning or taking it to a recycling centre. Many larvae are killed by woodpeckers, nematodes and other natural predators, and over 60 per cent never make it to maturity.


Aphids live in many tree and plant species. Signs of them include honeydew falling from trees, curled and deformed leaves and shoots, and the presence of ants. There will be darkening of the bark on younger elms caused by sooty mould growing in the honeydew.

Management tips

Spray aphids of a plant with a powerful stream of water. There are also many natural predators of aphids including ladybugs.

Visit Natural Resources Canada for more information on aphids.

Yellow-headed spruce sawfly

These can be found in a variety of spruce species, where you may notice larvae and defoliation of the ends of branches. Sawfly larvae can defoliate a tree if it is heavily infested. Bare branches and defoliation affect the aesthetics of a tree. Repeated defoliation is where the danger to the tree lies.

Management tips

You can remove by hand or spray larvae off with high-pressure water. Disturbing the ground under the tree may disrupt pupation. There are also a lot of natural predators that will eat the pupating Sawfly, including birds and ground beetles.

Visit Natural Resources Canada for more information on yellow-headed spruce sawfly.

Ash leaf cone roller

These caterpillars live in green, manchurian and black ash trees.

Management tips

Let nature take its course. This insect does not damage the leaf enough for it not to be able to perform photosynthesis. There are naturally occurring predators of this caterpillar, including a non-stinging wasp, birds and more.

Natural Resources Canada provides information on the following tree pests:

Further information:

County Connect

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Last updated: Thursday, August 11, 2022
Page ID: 40903