Trees Winnipeg
Education & Publications
Tree Health
DED Management
Setting Down New Roots
What's Bugging My Trees
Spruce Budworm
Setting Down New Roots
A Practical Guide to Pruning
Like us on Facebook
Follow us on Twitter

Tree Care

What's Bugging My Trees?



Aphids are soft-bodied insects that suck the sap from the plant leaves or roots. They are commonly found on trees, shrubs and flowers. Aphids come in many different colours such as green, yellow, brown, pink or black and sizes. Aphids excrete a sugary substance called honeydew on the leaves which overtime can cause sooty mold. In most cases aphids are found in the folded over part of leaves, known as a clutch. This is where the eggs and the next generation of new nymphs are present.

You can control aphids on trees, shrubs and flowers by frequently hosing plants with high water pressure. Hosing your plants knocks most of these insects off and kills many of them. It may require several treatments to reduce the population.

If you have a severe aphid infestation, mix one to two tablespoons of 100% pure dishwashing soap (not regular dish soap) to one gallon of water. Spray this mixture in the early morning or late evening and rinse the plant thoroughly the next morning. Some plants may be sensitive to the soap and cause the leaves to have a chemical burn. Test on a small area first before treating the entire plant. You can also purchase an Insecticidal Soap from home and garden centres.

We do not recommend using chemical insecticides for aphid control because the aphid population will rebound to the same level in about a week. Insecticides such as Malathion® or Sevin® can be used occasionally. Make sure to follow label directions.


CankerwormPlease note that Tanglefoot© Bands are designed only for cankerworm control, and have not been scientifically proven to prevent Dutch elm disease. For simple tips on how to help prevent DED, please click here.

Cankerworms are caterpillars that feed on the leaves of many trees including elm, ash, Manitoba maple, linden, oak and ornamental trees. They create small holes in the leaves but during severe outbreaks, trees may be completely stripped of leaves. This weakens the tree as it must develop a second set of leaves, usually by mid July. However, growth may be slower and resistance to other insect attacks and disease declines. Repeated defoliation year after year may lead to a tree's premature demise.

It is important to protect elm trees in particular as they may become more susceptible to deadly Dutch elm disease, transmitted by the elm bark beetle.(Tanglefoot bands also trap the beetle if applied at the beginning of September.)

Types of Cankerworms

There are two kinds of cankerworms in Winnipeg - fall and spring. They are both about 2.5 cm long and range in colour from light green to brownish green with a dark stripe down the back.The fall cankerworm (female) moth lays it eggs in the tree following the first severe autumn frost. The spring cankerworm (female) moth lays it eggs in spring after the first real thaw. The female moth is wingless and must climb up the tree to lay its eggs. When the tree is banded, the moths are trapped on the band before they have a chance to lay their eggs. This trapping method significantly reduces the number of cankerworms defoliating the trees, in late spring/early summer.


At times when cankerworms are very high in numbers or if you have a property with many trees, it may be necessary to use an insecticide. Trees Winnipeg recommends that property owners use only products containing Bacillus thuringiensis,a biological control product aimed specifically at caterpillar-type insects. The biological insecticide is sprayed on the leaves which are then ingested by the cankerworm larvae.

Obtain a number of estimates from tree service firms in advance of cankerworm season (usually the beginning of June). Ensure that a biological control will be applied Insecticides such as malathion kill other beneficial insects and are harmful to the environment.


Elm SpanwormThe elm spanworm is a serious defoliator of shade and forest trees in the eastern Canada and the United States. Generally, this native pest feeds on elm, hickory, oak, red and sugar maple, American beech, and ash. During July females lay eggs in irregular, single-layered, compact masses on the underside of twigs, large branches, or on tree trunks. Each female lays an average of 250 eggs. The eggs are oblong-elliptical, with a white serrated ring around the end. Elm span-worm spends winter in the egg stage.

Egg hatch may begin in mid- to late May. After hatching, the larval stage passes through five or six instars, and then it pupates. To prepare for pupation, mature larvae spin coarse, netlike cocoons of silken threads, often on partially defoliated leaves. In severely defoliated stands, cocoons may be spun on exposed branch tips, in bark crevices, or stumps in the undergrowth. The pupal period varies from 9 to 17 days. Adults emerge in late June through July. They are on the wing at night. If an infestation is close to a urban area, male moths may fly to lights in large numbers frequently described as resembling a snowstorm in the summer.

The elm spanworm adult is a powdery white moth with a wingspread of 30 to 37 mm (1 to 1 1/2 inches). Its body is fairly stout and hairy. Females lay the eggs incompact masses on the underside of twigs, large branches, or on tree trunks. The eggs are bright yellow green when first laid in summer but darken to dull olive gray or brown in Elm Span Worm Mothwinter. Mature larvae, some-times referred to as “loopers” or “inchworms,” are about 50 mm (2 inches) long. The body of the larval stage maybe dull or slate black and the head rust-colored. Some larvae may be light green with yellow head capsules. When population levels are low, there is a higher proportion of lighter colored larvae. The pupal stage is light brown, sometimes patterned with dark brown spots.

All damage is caused by the larval stage of the elm spanworm. As soon as egg hatch occurs, larvae begin to feed on the underside of leaves, causing a shot hole effect. As larvae mature, they eat all leaf material between the major veins. Larvae are capable of completely defoliating shade trees and large areas of mixed hardwood forest during outbreaks.

Trees should be monitored from mid-May through early June for signs of elm spanworm larvae. If necessary, a registered insecticide should be applied when larvae are small. Where possible, prune small twigs that are infested with masses of eggs. Two egg parasitoids, Telenomus droozi and Ooencyrtus entomophagus are known to keep this pest at low population levels. These small natural predators can destroy more than 80 percent of eggs during an outbreak.

Bacillus thuringiensis var. kurstaki (Btk) can also be used to control Elm Spanworm. Btk is a selective biological insecticide which controls lepidopterous larvae (caterpillars). Btk crystals release a toxic protein when dissolved in the alkaline digestive system of the insect. The caterpillar stops feeding soon after, and dies within five days. Other insects, mammals, birds and fish are not affected by Btk. Btk is most effective when larvae are in their first instars. Btk has to be applied when larvae are actively feeding, and applied so that all foliage is thoroughly covered. Rain washes Btk off the leaves and sunlight breaks it down within a short period of time, approximately one to two days.


Forest Tent CaterpillarForest tent caterpillars, Malacosoma disstria, are a serious threat to deciduous trees across Canada and the United States. During severe outbreaks, these destructive insects can eat all the leaves on hundreds of thousands of hectares on broadleaf trees and shrubs.

The caterpillars (larvae) prefer trembling aspen but also feed on various other broadleaved trees and shrubs. When starvation threatens they may also feed on conifers.

Learn about the life cycle, damage and control of the forest tent caterpillar, Malacosoma disstria.

PDF Full brochure


Spruce BudwormThe spruce budworm, Choristoneura fumiferana (Clemens), is the most destructive and widely distributed forest defoliator in North America. The destructive phase of this pest is the larval or caterpillar stage. Massive budworm outbreaks occur periodically, destroying hundreds of thousands of hectares of valuable fir and spruce. In eastern Canada the budworm’s preferred food is balsam fir, white spruce and red spruce. In Manitoba, the budworm feeds primarily on white spruce and balsam fir, and, less frequently, on black spruce.

PDF Full brochure

Donate to Trees Winnipeg
Thank you to Our Supporters
United Rentals

Green Drop
Great West Life
Cambrian Credit Union
The Winnipeg Foundation
University of Winnipeg
Government of Manitoba
The City of Winnipeg
Manitoba Hydro
Starbucks Coffee