You may have noticed your neighbourhood coated in a shiny and sticky coating recently. This is happening because of a surge in the aphid population. Aphids secret honeydew, a sugary, sticky liquid which can attract fungus and create a black scum on cars, sidewalks, and other surfaces. Honeydew is not harmful, but it can be a nuisance. Elm scale presents in a similar manner, read on for more information.

What are aphids & elm scale

Aphids are tiny pear-shaped insects that are generally green in colour; but some can be yellow, grey, black, brown and pink. Female aphids can reproduce without mating in the summer, so populations can rapidly increase.

Woolly elm aphids are a native pest that cause limited damage to elm trees and saskatoon shrubs. They are difficult to control due to their protective cover inside rolled leaves and their white waxy protective coating.

European elm scale (EES) is a pest that normally attacks elm trees. Elm scale feeds by piercing leaves and bark and sucking juices from the tree. The eggs begin to hatch in late June and start feeding on leaves in mid-July. By autumn, the pest moves onto branches and twigs where they prepare to overwinter. 

What is the City doing to control this issue?

To help manage population levels the City treated 2,347 trees in 2022 and another 433 trees in 2023. The tree injections to control against European Elm Scale are often effective for 2-4 years after treatment at reducing populations.

What can you do to manage aphids?

1. Assess the problem. Watch for aphids by looking closely at new shoots and underneath leaves. Early signs of plant damage are yellow or light red spotting, or curling leaves where the aphids are feeding.
2. Wash away the aphids. Use a strong direct spray of plain water to remove aphids without harming the beneficial insects. Get plants dripping wet and spray into the middle of the plant where aphids may be hiding. Beneficial insects will fly away to dry off and return later to eat more aphids. Adding insecticidal soap will also help. Water restrictions have made this difficult, but I am glad that as of today people can try and clean their trees.
3. Encourage beneficial insects. Predatory insects such as ladybugs will help to keep aphids under control.
4. Trap the aphids. Sticky yellow cards are effective traps for aphids. They can be hung in or around plants affected by aphids.
5. Keep trees healthy. Growing plants in a healthy condition in moist, fertile soil is the best way to avoid aphid infestations. Aphids can be a nuisance, as they cannot be fully eradicated, only controlled. The easiest way to prevent pests and diseases is to keep your trees healthy. One way to do this is to give your trees a deep watering on a monthly basis, beginning when the tree leafs out and ending when the tree drops its leaves in the fall.

Weather vs. aphids

Weather plays a factor in managing the pest population. We experienced a deep cold snap over the winter which provides a natural culling of insect populations. Thunderstorms also create a decrease in population levels by physically knocking them down. Unfortunately, cooler, moister weather can perpetuate the aphid cycle and they can grow.

Aside from the inconvenience they cause, aphids don’t do much harm to plants, and insects, including lady bugs use them as a food source.

This article provides some further facts and tips on dealing with aphids.

Learn more about tree pests and diseases in Calgary.

We know that Ward 12 loves their trees. Don't hesitate to reach out with questions or suggestions. The Parks & Open Spaces business unit does a lot with meagre funding and our urban canopy has suffered due to changing environmental conditions and a lack of care. 

Evan Spencer

Cllr. Ward 12

[email protected]