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Tree diseases in Quebec

Updated: Mar 20, 2023

Maladies des arbres du Québec

The Quebec Tree Diseases site provides you with all the information you need to identify the diseases that affect trees in Quebec

Quelques maladies connues

Thilleul gall - ash gall - rose disease

Thilleul gall - ash gall - rose disease

Red spiders (Tetranychus urticae) on their web: at this stage, the infestation is very advanced. Photo: David Cappaert, Michigan State University

The little mite known as a red spider is neither red (or only very rarely) nor a spider. It is called a "spider" because it spins fine webs when present in large numbers. And if it sometimes reddens in the fall, its coloring is generation more innocuous (beige, greenish, etc.) the rest of the year.

There are in fact several mites that are commonly called red spiders. The two-spotted spider mite (Tetranychus urticae), which infests both houseplants and garden plants, is the best known, but there are several other small mites with similar lifestyles: the spruce spider mite (Oligonychus ununguis), which prefers spruce, pine, and other needle-bearing conifers, the red apple mite (Panonychus ulmi), which is more common on fruit trees, and several others.

Red spider mites are often seen on tomatoes or squash or cedar hedges and roses, but in fact the range of hosts is very wide and they can theoretically be found on any plant.

These pest mites damage plants by boring tiny holes in the leaves and stems and sucking the sap from them, causing first small yellow spikes, then more generalized yellowing and eventually browning and dropping. The leaves and needles often look dusty. In very severe cases, they can kill the host plant.

Tap the branch and red spiders will fall onto the white sheet where they will be more visible. Photo: Jill O'Donnell, Michigan State University

Or, hold a sheet of white paper under a potentially infested branch and tap it. If the dust that falls on it starts to move, it's probably red spiders.

Red spiders are not insects and do not have wings. So they do not fly, but they are so light that the wind can carry them from one plant to another. They can also travel on clothes, tools or animals that brush against them or, since they are mobile, simply walk to a new plant.

When it's hot and dry, red spiders multiply rapidly. Illus. Clipart Panda

Associated with drought and heat

Red spiders are present throughout the growing season, but rarely cause problems under normal circumstances, as rain, the moths' worst enemy, washes the plant of most, lowering the population.

They proliferate during periods of drought and heat, multiplying at high speed. T. urticae, for example, goes from egg to adult in 36 days in cool weather; in 7 days in hot weather. Since each female can lay up to 100 eggs, it is understandable that the population can grow from a few hundred individuals that show no symptoms to hundreds of thousands or even millions, a number capable of killing an entire shrub in just a few weeks.

Le traitement le plus simple

A strong spray is often all it takes to chase away red spiders.

When rain is not available, simply spray infested plants with a strong stream of water. Direct the spray onto the webs to destroy them and rinse the foliage, branches and trunk thoroughly. Repeat weekly if dryness persists.

You can also spray with insecticidal soap or black soap. Note that, despite a persistent popular belief, so-called "dish soaps" no longer contain soap (they are more like detergents) and dish soap solutions will not be much more effective than a simple water treatment.

It is also possible to treat with a horticultural oil... but read the instructions carefully, because applying this product in very hot weather can damage the plant.

It may be useful to remove the very affected branches: if they are very damaged, they will not recover anyway.

There are also insecticides that are at the same time acaricides and that can then control red spiders. Neem oil, an organic product, is very effective, but is no longer available on the Canadian and French markets. Various chemical insecticides/acaricides are available in garden centers, but it is better to start with water treatments, which are less dangerous for you and for the environment.


It is essentially impossible to completely prevent red spiders: they are ubiquitous in nature. Even if you don't see them, they are around. But we can help keep the population so small that they don't cause harm.

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