Help! I have a spider mite infestation on the snowball
viburnum growing in my yard. We live in Toronto, Ontario
and the soil is basically clay, which we enrich with triple
mix yearly. The spider mites must have overwintered on the bark/branches of the tree and we did not even notice the damaged leaves until this spring when the snowballs were about to bloom. I sprayed the tree with a 3-in-1
miticide/fungicide/insect repellant – to no effect so far.
What would you recommend for treating this tree?
Can it be saved? Thank you in advance for your response
to my questions.
Spider mites, a common garden and landscape pest, can overwinter in trees and shrubs in Ontario, and indeed, can survive extremes of unfavourable weather conditions. Although Viburnum opulum is considered reasonably resistant to pests, it is possible that your Snowball Viburnum may have become weakened by the conditions of this past winter, and thus more vulnerable. Spider mite infestations are also attributed to plants under stress from drought conditions.
Although it may seem counterintuitive, pesticides can actually increase the spider mite population if they are toxic to the natural predators of the spider mite. These predators, including other predatory mites, thrips and the aptly named spider mite destroyer lady beetle, provide natural control of the spider mite population. In Ontario apple orchards, resistance by spider mites to miticides has been documented and is a concern.
Spider mites prefer hot, dry and dusty conditions. Many experts recommend using a strong, forceful stream of water aimed at the both the underside and the tops of the leaves to dislodge the mites and destroy their webs and eggs. This should be repeated regularly until you notice a decrease in the spider mite population. Insecticidal soaps are also said to be effective and are less toxic to beneficial insects.
An infestation of spider mites should not kill your Snowball, but repeated episodes may continue to weaken it, so it is important to continue to do what you are already doing: amend your soil with compost and other organic matter, make sure that the surrounding area is not dry and dusty (ideal spider mite breeding conditions) and ensure that your Viburnum is well irrigated.
The Ontario Ministry of Agriculture, Food and Rural Affairs has a publication on spider mites, produced specifically for apple growers, but interesting all the same: https://www.omafra.gov.on.ca/english/crops/facts/mite.htm