I recently discovered an aphid infestation on my 2 Golden Full Moon Maples. I’ve been spraying the undersides of leaves with water to deal with them. Then I noticed ants on the trunks that congregated around knots. That’s when I discovered what appear to be Calico scale. Please advise if this ID is correct and if applying horticultural oil to trunk and branches in fall will help. Thank you.
Thank you for contacting Toronto Master Gardeners.
It would appear that this is Calico scale (Eulecanium cerasorum), although we suggest you confirm the diagnosis with an arborist who can advise on the appropriate treatment.
The Calico scale, an invasive soft scale, is a severe pest of maples, sweetgum, honeylocust, crabapple, and other tree species in the eastern United States. Nymphs overwinter on bark, producing copious honeydew as they mature in spring. Eggs hatch in late May, approximately a week after adult coloration fades from bright black and white to dull brown, and crawlers migrate to leaves where they feed until autumn.
Based on information found, several foliar insecticides are active against settled crawlers, but insecticidal soap and summer or dormant oils were ineffective. An arborist can best advise and provide the appropriate treatment if this is calico scale. You can find an arborist in your area through the Landscape Ontario website noted below: