European elm scale (EES) is a pest that normally attacks fruit and ornamental trees.
Controlling elm scale
There are several ways we can control elm scale populations. Early and late season control involves applying dormant or horticultural oil in order to suffocate active nymphs. Early season control is applied shortly after tree bud-break, prior to the production of elm scale adults and crawlers. Late season control is undertaken just prior to leaf drop as nymphs move back to the branches. During peak crawler numbers, applying a soap and permethrin mix known as Trounce® will significantly reduce crawler numbers.
The City of Calgary is conducting an injection program to treat select, highly infested Elm trees with a systemic insecticide. This control approach permits the control of all parts of the pests lifecycle.
About elm scale
In order to understand how elm scale affects Calgary’s trees, it’s important to understand the lifecycle of the insect. Elm scale eggs begin to hatch in late June and early July each year. The early stages are known as nymphs and the first nymphal stage are crawlers. Crawlers begin feeding on leaves and crawler numbers peak in mid-July. By the beginning of autumn, they move onto branches and twigs where they prepare to overwinter - nymphs are forced to initiate overwintering prior to leaf drop. Elm scale feeds by piercing plant tissue (leaves and bark) and utilizing plant juices.
In Calgary, Elm trees are particularly affected and both the nymph and adult stages inflict damage to the tree.
How to tell if elm scale is present
Damage symptoms are not readily apparent on Elm trees unless the infestation level is extremely high. Damage consists of leaf discoloration and premature leaf drop, as well as twig and branch dieback. The presence of this insect is also detected by production of a sticky honeydew material falling as fine droplets from the tree canopy.