There are two species of elm leafminers in Grande Prairie:
- Fenusa elm leafminers (Fenusa ulmi) damage elm leaves during June and July. Their feeding damage crosses leaf veins, leaving large portions of the leaf brown and papery thin. Larvae are 4-5 mm long and adults are small, black, stingless wasps. As yet, these leafminers have only been found on Siberian elm in Grande Prairie. Outbreaks seem to follow aggressive pruning regimes or wounding.
- Agromyzid elm leafminers (Agromyza aristata) are active in May, almost as soon as the leaves emerge. They cause substantially less damage than Feunsa leafminers, since they create serpentine galleries, which don't generally join together but wind their way through the leaf. They are also smaller larvae than Fenusa, only 2-3mm long. Adults are small flies. These leafminers are found on both species of elm in Grande Prairie.
Watch for mining activity within elm leaves beginning in May. Agromyzid leafminers rarely cause severe damage, as their mines are so small. Fenusa leafminers can cause severe damage to trees in high populations, as their mines destroy large portions of leaf surface.
Agromyzid elm leafminers cause so little damage that no controls are necessary.
If only a few leaves are affected by Fenusa leafminers, they may be picked off and destroyed. Avoid aggressive pruning on your Siberian elm, as this triggers outbreak of the Fenusa leafminer. Keep your tree well watered and reduce stress as much as possible.