In July, Canadian Forest Service staff became aware of an insect infestation occurring on wild and cultivated saskatoon berry bushes in and around the Lac La Biche area.
Lac La Biche is situated in the east-central area of Alberta’s central mixed wood boreal forest region, a zone that typically contains poplars, white birch, white spruce, and shrubs including native fruits such as saskatoons.
Based upon leaf symptoms and a photo of an unusual adult moth taken in May by amateur photographer Stan Gosche, we suspect that maple leafcutters (Paraclemensia acerifoliella) are the insects causing the damage.
This species is not normally known to be found outside of southeastern Canada on its main host sugar maple (Acer saccharum), and it has never been reported on saskatoon. However, there are historical reports of maple leafcutter in Riding Mountain National Park in Manitoba and Kaslo, B. C. There are also unconfirmed historical reports of similar feeding damage from birch trees in Saskatchewan.
It is possible that the Alberta outbreak could be a related, unnamed species. Positive identification awaits DNA analysis and examination of next spring’s adult moths.
These insects are moths of the Incurvariidae family, which are also known as “leafcutters.” The damage to leaves occurs when the insect is in the form of a tiny caterpillar, about six mm long. The caterpillars cut small ovals of leaf, and then construct a protective case from the cut pieces. The caterpillar fastens this case to the leaf surface, lives inside it, and skeletonizes the leaf (strips away the green tissue to leave only the leaf veins).
The caterpillar feeds at the edges of its oval case so the skeletonizing has a characteristic curved or ringed shape, which can coalesce into large areas if many caterpillars are present. The damage becomes apparent in mid-July, and continues until mid-August, at which point the larvae drop to the ground in their cases.
The caterpillars then turn into small, dark pupae, which change into moths the following spring to repeat the cycle.