You might have noticed a lot of moths around lately (June 2017). You also may have noticed the brown ridgetops all along the north side of the West Bragg Creek Road from Saddle & Sirloin to Twopines and on to Wintergreen. The moths are also present quite far south along Highway 762. The trees should be brilliant green at this time of year. But, they are brown because there aren’t leaves on the trees. It’s a minor infestation, causing the defoliation of the Aspen trees.
According to Mary Reid and Ralph Cartar, two local entomologists and biologists – scientists, there are two culprits – the Bruce Spanworm and the Large Aspen Tortrix. Why is this? It could be the winter conditions, the abundance of food supply, moisture and maybe they were able to escape predators. Whatever the reason, last year’s moths were able to produce a large number of hungry caterpillar larvae this year. These larvae hatch from eggs then are deposited on the leaves of the Aspen (in the case of the tortrix), or in cracks or moss at the base of the trees (in the case of the spanworm). Larvae eat the leaves, the effect we see from afar. Often, larvae produce a retreat formed from two leaves wrapped together, or a single leaf partially rolled up. Sometimes they overwhelm small saplings or bushes and kill them. As they develop they curl up in the leaf and pupate to transform into a moth (tortrix), or drop to the ground and pupate in the leaf litter (spanworm). That’s an incredible transition from creepy crawler worm into a flying moth. Anyway, the moths don’t eat much. Most of the damage is done by caterpillars.
Large Aspen Tortrix have one generation each year. They overwinter as larvae and start feeding on buds before the leaves form. They have a wingspan of 25-35 mm. The Bruce Spanworm has a similar life cycle and size, but overwinters as adults, and only males have wings. You can distinguish the two species by examining for 3 regularly-spaced bands of dark grey in the wing: the tortrix has them, while the spanworm doesn’t.
There have been several severe outbreaks of Bruce Spanworm since 1903. In 1958, the outbreak covered 130,000 hectares of aspen forest in Alberta. In general, severe defoliation lasts 2-3 years. That these two bugs are here at the same time, is quite normal. Maybe it’s a conspiracy.
Eventually, this will end. The trees will likely survive; in fact, some trees are now producing a second growth of new leaves. The bugs will succumb to disease or predation. Apparently, spanworms are the same kind of moths that we typically see later in the fall, after the first frosts.
For more information see:
A field guide to Forest Insects and Diseases of the Prairie Provinces.
by Y. Hiratsuka, D.W. Langor, and P.E. Crane. 1995.
Natural Resources Canada