Royal Alberta Museum: Invertebrate Zoology - Bug Facts

[Caterpillars and Other Immature Insects]

Fact Sheets: Commonly Observed and Asked About Insects and Spiders Found in Alberta

Bagworm Moth (Dahlica triquetrella)

Bagworm moth showing the variety of debris on its bag
Credit: Terry Thormin

INTRODUCTION

Every fall in central Alberta the larvae of the bagworm moth, Dahlica triquetrella, are found on the sides of some buildings, often in large numbers, as they search for protected places to over winter.

IDENTIFICATION

It is the larval stage of this species that is most often noticed. The caterpillars make a silken bag into which they retreat. At maximum size they are about 7 to 8 mm (1/4") long. The bag is triangular in cross-section and covered with fine debris such as grains of sand and dust, and on occasion includes such things as the shells of small land snails.

DISTRIBUTION

This is a recent invader to Alberta, being native to Europe. In Canada, it was first noticed in 1941 in Vernon, British Columbia. It was only in the mid-1990s that it was first observed in Alberta. It is expected that this species will eventually move across most of North America.

TIME OF YEAR

Although bagworms are present from spring to fall, it is usually not until the fall, when the fully-grown larvae start to climb the sides of buildings in numbers, looking for a place to hibernate, that they are commonly seen. Larvae may be observed like this well into October.

HABITAT AND HABITS

The caterpillars of the bagworm moths feed on lichens. Although they are most common on trees, they are found on the sides of buildings, especially any building that might support a growth of lichens. The larvae pupate inside the bag in mid-April and the adult moths come out of the bag, and remain on the bag to lay their eggs, depositing them into the bag through the sides. The adults don't feed and probably only live three or four days. Once the eggs hatch, the larvae vacate the parent's bag and create their own bag. The population in North America is parthenogenic, that is there are no males in the population, and the females produce viable eggs without ever mating. In Europe there are both parthenogenic populations and populations that reproduce sexually. The males in the latter populations have functional wings.

SIMILAR SPECIES

Another species of bagworm moth, Dahlica walshella, is found in southern Alberta, in the Cypress Hills and Waterton Lakes National Park. It is almost identical in appearance to D. triquetrella, but reproduces sexually, so there are males in the population. There are no common names at the species level for bagworm moths, and the name applies to all members of the family Psychidae.

COMMENTS

Because bagworm moths feed exclusively on lichens, they are not considered a pest species. Having hundreds of these creatures climbing the sides on one's house, however, is not desirable. We would suggest that if you have this problem and want to get rid of them, you hit them with a hard blast of cold water. This may have to be done several times over a period of a few days as many of the ones you knock off may just climb back up.

Note: We cannot provide advice on controlling insect pests. If you need more information beyond what has been provided here, we suggest you either search further on the web, or talk to a local exterminator.

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Last update: August 19 2004