Snailcase Bagworms - June 15, 2011
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Last week, the Master Gardener on duty in the Prescott Cooperative Extension office was looking at a jar full of unique insects that had been brought in to our Prescott office and were eating vegetable seedlings. After inspecting them under the dissecting scope, I heard our Master Gardener say “it looks like a hermit crab moving around in its own little shell”. I looked and thought it resembled a bagworm because it was a somewhat secretive caterpillar living inside a snail-like case. We looked at through some books and on-line resources where we learned that it was a snailcase bagworm (Apterona helix). I had never seen one before!

The snailcase bagworm is an unusual moth in the family Psychidae. It is a native to Europe that may have been somewhat recently been introduced into Arizona. The insects have been in Colorado for several years and are expanding their range in that state. The developing insects feed on a wide variety of plants but rarely cause significant plant injury. Instead, problems occur as the full-grown larvae migrate to sites to pupate. During this time, large numbers may firmly attach themselves to sides of buildings, fences, mailboxes and other surfaces, creating a nuisance. I have included several photos below.

All stages of this insect take place within a coiled, snail-like case, approximately 1/8 inch in diameter. The caterpillars are greenish or reddish-gray with a black head. These are difficult to see because they stay inside the above-described case. Adults are wingless and nearly legless moths. Only females are known to occur in Colorado.

Snailcase bagworms survive winter as young caterpillars protected within the case of the mother insect. They become active in midspring and feed on the leaves of a wide variety of native and cultivated plants, including saltbush, rabbitbrush, willow, mountain-mahogany, various fruit trees, squash, tomato, wild mustards, and alfalfa. The feeding injuries appear as small areas progressively gouged out of the leaf surface. Serious plant injury is said to be rare.

As the larvae grow and develop, they produce a snail-like case of silk and soil particles. Later, they push their fecal matter out of an opening in the center of the case, allowing it to pile up on top of the insect. The larval insects are mobile and can carry the case upright. As they become full-grown, typically in late spring and early summer, snailcase bagworms migrate to high, shaded points. There they firmly attach themselves to an available surface and transform to the pupal stage.

Transition to the adult moth takes place in the pupal covering after attachment. As stated above, the moths are wingless, nearly legless, and do not feed. Only females are produced, but they can fertilize eggs asexually. About one to two dozen eggs are produced by the female. During midsummer, these eggs hatch. However, the young larvae remain in a dormant condition within the pupal covering throughout the winter. They emerge from the case the following spring and disperse, probably with the aid of wind, to new host plants.

I’d like to credit Dr. Whitney Cranshaw, Colorado State University Entomologist and Extension Specialist, for the above information on the life history of the snailcase bagworm. He also took the photos displayed on the web version of this column and will be a keynote speaker at the upcoming Arizona Highlands Garden Conference that will be held on October 22, 2011 at the Prescott Resort. Dr, Cranshaw is a legend in horticultural entomology and an excellent speaker. The conference agenda and registration information will be available very soon.

I have also sent the samples of pupating snailcase bagworms to Carl Olson, University of Arizona Insect Collection Curator, for further study and to be added to the collection. Carl is also checking with the Arizona Department of Agriculture to see if they have any previous records of the snailcase bagworm in Arizona. If anyone has seen these insects in the Verde Valley, I would appreciate knowing about it. Please e-mail me at or call 928-445-6590 ext. 224.

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Snailcase bagworms (photo by: Whitney Cranshaw).

Snailcase bagworm damage on tomato (photo by: Whitney Cranshaw).

Snailcase bagworms pupating under eaves (photo by: Whitney Cranshaw).

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: June 9, 2011
Content Questions/CJune 9,

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