Brief Summary

The snailcase bagworm (Apterona helicoidella) is a bagworm moth (family Psychidae), also known as the garden bagworm. Like about half of the approx. 1000 known bagworm species, the female snailcase bagworm is wingless. She lives her entire life cycle from egg to caterpillar to adult in a snail-like case that she constructs from soil particles and feces held together with silk. The snailcase bagworm has an interesting life history; clutches of up to about 50 eggs are laid within the mother’s pupal skin. These hatch late summer, and construct their own compartment within the pupal skin in which they go into diapause to overwinter (Davis 1964). In the spring these larvae drop to the ground on silk threads to disperse from the mother’s case, and start to build their own very small cases, which they carry around with them as they graze. Larvae undergo four instar stages, and they expand their case in curlicue whorls as they grow. When mature, the case is about 5mm in diameter and has about three coils. Larvae then pupate into a wingless, legless adult, which remains in her case to complete the cycle and lay eggs within her pupal skin. In most of its range, this species is asexual; all the individuals are female and reproduce parthogenetically (without mating). However, another form of this species, found in northern Europe, has small winged males which escape from a slit they make in the top whorl of their case when they mature. It is suggested that the sexual form may indeed be a distinct species, A. crenulella, but this is unclear. The parthogenetic form is known by two synonymous names: A. helicoidella and also A. helix.

Although native to Europe, the snailcase bagworm (parthogenetic form) was introduced to the western United States in the 1940s, and probably a second introduction from Europe populated the eastern United States in the 1960s. They are polyphagous and feed on an extensive range of host plants. Literature accounts have cited some damage of crops including, legumes, fruit trees, ornamental plants, douglas fir, alfalfa, barley, corn, cruciferous vegetables, sweet clover, and ornamental flower plants, however economic damage is minimal. When the larvae is mature and ready to pupate, it climbs upwards to find a good spot upon which to cement itself using its larval silk, often settling on the walls of houses. At this phase it is very difficult to remove the casing, and this presents a nuisance to humans.

(Jacobs 2003; Wheeler and Hoebeck 1988; Cranshaw 2007; Riands et al. 2009; Wikipedia 2011)

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Apterona helicoidella

The Snailcase Bagworm (Apterona helicoidella) is a moth of the Psychidae family. It is widely distributed in Europe, from Portugal through most of central Europe and the Alps, up to the Ural. It is also found on the Balkan and in Turkey. It was introduced in the United States by accident during the 1940s. It is now found in many mid-Atlantic states, including Pennsylvania, and has also been reported in the Pacific coastal states, as well as Colorado, Michigan, Nevada, Utah and Idaho. It has been collected in Ontario as well (Royal Ontario Museum Collection).

In southern Europe a bisexual form exists with small males that have light grey wings. North of the Alps the parthenogenetic species named Apterona helicoidella parth. is only present as females. They form a case that looks like a small snail. It transforms into a wormlike adult in the late summer. Unlike most bagworms, the case is constructed of soil particles and feces instead of leaves and twigs. The snailcase bagworm begins constructing its case at birth and remains inside for the rest of its life. This species is also very unusual in that it is parthenogenetic; all individuals are females, and they reproduce without mating. Each female produces one to two dozen eggs.

The case of a snailcase bagworm, not surprisingly, resembles a small snail about 4 mm in diameter with coloration similar to the soil. The larvae are greenish or reddish gray with a black head that protrudes from the case to feed.

Though the snailcase bagworm doesn't cause the damage of related species such as the evergreen bagworm, they are a nuisance to humans by attaching to the sides of homes and buildings. Once the larvae are full-grown and moving to pupate, they cannot be controlled with insecticides. The best way to keep them off the sides of buildings is by using temporary barriers like sticky tape or flanges. They can be knocked off buildings with a strong spray of water before they attach. After they are attached, removal is difficult.

They consume small areas of a leaf surface, rarely causing significant damage, though it has been found to feed on most vegetables, ornamentals, legumes, fruit and other trees, and many species of annual herbs. Reported food plants include Potentilla neumanniana, Erodium cicutarium, Teucrium, Artemisia vulgaris, Helianthemum nummularium and Alyssum montanum.

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