Larvae of the snailcase bagworm cause minimal feeding damage to leaves, and they are not considered economic pests of wine grapes. The large aggregations are easily detected on buildings, posts, wires and the trunks of vines.
This introduced moth (Apterona helix) remains, throughout most of its life, within a snail-like case made from particles of soil and sand bound together with silken threads. When fully grown, the coiled case is approximately 4 mm in diameter.
There are no males, and the wingless female moths remain inside the case to deposit their eggs without having mated (parthenogenetic reproduction). Large aggregations often form on posts, tree trunks or vine trunks in late summer.
Young caterpillars overwinter within the case formed by the mother. They drop to the ground in spring, form C-shaped cases, and begin to feed on a wide range of weeds, native plants and cultivated crops. Feeding damage consists of small excavated holes on the leaf surface.
As the larva grows, it eventually expands its case into the typical snail-like, helical shape. In mid-summer, the fully grown larvae will climb vines, posts or buildings, attach themselves firmly with silk, and pupate within the case. Attachment to overhead sprinkler rotators can occasionally cause them to malfunction.
Several weeks later, the females emerge, lay eggs and then exit the case to die. The eggs hatch in late summer, and the small first instar larvae spend the winter within the protective cases.
There is only one generation per year.
If contamination of table grapes is a concern, inspect vines and apply control measures before the overwintering aggregations form in late summer. The non-feeding overwintering stage, secure within the case, is not susceptible to insecticides.
For more information about chemical control, see the Pesticides page.
Because it has not yet been adequately studied, natural enemies of this pest in B.C. are unknown. Its protective case would provide some shelter from certain generalist predators, but predators, parasitoids and diseases undoubtedly take their toll.
Snailcase bagworm prefer moist areas with thick vegetation. Avoid excessive irrigation, and drain areas that puddle or remain damp. Bagworm are uncommon in vineyards planted on light soils with drip irrigation.
Proper drainage will also help reduce grapevine root diseases. Frequent mowing of areas of the vineyard where bagworm are a problem can help reduce infestations.
