Guide

Look closerInsects

Bagworm bags on arborvitae, juniper and spruce

November to March

Evergreen bagworm caterpillars build a hanging bag up to about 2 inches long and carry it as a shelter. In cold months many of the empty-looking bags on arborvitae, juniper and spruce hold the next generation's eggs.

Why this time of year

The bags stay on the branches all winter, with eggs sealed inside, and the eggs do not hatch until warm weather arrives in late spring.

How to notice it

  1. Walk slowly around an arborvitae, juniper or spruce in a yard or along a hedge and scan the branches for small brown bags hanging from the twigs, like tiny ornaments.
  2. Look closely at one bag from arm's length: notice its spindle shape, tapering toward the ends, and how firmly it is fastened to the twig.
  3. Count how many bags hang on one branch, then on the whole tree, to see whether a single tree is crowded with them.
  4. If the tree is yours and small, the bags can be picked by hand before the eggs hatch and dropped in a bucket of soapy water. Wash your hands afterward.

Try this with kids: Count the bags on one branch without touching them. Are they all the same size?

Word to know: A “bagworm” is a moth whose caterpillar builds and carries a portable bag, and the wingless female lays her eggs inside it.

Safety

  • Keep feet on the ground; do not climb or use a ladder to reach high bags.
  • Wash hands after handling bags.

Sources

  • umass.eduUMass Extension landscape insect guide, bagworm (Thyridopteryx ephemeraeformis): Massachusetts page; 'Extended warm autumns enhance its survival in New England.'; hosts include arborvitae and juniper; 'Females can produce 500-1,000 eggs which overwinter in the bag.'; 'bags may be handpicked from the host and destroyed'; hatch guide '600-900 GDD's, Base 50F, March 1st Start Date' (about mid-June). Pesticides listed there are not used here. Refute pass also read: 'Spindle-shaped bags' that 'may remain on the plant even after management has been successful'; bags '1.18-2 inches'; range 'eastern and parts of central US'.
  • yardandgarden.extension.iastate.eduIowa State Extension: 'The eggs remain in the bags on the trees until the following spring.'; bags removable by hand 'anytime before June'; hosts most often arborvitae, red cedar, juniper, spruce; bags reach almost 2 inches by late summer; hatch about mid-June.
  • extension.entm.purdue.eduPurdue Extension: 'Early in June, the insects hatch from eggs which wintered in the old bags attached to tree branches.'; 'simply pick the bagworms off and drown them in a bucket of soapy water'; bags about 2 inches long. Refute pass also read: 'The wingless female never leaves the bag and is fertilized by the winged male'; bags attached 'firmly to branches'.
  • api.inaturalist.orgPresence evidence only (iNaturalist API, taxon 121594, queried 2026-10-30): 302 research-grade observations in a rough eastern Massachusetts box (lat 41.2-42.9, lng -71.4 to -69.9; edges may clip slivers of RI and NH), 353 statewide; the 200 newest span 2023-02-10 to 2026-09-04, mostly Boston, Quincy, Arlington, Somerville, Attleboro, Norton, Medford, Cambridge, Woburn; 56 of the 302 observed Dec-Feb. Refute pass re-ran the query 2026-10-30: 302 in the box, 353 in Massachusetts; by month in the box Nov 13, Dec 21, Jan 19, Feb 16; 3 of the newest 200 tagged RI.

Checked against these sources on October 30, 2026.

In the guide for November, December, January, February, March

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