Bagworms
Thyridopteryx ephemeraeformis
Cones of silk and stuck-on foliage hanging from an arborvitae, each one a caterpillar in a case. They pass for cones until an evergreen is dead.
How to know it
A spindle-shaped bag two to five centimeters long, hanging from a twig and covered in fragments of the plant it is on, so it looks like a small ragged cone. Inside is a caterpillar that carries the bag around with it, poking its head and legs out to walk and feed. On a conifer they are extraordinarily easy to overlook.
What you'll see
- Small hanging cones on arborvitae, juniper, spruce or pine that turn out to be silk rather than wood.
- Bags moving very slowly along a branch, or a head visible at the open top end.
- Foliage browning and thinning in patches, worst at the top of the plant.
- Whole branches of an arborvitae or juniper dead, which on a conifer does not regrow.
- Dozens of old bags hanging on through the winter, each full of eggs.
Where it comes from
One generation a year. Eggs overwinter inside the mother bag and hatch in late spring; the tiny caterpillars balloon out on silk and immediately start building their own case. They feed and enlarge it all summer, then fix it to a twig and pupate. The male emerges as a small black moth; the female never leaves her bag, never develops wings, and lays inside it — which is why an infestation on one plant stays and multiplies rather than dispersing.
Warm regions and evergreen hedges and specimens, especially arborvitae, juniper and spruce planted in rows. Because the female cannot fly, a heavily infested plant reinfests itself year after year until it dies.
Dealing with it
- Pick the bags off by hand between autumn and mid-spring and burn or bin them — each one holds several hundred eggs, and this is the single most effective control there is.
- Cut the silk band cleanly; if you pull, you leave a girdling loop of silk around the twig.
- Spray Bt in late spring, about a fortnight after the young caterpillars emerge and while the bags are under a centimeter; after that nothing penetrates.
- Watch the timing rather than the calendar; emergence is late May to mid-June in most of the range.
- Accept that dead conifer branches will not refurnish, and prune them out.
Keeping it out
- Inspect evergreens every winter and pick off every bag you can find.
- Check new nursery evergreens carefully before planting, since bags travel on stock.
- Plant a mix rather than a single-species hedge of arborvitae.
- Keep the plants healthy and watered; a stressed evergreen dies far faster under the same load.
- Encourage the parasitic wasps and birds, which do take a real toll on smaller populations.
What it goes for
Arborvitae, juniper, spruce, pine and cypress above all, and over a hundred broad-leaved trees including honeylocust and holly.
Who hunts it for you
- Garden birdsPasseriformes Both things are true: birds strip your currants and a brood of blue tits takes several thousand caterpillars out of one fruit tree in a fortnight. Net what you must and welcome them everywhere else — a nest box near the apples is worth more than a spray. Grow to recruit it: Echinacea,Elderberry,English holly,Goldenrod,Rudbeckia,Sunflower
- Parasitic waspsBraconidae & Trichogramma Tiny, stingless wasps that lay eggs in or on caterpillars, aphids, and moth eggs — the white cocoons riding a hornworm are their calling card, so leave that caterpillar be. Umbel flowers with shallow nectar keep the adults fuelled. Grow to recruit it: Cilantro,Dill,Fennel,Lovage,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com