Bacillus thuringiensis (Bt) kills young bagworm larvae when applied at the right time, but it won’t work on large, mature bagworms inside their silk bags.
An arborvitae turning brown can mean bagworms have taken hold, and the window for stopping them with BT for bagworms is narrow but effective. This biological insecticide targets young, feeding larvae, but it only works when you hit the right stage and use the right technique. Understanding how Bt works—and when it doesn’t—keeps you from spraying too late and losing the tree.
How Bt Works on Bagworms
Bt is a biological insecticide that must be eaten to work—it’s not a contact killer. Young bagworm larvae under 1 inch long ingest the treated foliage, and the bacteria attacks their gut, stopping feeding within hours. Death follows within a few days.
The active strain is Bacillus thuringiensis subsp. kurstaki. It’s specific to caterpillars and has no effect on birds, earthworms, honeybees, or people when used as directed. Because Bt kills all caterpillars, not just bagworms, only spray plants with active infestations. Clemson Extension’s IPM guide for evergreen bagworms notes that Bt is safe for beneficial insects when applied correctly.
Once bagworms reach roughly 1 inch and seal themselves inside their silk bags, they stop feeding entirely. Bt has no effect on them at this stage. The bag protects them, and they’re no longer ingesting anything from the foliage. This is why timing matters more than which product you choose.
When to Apply Bt for Bagworms
Timing is everything. The primary application window is late May to mid-June, when eggs hatch and larvae first emerge. A second window opens at the end of June after all eggs have hatched and larvae finish ballooning to new branches.
Apply two treatments spaced 10 to 14 days apart to catch the full hatching window. Scouting weekly from winter through early summer helps you spot new bags before they grow. By mid-July, larvae approach 1 inch and Bt’s effectiveness drops sharply—switch to a chemical option like bifenthrin or permethrin if you’re past that point.
| Timing Window | Larval Stage | Bt Effectiveness |
|---|---|---|
| Late May – mid-June | Newly hatched, under 1/2 inch | High — ideal window |
| Late June (post-hatch) | Under 1 inch, still feeding | Moderate — worth applying |
| After mid-July | Over 1 inch, inside silk bags | Low — use chemical control |
How to Apply Bt the Right Way
Mix liquid Bt concentrate with water in a standard pump sprayer, backpack sprayer, or hose-end sprayer. Use a fan spray setting for uniform coverage. Start at the top of the plant and work down, directing spray into the interior and the undersides of foliage—that’s where larvae feed.
Adding a spreader-sticker improves adhesion and helps the spray penetrate silky webs. Avoid application if rain is expected within 24 hours. Recheck the plant after your two treatments spaced 10 to 14 days apart; if larvae are still active, a third application may be needed. While you’re at it, hand-pick and destroy old bags before new eggs hatch in late May, cutting the silk attachment band rather than just pulling the bag—this prevents branch girdling later.
Choosing the right product matters too. For a head start, the best Bt products for bagworm control can save you time reading labels at the store.
If Bt fails because the timing was off, switch to a chemical insecticide after July 15. Broad-spectrum pyrethroids work on larger larvae, but they’re hazardous to pollinators and should only be used for severe infestations.
FAQs
Is Bt safe for bees and other beneficial insects?
Yes. Bt kurstaki targets only caterpillars and has no effect on honeybees, ladybugs, or earthworms when used as directed. Still, only spray infested plants to avoid harming non-pest caterpillar species that butterflies need.
Can I spray Bt right before rain?
No. Bt needs at least 24 hours of dry weather after application to work. Rain washes it off the foliage before larvae can ingest it, and you’ll have to reapply once the foliage dries.
What should I do if Bt doesn’t stop the bagworms?
If larvae are already over 1 inch or it’s past mid-July, switch to a chemical insecticide containing bifenthrin, permethrin, or cyfluthrin. These work on larger larvae but are harder on beneficial insects—reserve them for severe infestations only.
References & Sources
- Clemson Cooperative Extension. “Integrated Pest Management (I.P.M.) for Evergreen Bagworms.” Covers Bt timing, application, and safety for beneficial insects.