Which Bt Kills What: A Plain Guide to Bacillus thuringiensis

The short answer

Bt stands for Bacillus thuringiensis, a bacterium that lives in soil. Different strains produce different toxins, and each toxin only works on a narrow group of insects. The label will name the strain in small print, usually near the active ingredients.

Match the strain to the pest:

• Bt kurstaki (often written Btk): caterpillars only. Cabbage worms, tomato hornworms, corn earworm, cutworms, squash vine borer, European corn borer.
• Bt tenebrionis (also sold as the San Diego strain): certain beetle larvae. In a vegetable garden that means Colorado potato beetle grubs.
• Bt israelensis (Bti): larvae of mosquitoes, fungus gnats, and black flies. This is what mosquito dunks are. It does nothing on plants.
• The one on the shelf at most garden centers is kurstaki. If the label does not name a strain, assume kurstaki and assume it is for caterpillars.

Bt only works if the pest eats it

This is the single fact that explains almost every Bt failure. Bt is a stomach poison, not a contact spray. The insect has to chew treated plant tissue and swallow it. Once eaten, the toxin ruptures the gut lining and the caterpillar stops feeding within hours and dies in a day or two.

Three things follow from that, and all three are practical:

• Insects that pierce and suck instead of chewing never ingest it. That rules out aphids, squash bugs, thrips, and spider mites completely.
• Coverage is everything. Bt has to be on the leaf surface the pest is actually eating, which is usually the underside. A spray that only wets the tops of leaves mostly misses.
• Anything hidden inside the plant is protected. Once a squash vine borer or corn borer is tunneling inside a stem, a surface spray cannot reach it.
• Sunlight breaks Bt down in a few days, so it does not build up. Reapply on the label's schedule and after heavy rain.

What Bt will not kill

This list matters more than the first one, because this is where people waste a season. None of these are touched by any Bt strain sold for garden use:

• Squash bugs. They pierce and suck sap, so they never eat the spray. Bt is useless on them at any dose.
• Aphids, thrips, and spider mites. Same reason. All sap feeders.
• Adult beetles of any kind, including cucumber beetles, Mexican bean beetles, Japanese beetles, and flea beetles.
• Slugs and snails. Not insects at all.
• Colorado potato beetle, if you bought ordinary kurstaki. You need the tenebrionis strain specifically.

Squash bugs and squash vine borers are not the same pest

This is the mixup behind most of the bad Bt advice about squash, and the two pests could not be more different.

Squash bugs are flat, gray-brown, shield-shaped insects that cluster on leaf undersides and lay bronze egg rows. They suck sap and wilt the leaves. Bt does nothing to them. What works is hand-picking, scraping the egg clusters off twice a week to beat the 10-day hatch, a board laid flat overnight to trap them at dawn, and insecticidal soap or neem on the small gray nymphs.

Squash vine borer is completely different. It is the larva of a moth, which makes it a caterpillar, which means Bt does work on it. The catch is timing. Once the grub is inside the vine it is out of reach, so surface sprays only help during egg hatch. After it is in, the fix is to slit the stem and remove the grub, or inject Bt or spinosad into the stem a couple of inches above the entry hole with a syringe, following the product label.

So if your squash is wilting, identify which one you have before you buy anything. Our pest guides for both are linked at the bottom.

How to spray it so it works

Assuming you have caterpillars and kurstaki, the application is where most of the remaining failures happen.

• Spray small caterpillars. Bt hits young larvae hardest, and a big fat hornworm may shrug it off. Time it to fresh egg hatch.
• Hit the leaf undersides. That is where the eggs are and where the small caterpillars feed.
• Spray in the evening or early morning. Sunlight degrades it, and cooler hours are also easier on pollinators.
• Expect feeding to stop within hours and death in a day or two. The caterpillar staying put on the leaf for a while is normal, it has already stopped eating.
• Reapply per the label after rain or every 5 to 7 days while eggs keep hatching.
• Mix only what you will use that day. Bt does not store well once mixed.

The honest downside

Bt kurstaki does not tell good caterpillars from bad ones. It kills every caterpillar that eats it, including monarch larvae on milkweed and black swallowtail larvae on dill, parsley, carrot, and fennel.

That last one is worth stopping on. The striped caterpillar on your dill is a parsleyworm, and it becomes a black swallowtail butterfly. On most home plantings the damage is trivial. Grow a little extra dill or parsley, move the caterpillar to it, and let it finish. Do not blanket-spray herbs with Bt unless you have a real infestation you cannot hand-pick.

The test: if you can pick the caterpillars off in under five minutes, do that instead. Bt is for when the count beats your hands.

Plan the garden so you spray less

Most caterpillar damage is predictable by crop and by week. Cabbage worms show up on brassicas, hornworms on tomatoes, borers on squash at a fairly reliable point in the season.

The planner lays out what you are growing against your ZIP code and frost dates, so you know which weeks to be checking leaf undersides on which crops. Row cover put on before the moths arrive prevents more caterpillars than any spray kills after.