Best Organic Caterpillar Control (BT Spray Guide)

Caterpillars — cabbage loopers, tomato hornworms, imported cabbageworms, corn earworms — can strip a bed in days. The good news is that the most effective organic control is also one of the most targeted: Bacillus thuringiensis var. kurstaki (Bt). Bt is a naturally occurring soil bacterium that, when eaten by a caterpillar, releases proteins that stop the insect from feeding within hours. It affects almost nothing else — not bees, not butterflies as adults, not birds, not you.

Below are the Bt products we recommend most often, based on active-ingredient strength, coverage, ease of use, and what owners consistently report, plus a buying guide so you pick the right one the first time.

At a Glance: Our Top Picks

• Best overall: Monterey BT Caterpillar Killer — concentrated, fast-acting, and the one most gardeners keep on the shelf.
• Best value: Bonide Thuricide — the same reliable Bt strain in a big, budget-friendly bottle.
• Best ready-to-spray convenience: Safer Brand Caterpillar Killer — no mixing, straight from the hose-end bottle.
• Best for hard-to-kill caterpillars: Bonide Captain Jack’s Deadbug Brew (spinosad) — a different organic mode of action when Bt alone is not enough.

The Products, Reviewed

1. Monterey BT Caterpillar Killer — Best Overall

What it is: A liquid concentrate of Bacillus thuringiensis var. kurstaki that you dilute and spray on foliage. Monterey’s formulation is widely available and consistently praised for good coverage and shelf life.

Who it is for: Most vegetable gardeners with leafy greens, brassicas, tomatoes, or corn. One bottle lasts most home gardens a season or more.

• Pros: kills actively feeding caterpillars fast; safe for bees, birds, and beneficial insects; concentrated so it is economical.
• Cons: must be sprayed on the foliage the caterpillar is actually eating; washes off in heavy rain and needs reapplication.

2. Bonide Thuricide — Best Value

What it is: The same Btk strain in a liquid concentrate sold at a very accessible price point, including large multi-pack sizes.

Who it is for: Gardeners treating big plantings — whole brassica patches, long rows of beans — where cost per gallon matters.

• Pros: excellent price per dose; effective on the same caterpillar species; easy measuring cap.
• Cons: same coverage limitations as all Bt — timing and thorough spraying are on you.

3. Safer Brand Caterpillar Killer — Best Ready-to-Spray

What it is: A ready-to-use Bt spray, often in a hose-end bottle, so there is no mixing or measuring.

Who it is for: Newer gardeners, or anyone treating a small number of plants who wants a five-minute application.

• Pros: zero setup; great for spot treatment; safe around kids and pets when used as labeled.
• Cons: costs more per gallon than concentrate; hose-end application can overspray and waste product.

4. Bonide Captain Jack’s Deadbug Brew — Best Alternative Mode of Action

What it is: Spinosad, a different organic insecticide derived from soil bacteria. It is one of the few organic options that also tackles sawfly larvae, thrips, and Colorado potato beetle larvae.

Who it is for: Gardeners dealing with multiple chewing pests, or caterpillars that keep surviving repeated Bt sprays.

• Pros: broader target list; also effective on tough foliage feeders; good residual activity.
• Cons: more toxic to bees while wet — spray in the evening; a bit pricier than straight Bt.

How to Spray Bt So It Actually Works

Bt only works if a caterpillar eats it, so coverage and timing decide your results:

• Spray small caterpillars early. Young larvae die quickly; large hornworms are much harder to kill and eat more before they die.
• Cover both leaf surfaces, especially the underside where young caterpillars feed.
• Reapply after rain and every 7–10 days while pests are active, since Bt breaks down in sunlight and water.
• Do not spray butterfly host plants like milkweed or dill unless you have a genuine infestation — Bt kills all caterpillars that eat the treated foliage.
• Spray in the evening to protect bees and slow UV breakdown.

Keeping plants healthy and reducing pest habitat makes sprays work better. No-till gardening preserves the ground beetles and other predators that eat caterpillars and pupae, and good mulching keeps pest pressure lower overall.

Buying Guide

• Check the active ingredient: you want Bacillus thuringiensis var. kurstaki (Bt-k) for caterpillars, not Bt israelensis, which targets mosquito and fungus gnat larvae.
• Concentrate vs. ready-to-use: concentrates are cheaper per spray; RTU bottles win on convenience for small gardens.
• Storage: keep Bt out of heat and freezing; a bottle that sat in a hot shed for two summers may be dead.
• Add a spreader-sticker if you have one; it helps Bt cling to waxy brassica leaves through light rain.

FAQ

Is Bt safe for bees and butterflies?

Bt-k targets caterpillar larvae specifically. Bees, birds, and adult butterflies are not affected by eating sprayed foliage. The honest caveat: it will kill the caterpillars of desirable butterflies that feed on the treated plants, so avoid spraying known host plants.

How fast does Bt work?

Caterpillars stop feeding within hours of eating Bt and usually die within a few days. You will still see them on the plant briefly — they just are not eating anymore.

How often should I reapply Bt?

Every 7–10 days while caterpillars are active, and right after heavy rain. UV light degrades Bt, so evening sprays last longer than midday ones.

Does Bt kill tomato hornworms?

Yes. Bt-k is very effective on hornworms, especially when they are young. For large, late-instar hornworms, hand-picking plus a spinosad product often works better.

Bottom Line

For organic caterpillar control, a Bt concentrate like Monterey or Bonide Thuricide is the standard for good reason: it is effective, affordable, and remarkably safe for everything that is not eating your leaves. Spray early, spray thoroughly, reapply after rain, and the caterpillars stop eating before they can finish your crop.

Check Current Prices on Amazon

Monterey BT Caterpillar Killer on Amazon — Bonide Thuricide — Captain Jack’s Deadbug Brew

Prices change often, so check current listings before you buy.

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