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Bagworms can strip a tree or shrub bare in weeks, and by the time you spot them, they have already built their tough little pouches. Biological sprays using Bacillus thuringiensis (a natural soil bacterium) work by targeting only leaf-chewing caterpillars, making them your cleanest, most effective weapon for this specific pest. This guide walks you through the concentrated liquids, ready-to-use bottles, and powdered options that actually stop bagworms — without worrying about toxic drift on your vegetables or pets.
I’m Rikta — the founder and writer behind Lawn Gear Lab. This guide is built by comparing the manufacturers’ published specifications and the patterns across verified customer reviews, so you get each pick’s real strengths and trade-offs instead of marketing spin.
It covers six products that are proven to work on bagworms, from a compact pint for a few small trees to a full gallon for larger properties, so you can match the right bt for bagworms to your specific yard size and sprayer setup.
Our Picks at a Glance
How To Choose The Best BT For Bagworms
Bagworms build tough silk pouches that shield them from many chemical sprays, but Bacillus thuringiensis (a natural bacterium that attacks caterpillar guts) works because they eat treated foliage. The right product for you depends on the size of your property, how you want to apply it, and whether you prefer a concentrate or a bottle you can grab and spray.
Liquid volume and coverage area
A small bottle like a 16-fluid-ounce pint is plenty if you are treating a couple of ornamentals or a few small trees. If you have a large row of arborvitae or acreage, a gallon jug (128 fluid ounces) of concentrate that mixes at 1 to 4 ounces per gallon of water will stretch much further and save you trips to the store.
Concentrate vs. ready-to-use vs. powder
Concentrates (you mix with water) are the most economical for larger yards, but require a sprayer. Ready-to-use (RTU) bottles let you spray immediately from a trigger bottle, ideal for quick spot treatments on a few bushes. Powders disperse in water and often come in larger bags for serious coverage—just check that your sprayer can handle dissolved solids without clogging.
Active ingredients and OMRI listing
All effective bagworm products use the *kurstaki* subspecies of Bt (Bacillus thuringiensis subsp. kurstaki, or Btk). An OMRI (Organic Materials Review Institute) listed product confirms it meets organic gardening standards, so you can spray edibles right up to harvest with no time restrictions.
Quick Comparison
| Model | Best For | Volume | Form | Item Weight | Amazon |
|---|---|---|---|---|---|
| Southern Ag Thuricide 16 oz (Pint)★ Best Overall | Small gardens with a few bagworm trees | 16 oz | Liquid concentrate | 9.9 oz | Amazon |
| Monterey B.t. 32 oz (RTU)Hose-End Power | Large-scale spot treatment with hose-end ease | 32 oz | Ready-to-spray | 2.46 lb | Amazon |
| Valent Dipel Pro DF 1lb Bag | Serious powder firepower for acres | 1 lb | Powder | — | Amazon |
| Southern Ag Thuricide 1 Gal | Long-run economy for big properties | 128 oz (1 gal) | Liquid concentrate | — | Amazon |
| Monterey BT 16 oz (Concentrate) | Teardrop bagworms on smaller trees | 16 oz | Liquid concentrate | 1 lb | Amazon |
| Fertilome Caterpillar Killer RTU 32 oz | Immediate grab-and-go for bagworm breakouts | 32 oz | Ready-to-use | 1.5 lb | Amazon |
In‑Depth Reviews
1. Southern Ag Thuricide BT Caterpillar Control, 16oz – Pint
Our pick — over 4.5★ from 3,500+ verified ratings; the strongest balance of quality and price.
The smallest, lightest bottle that still gives you full organic Bt power.
At 9.9 ounces of liquid (16 fluid ounces in volume), this pint is the lightest weight on the list and the most compact option for a first-time buyer. Southern Ag Thuricide concentrate uses the Btk strain (Bacillus Thuringiensis Subsp. Kurstaki Strain SA-12) and is OMRI certified for organic use — safe on all edible crops right up to harvest. It is ideal for a few bagworm-infested shrubs or a small vegetable patch where caterpillars have moved in.
Reviewers point out it is fast-acting: pests stop eating immediately after spraying. One reviewer with collard greens said the product “worked perfectly” after bagworms and caterpillars had shredded their leaves overnight. If you compare it to the Monterey 16-ounce concentrate (1 pound, 16 fluid ounces, with a measuring spoon), the Southern Ag pint is slightly cheaper and already well-proven with over 3,700 ratings, but it does not include a measuring spoon — so you will need to measure the 1 to 4 ounces per gallon yourself.
simple ready-to-use: low initial investment, small footprint, and a massive base of real owner feedback (4.6 out of 5 stars from nearly 4,000 reviews).
limited coverage area: you need a separate sprayer and a measuring tool — and at 16 fluid ounces versus 32 fluid ounces for the Fertilome RTU, you get less total solution from one purchase if you do not dilute properly.
beginners: you have a small garden patch or a couple of bagworm-prone trees and want the cheapest entry into reliable Bt. experienced users: you prefer ready-to-use convenience or are covering more than three to four trees.
2. Monterey – B.t. Bacillus Thuringiensis Bundled with Garden Measuring Spoon – Ready to Spray Worm & Caterpillar Killer Insecticide, OMRI Listed – 32 oz
Attach this 32-ounce bottle to your garden hose and walk the perimeter in minutes.
If you have a long row of arborvitae or a line of shrubs that bagworms keep returning to, the quickest way to cover every branch is a hose-end setup. This Monterey B.t. bottle screws directly onto your garden hose, so the concentrate mixes as you spray — the label says it works on cabbage looper, gypsy moth, fall cankerworm, and bagworm. It is OMRI Listed (certified for organic gardening) and safe for earthworms and honeybees when used as directed.
Buyers report that two sprayings took care of bagworms on their arborvitae, with one owner noting the product generates new growth after the pests die. Unlike the 16-ounce concentrate from Monterey that needs a separate sprayer, this 32-ounce ready-to-spray format (the same brand, but in a larger hose-end container) means you never have to mix a drop — just set the dial and move.
A reviewer who bought a second bottle described his wife seeing the results on trees that had lost their leaves: the bagworms died and fresh leaves started growing back. The primary catch is the 2.46-pound filled bottle size — you carry the whole setup, so small-target spraying on a single rose bush feels a bit heavy-handed. But for covering a whole hedge line? This is the most time-efficient option in the list.
broad area coverage: If bagworms are attacking a long row of evergreens or multiple trees, this hose-end bottle saves mixing time and reaches high branches easily.
needs frequent reapplication: You cannot adjust the concentration on the fly, so for very small spot treatments a ready-to-use trigger bottle may waste less product.
large gardens: you have a large property and want to hit every branch without pausing to mix. small plots: you only have one small shrub and would prefer a pint-sized concentrate instead.
3. Valent USA Dipel Pro DF Biological Insecticide BT 54%, 1lb Bag
A dry powder that dissolves fast and puts 54% active Bt to work on every leaf.
Most liquid concentrates contain around 0.5% to 2% active Bt, but this Valent USA Dipel Pro DF powder delivers a 54% concentration of the active strain — far more punch per spoonful. It is OMRI Listed (organic approved), and the label states there are no time restrictions before harvest, so you can spray vegetables in the morning and pick them that evening. The included one-pound bag gives you enough to treat a sizeable property for multiple seasons.
Owners mention that after just a few applications, caterpillars are gone entirely. One reviewer in Louisiana noted it saved their blueberry bushes after bagworms and other caterpillars had stripped the leaves the previous year. Unlike the ready-to-use Fertilome bottle (which is pre-mixed at 32 fluid ounces), this powder lets you mix exactly the concentration you need — use 1 to 4 ounces per gallon depending on the pest pressure — and a bag weighs far less than carrying gallon jugs of liquid.
concentrated long supply: You get a far higher active percentage per dollar, and a single bag can last several seasons if you only have a few trees.
mixing requires care: You need a sprayer that handles dissolved powders without clogging, and a user caution: this Btk strain will not kill fungus gnat larvae (that is a different Bt strain, Bti, found in mosquito dunks).
bulk users: you want the most concentrated bang per pound and already own a pump sprayer. casual use: you only need a one-time spot spray on potted plants — a liquid concentrate or RTU will be simpler.
4. Southern Ag Thuricide BT For Control of Caterpillars & Worms, 1 Gallon – 128oz
One gallon of concentrate that keeps spraying all season long without restocking.
If you have multiple large trees or a whole landscape full of bagworm-prone shrubs, buying a single gallon eliminates the hassle of running out mid-season. Southern Ag Thuricide is a liquid concentrate that calls for 1 to 4 ounces per gallon of water, depending on the target. The 128-fluid-ounce container is the largest volume on this list, and the most economical per ounce for people who spray heavily.
One reviewer noted that it killed bagworms by the next morning, but stressed the importance of thorough coverage of the entire tree, not just the outer branches. Another reviewer said that using only about 3 teaspoons per gallon of water, the jug will “probably outlast me.” It carries the same Bt active ingredient as the smaller Southern Ag pint (16 fluid ounces), but holds exactly 8 times the volume — so you save on cost per ounce and on shipping weight per application.
targets many caterpillars: treat multiple trees, garden rows, and ornamentals with one purchase.
kills non-target larvae: a gallon is heavy to pour and requires a separate pump or hose-end sprayer — this is not a grab-and-go bottle.
skip it if: you have only a couple of shrubs — the pint size from the same brand will suffice and store more easily.
5. Monterey BT – Bacillus Thuringiensis for Organic Worm and Caterpillar Control Bundled with Measuring Spoon – Concentrate for BT Spray – 16 oz
A small jug that packs enough concentrate for several rounds of bagworm spray.
Sometimes you do not need a gallon — a manageable 16-fluid-ounce concentrate that you mix yourself gives you total control over strength and waste. This Monterey BT targets bagworm, cabbage looper, gypsy moth, and cankerworm, and it is OMRI Listed (the Organic Materials Review Institute guarantees it meets organic standards). It comes with a measuring spoon so you are not guessing the dose from the first spray.
Customers note that after spraying tomatoes according to the directions, they found several caterpillars dead within two days. Unlike the 32-ounce ready-to-spray version from the same brand, this concentrate makes you add water — which means a single bottle actually yields far more total spray solution. At 1 pound of liquid concentrate vs. the Fertilome ready-to-use (which weighs 1.5 pounds and cannot be diluted), you get more flexibility from a lighter bottle.
precise small batches: the included measuring spoon takes the guesswork out of every batch, so you use exactly as much as needed.
cost per use higher: this is a 16-ounce bottle, so if you are covering a whole acre of bagworm-infested trees, you will finish it faster than a gallon.
exact dosing: you have moderate bagworm pressure on a few trees and want the freedom to mix fresh each time. quick spraying: you prefer zero-mix convenience — grab the ready-to-spray from the same brand.
6. Fertilome (16016) Caterpillar Killer with Bt Biological Insecticide, RTU, OMRI Listed (32 oz.)
A trigger bottle you can pull off the shelf and aim at bagworms in ten seconds.
No mixing, no measuring, no sprayer to rinse out. Fertilome’s 32-ounce ready-to-use (RTU) bottle is pre-mixed with Bacillus thuringiensis and labeled for bagworms, tent caterpillars, tomato hornworms, cabbage loopers, and gypsy moths. At 1.5 pounds total weight it is heavier than a concentrate pint, but you pay for pure convenience — it is OMRI Listed for organic produce and safe around people and pets.
One buyer mentioned that after two days of applying it to broccoli leaves that were being eaten, they saw no more worms or eaten leaves until the end of fall. Another reviewer used it on boxwood moths with success. This is the only ready-to-use option in the table that names bagworms directly on the label, making it the easiest entry point for a homeowner who just noticed their arborvitae turning brown. Unlike the Southern Ag concentrate pint (which you must dilute at 1–4 oz per gallon), this bottle is ready the moment you squeeze.
works fast on contact: no setup, no sprayer, no measuring — just point and spray on visible bagworms.
short residual effect: you cannot adjust the strength; if you need heavy coverage on a tall tree, a concentrate goes further for the same money.
immediate results: you are a casual gardener who wants to stop bagworms fast without buying extra equipment. preventive use: you plan to treat dozens of trees — the concentrate options give more spray solution per dollar.
Understanding the Specs
Bacillus Thuringiensis kurstaki (Btk)
This is the specific subspecies of the soil bacterium that targets leaf-chewing caterpillars like bagworms. When an insect eats treated foliage, the bacterium produces proteins that damage the insect’s gut lining, stopping it from feeding within hours. The key point: Btk does not affect honeybees, earthworms, birds, or mammals — it is a selective biologic, not a blanket poison.
Concentrate vs. Ready-to-Use (RTU)
A concentrate (usually 16 to 128 fluid ounces) tells you to mix 1 to 4 ounces per gallon of water. This gives more total spray volume for the money, but requires a separate sprayer. An RTU bottle comes pre-mixed — you squeeze the trigger and spray directly, which is faster but pricier per ounce of spray and cannot be made stronger.
OMRI Listed and Organic Standards
The Organic Materials Review Institute (OMRI) reviews products to ensure they comply with USDA National Organic Program standards. An OMRI Listed product has no synthetic pesticides, so it is allowed on certified organic farms. For home gardeners, it simply means you can spray vegetables and fruit right up to harvest without waiting.
Volume and item weight
Liquid volume (fluid ounces or gallons) tells you how much concentrate or RTU fluid is in the bottle. Item weight (pounds or ounces) includes the weight of the container. A lighter bottle is easier to carry around a large property, but a heavier bottle like the 1.5-pound Fertilome RTU holds its own sprayer pump so you do not have to carry a separate one.
FAQ
Will BT for bagworms kill other caterpillars too?
How long after spraying do bagworms die?
Can I spray BT on vegetables that I plan to eat?
How often should I reapply BT for bagworms?
What is the difference between Btk, Bti, and Bt?
Is BT safe for birds and pets around the garden?
What is the best way to apply BT to tall trees for bagworms?
Does temperature or rain affect how well BT works?
How do I store leftover BT concentrate?
Can I use BT on houseplants infested with caterpillars?
Final Thoughts: The Verdict
For most homeowners, the bt for bagworms pick that balances convenience and reach is the Monterey B.t. Ready to Spray 32 oz — its hose-end design makes treating a row of arborvitae fast, and the OMRI listing lets you use it on vegetables without worry. If you want the absolute most concentrated power and already own a sprayer, the Valent Dipel Pro DF Powder packs 54% active Bt, enough to treat an acre for less. And for small gardens and first-time bagworm fighters, the Southern Ag Thuricide Pint gives you a proven formula backed by thousands of happy gardeners.
How We Picked
We do not accept paid placement. Every pick is matched to a real buyer and a real use-case; we do not hands-on test units.
Sources & Methodology
Specifications: manufacturer listings and product documentation. Review insights: verified customer reviews, as of July 2026. Pricing: not shown on this page (it changes often); check the current price via the retailer link.
As an Amazon Associate, Lawn Gear Lab earns from qualifying purchases. This does not affect which products we feature.