Safer Brand Caterpillar Killer II Concentrate mixes at a rate of 4 teaspoons per 1 gallon of water. That single ratio covers most home garden applications, including fruit trees, vegetables, and ornamentals. The mixing process is straightforward, but getting the timing, coverage, and reapplication right makes a real difference in how well it works.
Standard Mixing Ratio
The label calls for 4 teaspoons of concentrate per gallon of water. Measure the concentrate first, add it to your sprayer, then fill with water and mix thoroughly. A 16-ounce bottle of concentrate will make roughly 24 gallons of spray solution at this rate, which is enough to treat a sizable home garden multiple times through the season.
Use the solution the same day you mix it. The active ingredient, a naturally occurring soil bacterium called Bt (Bacillus thuringiensis var. kurstaki), begins to break down once diluted and exposed to light. Mixing fresh batches each time you spray gives you the strongest results.
How the Product Actually Works
Bt doesn’t kill caterpillars on contact. Instead, caterpillars and worms must eat leaf material coated with the spray. Once ingested, a protein toxin in the bacterium shuts down their ability to feed. Affected caterpillars stop chewing almost immediately, but they don’t drop dead right away. They die of starvation over the next few days. This lag can make it seem like the product isn’t working at first, but visible leaf damage should stop within hours of a thorough application.
The concentrate is 98.35% Bt, and it’s OMRI Listed for organic gardening. Because it targets the digestive systems of caterpillars and moth larvae specifically, it won’t harm bees, birds, or pets that come into contact with treated foliage. Studies have found it to be practically nontoxic to saltwater organisms like grass shrimp and sheepshead minnows. Freshwater fish can be affected at high concentrations in lab settings, but normal garden use at label rates doesn’t produce those exposure levels.
Spraying for Best Coverage
Coverage is the single biggest factor in whether this product works. Caterpillars often feed on the undersides of leaves, so spraying only the tops misses where the pests actually eat. The label directs you to coat all leaf surfaces, including undersides, until they’re thoroughly wet. A pump sprayer with an adjustable nozzle makes it much easier to angle the spray upward beneath the foliage.
Apply late in the afternoon or on overcast days. Bt breaks down in direct sunlight, so spraying at midday on a clear day means the product degrades before caterpillars get a full dose overnight, which is when many species do their heaviest feeding. An evening application gives you maximum potency through the night.
When to Reapply
One application often isn’t enough for a full infestation. If you still see caterpillars feeding after the initial spray, reapply in 5 to 7 days. Rain washes the product off leaf surfaces, so a heavy downpour between applications means you should spray again sooner rather than waiting the full week. New growth that emerges after you spray is also unprotected and will need a fresh coat if caterpillars are still active in the area.
For ongoing pressure from pests like tomato hornworms, cabbage loopers, or tent caterpillars, plan on spraying every 7 days through the active feeding period. The product has no pre-harvest interval for most crops, meaning you can spray up to the day of harvest on vegetables and fruit.
Safety During Mixing and Spraying
Bt is one of the lowest-risk pesticides you can use, but basic precautions still apply. Nearly all pesticide exposure during spraying happens through skin contact, so wear waterproof or chemical-resistant gloves (not cotton or leather) while mixing and applying. Long sleeves and pants reduce skin exposure further. If your sprayer tends to produce fine mist or splash back, safety glasses or indirectly ventilated goggles protect your eyes. Polycarbonate lenses offer the best combination of chemical splash resistance and impact protection.
Keep pant legs outside your boots or shoes rather than tucked in. This prevents spray from funneling down into your footwear, which is a common and overlooked route of prolonged skin contact. Wash your hands and any exposed skin after you finish spraying, and rinse out your sprayer with clean water so residue doesn’t clog the nozzle before your next application.
Quick-Reference Mixing Chart
- 1 gallon of water: 4 teaspoons of concentrate
- 2 gallons of water: 8 teaspoons (roughly 2.5 tablespoons)
- 5 gallons of water: 20 teaspoons (roughly 6.5 tablespoons)
Always check the product label on your specific bottle, as formulations can change between production runs. If you’re using the ready-to-use spray version rather than the concentrate, no mixing is needed.