Monterey B.t. liquid concentrate mixes at 1 tablespoon per gallon of water for fruits and vegetables, and 4 teaspoons per gallon for ornamental plants. Those two ratios cover most home garden situations, but getting the timing, frequency, and technique right matters just as much as the mix itself.
Mixing Ratios by Plant Type
For fruits and vegetables, the standard rate is 1.5 ounces per 3 gallons of water, which works out to 1 tablespoon per gallon. This covers roughly 1,000 square feet of planted area per 3 gallons of diluted spray.
For ornamental plants, shade trees, and flowering or bedding plants, the rate is slightly higher: 2 ounces per 3 gallons, or about 4 teaspoons per gallon. The higher concentration accounts for the thicker, waxier foliage on many ornamentals, which needs more product for adequate coverage.
For lawns, the label groups the rate with ornamentals at 4 teaspoons per gallon. If you’re targeting sod webworms or other lawn caterpillars, use that same ratio and aim for even coverage across the turf.
How to Mix and Spray
Shake the concentrate bottle well before measuring. B.t. is a biological product containing living bacteria and their protein crystals, and settling is normal. Measure your dose, add it to your sprayer already partially filled with water, then top off to the final volume. This helps distribute the product evenly without excessive foaming.
Use your mixed solution quickly. The active ingredient in B.t. can degrade in water, especially if your water is alkaline (pH above 7). High temperatures speed up that breakdown. Mix only what you plan to spray that day, and never let a diluted batch sit overnight. If you have well water or suspect high pH, using it promptly is even more important.
Spray to wet both the upper and lower leaf surfaces thoroughly. Caterpillars need to actually eat the treated foliage for B.t. to work, so coverage is everything. A pump sprayer with an adjustable nozzle set to a fine mist gives you the best leaf contact.
When and How Often to Apply
Spray at the first sign of caterpillar activity or feeding damage. B.t. works best on young, actively feeding caterpillars. Larger, more mature larvae are harder to kill, so early detection pays off.
Repeat applications every 5 to 7 days while caterpillars are active. For heavy infestations, you can spray more frequently. The product breaks down in sunlight and washes off in rain, so it doesn’t persist on the plant the way synthetic pesticides do. Reapply after any heavy rain, even if you sprayed recently.
For certain pests where eggs hatch over an extended period (like bagworms on evergreens), a second application about 14 days after the first may be needed to catch the late hatchers. The key timing window for bagworms and similar pests is when buds are swelling but before leaves fully open, since that’s when young larvae are most exposed and vulnerable.
Spray in calm weather when no rain is expected for at least 24 hours. This gives the product time to dry on the foliage and avoids drift onto areas you didn’t intend to treat. Late afternoon or early evening is ideal, since UV light breaks down B.t. and caterpillars often feed more actively at dusk and overnight.
What Monterey B.t. Controls
Monterey B.t. targets the larval (caterpillar) stage of moths and butterflies. Common garden pests it handles include tomato hornworms, cabbage loopers, cabbage worms, tent caterpillars, gypsy moth larvae, bagworms, leafrollers, and sod webworms. It works on any caterpillar that feeds on treated foliage.
Bagworms are a particularly good use case. These caterpillars are devastating to arborvitae, cedar, juniper, Leyland cypress, and pines. Because bagworms hide inside their silk cases, timing the spray for when young larvae are still actively crawling and feeding (typically late spring to early summer) is critical for good results.
Safety for People, Pets, and Beneficial Insects
B.t. does not harm most non-target organisms. It’s specific to caterpillars that ingest it. The protein crystals in B.t. only activate in the alkaline gut of caterpillar larvae, which means it won’t affect honeybees, ladybugs, predatory wasps, or other beneficial insects that don’t eat your plant leaves.
Monterey B.t. has a zero-day pre-harvest interval on food crops, meaning you can spray and harvest the same day after the spray has dried. This makes it one of the safest options for vegetable gardens where you’re picking tomatoes, peppers, or greens regularly throughout the season. Pets and children can return to treated areas once the spray dries on the foliage.
One consideration: B.t. will kill any caterpillar that eats treated leaves, including butterfly larvae you might want to keep around. If you grow milkweed for monarchs or host plants for swallowtails, avoid spraying those specific plants or the area immediately surrounding them.
Storage and Shelf Life
Store the undiluted concentrate in a cool, dry place out of direct sunlight. B.t. is a living organism, and heat and UV exposure reduce its potency over time. A garage shelf or garden shed that doesn’t get extremely hot works fine. Shake the bottle before each use, since the active ingredients settle between applications. Check the label for an expiration date, and replace old product if you notice reduced effectiveness against caterpillars you’re treating.