Bacillus thuringiensis kurstaki
A soil bacterium sold as a spray for caterpillars. It only works if the caterpillar eats it: toxins activated in the caterpillar's alkaline gut break down the gut lining, feeding stops quickly, and the larva dies over the next few days. The kurstaki strain affects moth and butterfly larvae and has little or no toxicity to bees and other beneficial insects. Sunlight breaks it down on the leaf within days, so it has to be on the foliage while young caterpillars are actively feeding.
At a glance
- Mode
- Stomach poison — must be eaten
- Label rate
- Thuricide 1–3 tsp/gal for tomato hornworm; Monterey B.t. 4 tsp/gal
- Speed
- Feeding stops within the hour; death in hours to 3 days
- On the leaf
- Half-life 1–4 days in sunlight
- Harvest
- Up to the day of harvest
- REI
- 4 h on agricultural labels
How it works
Bt is a bacterium whose spores carry crystal proteins toxic to insect larvae. A caterpillar eating a treated leaf swallows the spores, and the high pH in its gut (9–10.5) activates the toxin, which breaks down the gut lining. The larva dies of infection and starvation. Human stomachs are acidic, not alkaline, which is one reason Bt is very low in toxicity to people and other mammals. Different strains target different groups: kurstaki and aizawai affect caterpillars, israelensis affects fly and mosquito larvae, and tenebrionis affects beetle larvae.
What you see is that feeding stops, often within the hour, while the caterpillar can look normal for several days. Death takes a few hours to three days on the DiPel label and up to five days in other sources, and smaller larvae die faster. Because it has to be eaten, only caterpillars that are actively feeding on treated leaves are affected. It also does not persist: sunlight and weather halve the deposit on the leaf in one to four days. Timing is therefore everything — it has to be on the leaves when young caterpillars are eating.
Applying it
Home-garden Btk is a liquid concentrate diluted in a hand or hose-end sprayer. The rate is set by the product label, not by area, and it differs between brands, so read your own label. The figures below come from the Bonide Thuricide, Monterey B.t. and DiPel DF labels.
Mix what you will use
Shake the concentrate first. Use the diluted spray within 12 hours (Thuricide) to 24 hours (Monterey B.t.).
Both sides of the leaf
Spray upper and lower leaf surfaces where the caterpillars are feeding, but not to heavy run-off.
First sign, small larvae
Start at the first sign of feeding, while the larvae are small. It works best on very young caterpillars, and smaller larvae die faster.
Weekly, and after rain
Repeat every 5–7 days while caterpillars are active (label intervals range from 3 to 14 days). Reapply after heavy rain.
When to choose something else
Bt kurstaki does one thing well: it kills young caterpillars that are feeding on the surface of the leaf. Choose something else when:
- The pest is not a caterpillar — squash bugs, aphids and beetles are untouched
- The larvae are beetle grubs (Bt tenebrionis) or fungus gnats and mosquitoes (Bti), not caterpillars
- The caterpillar is inside the fruit — the Thuricide home label claims only suppression of tomato fruitworm
- The caterpillars are already full-grown — in a garden, hand-picking is the more reliable control
- The plant feeds caterpillars you want to keep — Btk can harm non-target butterfly and moth larvae as well
Where to buy
These are the registered Btk products whose labels we checked, in alphabetical order. They are not ranked. helpfulbugs takes no commission and does not sell any of them.
- Bonide Thuricide (EPA Reg. 4-226)Liquid concentrateUS · HOME
- DiPel DF (EPA Reg. 73049-39)Dry flowable, 0.5–2 lb/acreUS · FARM
- Monterey B.t. (EPA Reg. 70051-106-54705)Liquid concentrateUS · HOME
Bacillus thuringiensis kurstaki by target
Rate and timing differ by what you are releasing it against. Each page below is the program for one pest.