If caterpillars are eating your home garden, one safe and effective way to get rid of them is by using the bacteria Bt or Bacillus thuringiensis. You can apply Bt as a liquid or powder, and many of the varieties are certified organic. Read on to learn more about how you can maximize DIY pest control for your garden without using harmful insecticides.
About Bt (Bacillus thuringiensis)
Bt (Bacillus thuringiensis) is a natural bacteria that occurs in soil. When eaten by caterpillars, the bacteria produce proteins that paralyze the caterpillar’s digestive system, which causes them to stop feeding and die.
Bt is safe for use in organic gardens because it has a specific target and is nontoxic to humans, animals, and beneficial insects. It’s approved for use in vegetable gardens up to the day of harvest, though all vegetables and herbs should still be thoroughly washed you eat them.
Certified Organic Bt
While Bt is a natural bacteria, not all commercial Bt products are certified organic. Some products are made with genetically modified Bt (which is not allowed in organic gardens), and many of the liquid forms are mixed with a non-organic solvent.
If you want an organic Bt, be sure to look for the certification stamp from the Organic Materials Research Institute (OMRI). Certified organic Bt is easiest to find in powdered form.
Types of Bt
There are a variety of Bt products available, from powders to baits to liquid concentrates. Different strains of Bt are also available for different purposes, such as:
- Btk (var. kurstaki): This is the most common strain of Bt. It is effective for killing bagworms, cabbage worms, gypsy moth larvae, tent caterpillars, leafrollers, tobacco and tomato hornworms, European corn borers, and other leaf and needle-eating caterpillars. Brand names for this strain include DiPel and Thuricide.
- Bti (var. israelensis): This strain of Bt targets mosquitoes, gnats, and black flies. Brand names for this strain are Mosquito Dunks and Bactimos. You apply this to standing water or as a soil drench rather than as a leaf spray.
- Bt (var. tenebrionis/San Diego): This special strain of Bt targets the elm leaf beetle, Japanese beetles, cottonwood leaf beetle, and potato beetle. Trident and M-1 are brand names for this strain.
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How To Apply Bt
Mix concentrated or wettable forms of regular Bt in a small spray bottle or garden sprayer, then carefully apply to the foliage of affected plants. Be sure to follow application instructions for proper dosage and mixing.
There are some general guidelines to follow when applying Bt. For Bt to work, the caterpillar has to eat it. Be sure to cover both the tops and bottoms of the leaves since they feed on both, with an even coating. Don’t drench plants to the point of dripping, just spray affected plants so you only target the problem caterpillars.
Bt takes several days to work, so be patient. It degrades quickly in sunlight and only lasts about a week or so. Apply as soon as you spot an infestation, and reapply every week if the caterpillars return. To help reduce the buildup of resistance to Bt, use it only if necessary. Use Bt as part of an integrated pest management program that includes beneficial predators and other natural controls.
Bt products only last a couple of years, with the powdered forms lasting longer than liquids. Store Bt in a cool, dry, shaded place for maximum shelf life.
Protect the Butterflies
The major strain of Bt kills all leaf-eating caterpillars. This, unfortunately, includes the larvae of monarch butterflies and other important and beneficial butterflies and moths. Each species of butterfly larvae feeds on specific plants. For example, monarch butterfly larvae feed on milkweed.
When applying Bt, be extremely careful to spray only affected plants. Don’t overspray or allow the product to drift over to other plants that might be hosting an insect that’s harmless and beautiful.
Final Thoughts on Bt, a Versatile Pesticide for Your Garden
Bt is an organic pesticide that produces proteins that paralyze a caterpillar’s digestive system when they eat it. This causes them to stop feeding and die, making Bt an effective and safe pest control method for your home gardens.
Bt, based on the strain, has a specific target and is nontoxic to humans, animals, and full-grown beneficial insects. However, practice caution when spraying Bt in your garden, making sure to apply it only to the areas with caterpillars you want to kill. Some strains can harm the larvae of these beneficial insects you want to keep, like monarch butterflies. Bt is safe to use in your vegetable gardens as well, just be sure to wash your vegetables before you eat them.
FAQs About Using Bt for Organic Caterpillar Control
Yes, Bt (Bacillus thuringiensis) can be used in organic gardening. It’s a naturally occurring soil bacterium that produces proteins that are toxic to certain insect larvae, particularly caterpillars. Bt is considered an organic pesticide because it’s naturally derived, targets specific pests without harming most beneficial insets (like pollinators), and has low toxicity to humans and other mammals.
Bt is approved for use in certified organic agriculture by many regulatory bodies, including the USDA’s National Organic Program in the United States. However, there are some things you should consider. Different strains of Bt target different garden pests, including tomato hornworms, webworms, fungus gnats, aphids, imported cabbage worms, and more. This versatile natural pest control option is a helpful alternative to chemical pesticides, but be sure to choose the right type for your problem.
You should also consider that overuse can lead to insect pest resistance. It’s advised to use Dt as one part of a pest management strategy. You might also use diatomaceous earth or neem oil as other organic insecticide options.
Yes, Bt is safe to use on vegetables when used as directed. Bt has low toxicity to humans and other mammals and isn’t harmful when consumed on treated vegetables. Bt breaks down quickly, usually within a few days to two weeks, so it won’t stay around long. As with any pesticide, insecticide, or fungicide use, be sure to wash your vegetables before consumption.
The best time to apply Bt for caterpillar control in your garden depends on several factors. Bt is most effective on young, actively feeding caterpillars. Young larvae are more susceptible to Bt than older larvae. For some pests, like cabbage loopers, application is most effective when caterpillars are less than 1/2 inch long.
UV exposure can degrade Bt, so applying in the late afternoon or evening is ideal. You should also check the forecast to make sure there will be no rain for at least 24 hours after your application.
