Using Bt for Organic Caterpillar Control in Your Garden

If caterpillars are devouring your garden, one safe and effective way to get rid of them is by applying the naturally occurring bacteria known as Bt, or Bacillus thuringiensis. Bt can be applied as a liquid or powder, and many of the varieties are certified organic.

Here’s what you need to know about using Bt to control caterpillars in your garden.

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About Bt (Bacillus thuringiensis)

Bt (Bacillus thuringiensis) is a natural bacteria that is commonly found 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 before eaten.

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 organic Bt, be sure to look for the certification stamp of the Organic Materials Research Institute (OMRI). Certified organic Bt is easiest to find in powdered form.

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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:

• Bt (var. kurstaki): This is the most common strain of Bt. It kills leaf and needle-eating caterpillars such as bagworms, cabbage worms, gypsy moth larvae, tent caterpillars, leafrollers, tomato and tobacco hornworms and European corn borers. 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, Bactimos, or Gnatrol. 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, cottonwood leaf beetle, and potato beetle. Trident and M-1 are brand names for this strain.

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How to Apply Bt

The concentrated or wettable forms of regular Bt can be mixed in a small spray bottle or garden sprayer, then carefully applied to the foliage of affected plants. Be sure to follow application instructions for proper dosage and mixing.

When applying Bt:

• Apply Bt Thoroughly: In order 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. It’s not necessary to drench plants to the point of dripping.
• Apply Bt Carefully: Be sure to limit your spraying to the affected plants, so that you only target the problem caterpillars.
• Give Bt Time to Work: Bt takes several days to work, so be patient.
• Apply Multiple Doses: Bt 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.
• Use Bt Only When Needed: To help reduce the buildup of resistance to Bt, use it only if absolutely necessary. Use Bt as part of an integrated pest management program that includes beneficial predators and other natural controls.
• Store Bt Properly: 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.

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Protect the Butterflies

The major strain of Bt kills all leaf-eating caterpillars, which 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!

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Further Information

• Organic Pest Control (video)
• Organic Mosquito Control (video)
• Bacillus thuringiensis (Colorado State University Extension)
• Bacillus thuringiensis (Cornell University Resource Guide)