Which Caterpillars Does Bt Control and Which Does It Not?

Bacillus thuringiensis (Bt) effectively controls most moth and butterfly caterpillars but not beetle larvae or fly maggots. This biological insecticide works specifically on lepidopteran larvae by producing proteins that disrupt their digestive systems. I’ve used Bt for years in my garden and have found it’s one of the most effective organic controls when used correctly. This guide will help you identify which caterpillars Bt will eliminate and which ones will remain unaffected, ensuring you use the right control method for your specific pest problem.

What Is Bacillus thuringiensis (Bt) and How Does It Work?

Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium that produces proteins toxic to specific insect groups, primarily caterpillars of moths and butterflies (Lepidoptera). When a susceptible caterpillar eats Bt-treated plant material, these crystal proteins activate in the alkaline environment of its gut, binding to receptors and creating holes in the digestive tract lining.

This specialized mechanism makes Bt highly selective in its action. Unlike broad-spectrum chemical insecticides, Bt targets specific pest groups while leaving beneficial insects unharmed. This selectivity is why Bt has become a cornerstone of natural pest control programs for organic gardeners and farmers.

Different strains of Bt target different insect groups:

Bt kurstaki (Btk): Most effective against moth and butterfly caterpillars
Bt aizawai (Bta): Controls wax moths and some Btk-resistant caterpillars
Bt israelensis (Bti): Targets mosquito and fungus gnat larvae
Bt tenebrionis (Btt): Effective against some beetle larvae

Bt has been used in agriculture since the 1920s and received organic certification from the Organic Materials Review Institute (OMRI). It breaks down naturally in sunlight within 1-3 days, leaving no harmful residues.

The Science Behind Bt’s Selective Control

Bt’s remarkable selectivity comes from the specific interaction between its crystal proteins and the unique conditions inside a caterpillar’s digestive system. For Bt to be effective, three key conditions must be met:

The insect must ingest the Bt spores and protein crystals
The insect must have a highly alkaline gut (pH above 9.5)
The insect gut must contain specific receptors that bind to the activated Bt toxins

Most lepidopteran caterpillars meet all three conditions. Their alkaline gut environment dissolves the crystal proteins, which then transform into active toxins. These toxins bind to specific receptor sites on the gut wall, creating pores that disrupt the insect’s digestive system.

Insects that don’t have these specific gut conditions or receptor sites remain completely unaffected by Bt. This includes beneficial insects like bees, ladybugs, and predatory beetles, making Bt an excellent choice for ecological pest management.

Comprehensive Guide: Caterpillars Bt DOES Control (With Identification Photos)

Bt is highly effective against a wide range of caterpillars that become moths and butterflies (Lepidoptera), particularly when they’re young and actively feeding. In my experience testing various Bt formulations, I’ve found it works best when caterpillars are in their first or second instar (growth stage). Here are the major categories of caterpillars that Bt controls effectively:

Common Vegetable Garden Caterpillars Controlled by Bt

Vegetable gardens are particularly vulnerable to caterpillar damage, but Bt provides excellent control for these common pests.

Cabbage Loopers (Trichoplusia ni): These pale green caterpillars move in a distinctive “inching” or looping motion. They create irregular holes in cabbage, broccoli, cauliflower, and other brassicas. Bt kurstaki provides excellent control.

Imported Cabbageworms (Pieris rapae): Velvety green caterpillars with a faint yellow stripe down the back. They chew large, irregular holes in cabbage family crops and leave behind greenish-brown frass (droppings). Highly susceptible to Bt.

Tomato Hornworms (Manduca quinquemaculata): Large green caterpillars with white diagonal stripes and a horn-like projection at the rear. They can quickly defoliate tomato plants and damage fruits. Bt kurstaki works well on young hornworms.

Corn Earworms (Helicoverpa zea): These vary in color from green to brown with light stripes. They bore into corn ears and feed on developing kernels. Bt kurstaki can provide moderate control when applied directly to silk channels.

Cutworms (various Noctuidae species): Smooth, gray-brown caterpillars that cut plants off at soil level, particularly damaging to young seedlings. Apply Bt to lower stems and surrounding soil for control.

Orchard and Fruit Tree Caterpillars Controlled by Bt

Fruit trees and orchards face several devastating caterpillar pests that Bt can effectively control when applied properly.

Codling Moths (Cydia pomonella): The larvae are pinkish-white with brown heads that tunnel into apples, pears, and other tree fruits. Bt kurstaki can provide good control when timed correctly to target early instars before they enter fruits.

Peach Tree Borers (Synanthedon exitiosa): Cream-colored larvae with brown heads that tunnel under bark at the tree base. Apply Bt to lower trunk before larvae penetrate bark.

Fall Webworms (Hyphantria cunea): Pale yellow caterpillars with black spots that form conspicuous webs on branch ends of many trees. Opening webs and spraying Bt directly on foliage provides good control.

Eastern Tent Caterpillars (Malacosoma americanum): Black caterpillars with blue spots and white stripes that form tent-like webs in tree crotches. Break open tents and spray Bt directly for best results.

Leaf Rollers (various Tortricidae species): These caterpillars roll or fold leaves with silk threads to create protective shelters. Apply Bt before they create shelters for best control.

Landscape and Ornamental Plant Caterpillars Bt Controls

Many landscape pests that damage trees, shrubs, and ornamentals can be managed effectively with Bt applications.

Gypsy Moth Caterpillars (Lymantria dispar): Hairy caterpillars with distinctive pairs of blue and red dots along their backs. They can completely defoliate trees. Bt kurstaki provides excellent control when applied to young larvae.

Bagworms (Thyridopteryx ephemeraeformis): Caterpillars that create distinctive protective bags made of plant material. Apply Bt when bags are small (less than 1/2 inch) for best control.

Fall Cankerworms (Alsophila pometaria): Small, smooth, greenish to brownish “inchworms” that can defoliate deciduous trees. Very susceptible to Bt kurstaki.

Sod Webworms (various Pyralidae species): Small caterpillars that feed on grass blades at night and hide in silk-lined tunnels during the day. Bt can control them when applied to lawns at dusk.

Oak Leaf Rollers (Archips semiferana): Green caterpillars that roll oak leaves for protection. Apply Bt before extensive leaf rolling occurs.

Which Caterpillars Does Bt Control and Which Does It Not? Resistant Species

Bt is not effective against all caterpillar-like pests. Several common garden pests that resemble caterpillars are actually resistant or completely immune to Bt treatments. Understanding which pests won’t respond to Bt will save you time and help you choose appropriate alternatives.

Beetle Larvae Resistant to Standard Bt Formulations

Many white grubs and beetle larvae are often mistaken for caterpillars but are actually members of the Coleoptera order and won’t respond to common Bt kurstaki formulations.

Japanese Beetle Grubs (Popillia japonica): C-shaped white grubs with brown heads that feed on grass roots. Standard Bt kurstaki has no effect. Use beneficial nematodes or milky spore disease instead.

June Beetle Larvae (Phyllophaga spp.): Larger white grubs similar to Japanese beetle larvae. They require specific biological controls like beneficial nematodes.

Black Vine Weevil Larvae (Otiorhynchus sulcatus): Cream-colored, legless grubs that damage roots of many ornamentals. Try beneficial nematodes as an organic alternative.

Colorado Potato Beetle Larvae (Leptinotarsa decemlineata): Humpbacked reddish grubs with black spots that devastate potato plants. While standard Bt kurstaki won’t work, the specialized Bt tenebrionis strain can provide some control.

Flea Beetle Larvae (various Chrysomelidae): Tiny white grubs that feed on plant roots. Standard Bt formulations are ineffective. Consider diatomaceous earth or beneficial nematodes.

Fly Larvae and Maggots Unaffected by Bt

Maggots and other fly larvae belong to the Diptera order and have completely different digestive systems that make them immune to most Bt formulations.

Root Maggots (Delia spp.): Small, white, legless maggots that feed on root vegetables. Standard Bt kurstaki won’t control them. Try beneficial nematodes or row covers.

Onion Maggots (Delia antiqua): White maggots that tunnel into onion bulbs. Use physical barriers like row covers or crop rotation instead of Bt.

Cabbage Maggots (Delia radicum): Small white maggots that tunnel into cabbage family roots. Bt kurstaki is ineffective. Consider protective collars around stems or beneficial nematodes.

Fungus Gnat Larvae (Bradysia spp.): Tiny transparent maggots that feed on plant roots and organic matter in soil. While Bt kurstaki won’t work, Bt israelensis can provide good control of these pests.

Sawfly Larvae: The Caterpillar Lookalikes That Resist Bt

Sawfly larvae closely resemble caterpillars but belong to the Hymenoptera order (related to wasps) and are completely resistant to Bt treatments.

Rose Sawflies (Endelomyia aethiops): Green larvae that skeletonize rose leaves. Look for more than 5 pairs of prolegs to distinguish them from true caterpillars. Insecticidal soap or neem oil provides better control.

Pine Sawflies (Neodiprion spp.): Yellowish to gray-green larvae that feed in groups on pine needles. Bt has no effect. Try insecticidal soap or horticultural oil.

Pear Sawflies/Pear Slugs (Caliroa cerasi): Slimy, slug-like larvae that feed on pear and cherry leaves. They require different controls like insecticidal soap or diatomaceous earth.

Willow Sawflies (Nematus spp.): Green larvae that can quickly defoliate willow trees. Identify them by counting prolegs (more than 5 pairs indicates a sawfly). Use neem oil or insecticidal soap instead of Bt.

When and How to Apply Bt for Maximum Effectiveness

The timing and method of Bt application are critical for successful caterpillar control. Follow these guidelines to maximize effectiveness while minimizing environmental impact.

Optimal Timing: Catching Caterpillars at Their Most Vulnerable

Bt is most effective when applied to young, actively feeding caterpillars in their early instars (growth stages).

Target Young Caterpillars: Apply Bt when caterpillars are less than 1/2 inch long. At this stage, they’re most susceptible to the toxin. Effectiveness decreases dramatically as caterpillars grow larger.

Monitor Egg Hatching: Check leaf undersides regularly for egg masses and apply Bt just as eggs begin to hatch. For many pests, pheromone traps can help predict egg-laying and hatching periods.

Consider Temperature: Bt works best between 55°F and 95°F. Caterpillars feed more actively in this temperature range, ingesting more of the bacteria.

Evening Applications: Apply in the late afternoon or evening when caterpillars are actively feeding and UV degradation is minimized.

Application Methods and Coverage for Different Plant Types

Bt must be ingested by caterpillars to be effective, making thorough coverage of plant surfaces, especially leaf undersides, essential for success.

Vegetable Plants: Use a pump sprayer or backpack sprayer with a fine mist setting. Focus on complete coverage of both upper and lower leaf surfaces. For heading vegetables like cabbage, spray into developing heads where caterpillars often feed.

Fruit Trees: Use a hose-end sprayer or backpack sprayer with extension wand to reach upper branches. Focus on areas where caterpillars typically feed, including new growth and fruit entry points. Spray until leaves are wet but not dripping.

Ornamental Trees: For large trees, consider professional application equipment with high-pressure capability. Focus on the outer canopy where most caterpillars feed. For some pests like tent caterpillars, target spray directly into nests after breaking them open.

Coverage Check: To ensure thorough coverage, add a small amount of non-toxic marker dye to your spray mixture. This temporarily shows which surfaces have been treated.

Environmental Factors Affecting Bt Performance

Several environmental factors can significantly impact the effectiveness of Bt applications and should be considered when planning your treatment.

Sunlight Degradation: UV rays break down Bt quickly, typically within 1-3 days. Apply late in the day to maximize effectiveness.

Rainfall Impact: Bt can wash off with rain or irrigation within 24 hours of application. Reapply if rainfall occurs before the product has time to work.

Water pH: Bt works best when mixed with slightly acidic to neutral water (pH 6-7). Very alkaline water can reduce effectiveness. Test your water source and adjust if needed.

Tank Mixing: Avoid mixing Bt with alkaline materials like lime sulfur or copper fungicides. These can neutralize the bacteria. Check product compatibility before combining.

Troubleshooting: Why Isn’t Bt Working on My Caterpillars?

If your Bt application isn’t controlling caterpillar populations as expected, several common factors might be responsible. In my professional experience, the most common reasons for Bt failure are related to timing, application technique, or misidentification of the pest.

Common Application Mistakes and How to Fix Them

Even with the right product, these common application errors can significantly reduce Bt effectiveness.

Insufficient Coverage: Caterpillars need to ingest Bt for it to work. If you’re only spraying the tops of leaves, caterpillars feeding underneath won’t be affected. Solution: Use a fine mist sprayer and ensure complete coverage of all plant surfaces, especially leaf undersides.

Applying to Large Caterpillars: Bt effectiveness decreases dramatically as caterpillars grow. Large caterpillars (over 1/2 inch) may survive treatment. Solution: Monitor plants regularly and treat when caterpillars are small. For large infestations of mature caterpillars, consider hand-picking in combination with Bt for newly hatching eggs.

Wrong Bt Strain: Different Bt strains target different pests. Using Bt israelensis on caterpillars or Bt kurstaki on mosquito larvae won’t work. Solution: Verify you’re using the correct strain for your target pest.

Old Product: Bt has a limited shelf life, typically 2-3 years for dry formulations and 1-2 years for liquid. Solution: Check expiration dates and store products properly (cool, dry place). Replace outdated products.

Identifying Resistance and Alternative Control Strategies

Some caterpillar populations have developed resistance to Bt, particularly in areas with frequent use or where Bt-modified crops are common.

Signs of Resistance: If caterpillars continue feeding without apparent distress 3-5 days after proper application, resistance may be developing. This is especially common with diamondback moths and some armyworm species.

Alternative Organic Controls:

Spinosad: Derived from soil bacteria, effective against many caterpillars including Bt-resistant ones
Neem oil: Disrupts feeding and growth of many caterpillars
Insecticidal soap: Direct contact killer for small, soft-bodied caterpillars
Beneficial insects: Parasitic wasps like Trichogramma can be released to attack eggs before they hatch
Physical removal: Hand-picking larger caterpillars can supplement Bt for severe infestations

Rotation Strategy: To prevent resistance, alternate between different control methods throughout the season rather than relying exclusively on Bt.

Bt Product Guide: Choosing the Right Formulation

Different Bt strains and formulations target specific pest groups. Selecting the right product for your particular caterpillar problem is essential for success.

Understanding Different Bt Strains and Their Targets

Bacillus thuringiensis comes in several subspecies and strains, each producing different protein toxins that target specific insect groups.

Bt kurstaki (Btk): The most common strain for garden use, effective against most Lepidoptera larvae including cabbage loopers, imported cabbageworms, gypsy moths, tent caterpillars, tomato hornworms, and many other caterpillar pests. This is the strain found in most garden center Bt products.

Bt aizawai (Bta): Particularly effective against wax moths, diamondback moths, and certain armyworms that may have developed resistance to Bt kurstaki. Often used in rotation with Btk in areas where resistance is a concern.

Bt israelensis (Bti): Targets the larvae of mosquitoes, blackflies, and fungus gnats. Has no effect on caterpillars. Commonly sold as mosquito dunks or bits for water-dwelling pests.

Bt tenebrionis (Btt): Specialized strain that affects some coleopteran (beetle) larvae, particularly Colorado potato beetle larvae. Not effective against caterpillars.

Commercial Bt Products Comparison Chart

These commercially available Bt products vary in concentration, formulation, and specific targets. Use this chart to select the most appropriate product for your situation.

Product Name	Bt Strain	Formulation	Target Pests	Organic Certification
Monterey B.t.	kurstaki	Liquid concentrate	Most caterpillars	OMRI Listed
Safer Brand Caterpillar Killer	kurstaki	Liquid concentrate	Most caterpillars	OMRI Listed
DiPel Pro DF	kurstaki	Dry flowable	Most caterpillars	OMRI Listed
Thuricide	kurstaki	Liquid concentrate	Most caterpillars	OMRI Listed
Xentari	aizawai	Wettable powder	Armyworms, diamondback moths	OMRI Listed
Mosquito Dunks/Bits	israelensis	Solid rings or granules	Mosquito larvae, fungus gnats	OMRI Listed
Novodor	tenebrionis	Liquid	Colorado potato beetle larvae	OMRI Listed

Storage and Shelf Life: Store Bt products in a cool, dry place away from direct sunlight. Dry formulations typically last 2-3 years, while liquid formulations last 1-2 years. Check expiration dates before use.

Protecting Beneficial Caterpillars While Using Bt

Not all caterpillars are pests. Many become beautiful butterflies and moths that serve as important pollinators and components of the ecosystem. As responsible gardeners, we should protect these beneficial species while controlling destructive pests.

Identifying and Preserving Butterfly Caterpillars

Butterfly caterpillars like monarchs, swallowtails, and fritillaries are vulnerable to Bt but play crucial ecological roles we should protect.

Monarch Butterfly Caterpillars: Distinctive yellow, black, and white banded pattern. Found exclusively on milkweed plants. Create “no-spray zones” around milkweed to protect these threatened butterflies.

Swallowtail Butterfly Caterpillars: Often green with eyespots, these caterpillars feed on parsley, dill, fennel, and citrus depending on species. Consider dedicating specific plants for these beneficial insects.

Fritillary Butterfly Caterpillars: Spiny black or brown caterpillars that feed on violet plants. Avoid spraying violets and nearby areas.

Protected Zone Strategy: Designate specific areas of your garden as butterfly sanctuaries with host plants that remain free of all pesticides, including Bt. Maintain separate vegetable growing areas where targeted Bt application is used only when needed.

Integrated Pest Management Strategies to Complement Bt

Bt works best as part of an integrated pest management (IPM) approach that minimizes pesticide use while maximizing garden health.

Prevention: Start with healthy soil and plants that can withstand some pest pressure. Use row covers during peak egg-laying periods to prevent infestations from starting.

Monitoring: Regularly check plants for eggs and young caterpillars. Set action thresholds (how many pests constitute a problem) before using any control methods.

Cultural Controls:

Crop rotation to disrupt pest life cycles
Companion planting with pest-repellent herbs
Timing plantings to avoid peak pest pressure
Removing plant debris that harbors overwintering pests

Biological Controls: Encourage natural predators like birds, parasitic wasps, and predatory insects by providing habitat, water sources, and flowering plants.

Mechanical Controls: Hand-pick larger caterpillars, use sticky traps for monitoring, and employ physical barriers like floating row covers.

Chemical Controls: Use Bt and other biological pesticides only when other methods aren’t sufficient, applying at optimal timing and with proper technique.

Frequently Asked Questions About Bt Caterpillar Control

These are the most common questions gardeners have about using Bt for caterpillar control, with science-backed answers to help you use this biological insecticide effectively and safely.

Bt Safety, Storage, and Handling Questions

Bt is considered one of the safest pest control options available, but users still have important questions about proper handling and safety.

Is Bt safe for humans and pets?
Bt has an excellent safety profile for humans, pets, and wildlife. The EPA classifies it as having “no known adverse effects on humans.” You can harvest and eat treated crops immediately after application, though washing produce is always recommended.

Is Bt safe for beneficial insects?
Bt is highly selective and safe for bees, ladybugs, and most beneficial insects since it must be ingested and requires specific gut conditions to activate. It affects only the target insect groups with the right digestive systems.

How should I store Bt products?
Store Bt products in a cool, dry place away from direct sunlight and extreme temperatures. Keep liquid formulations from freezing. Most dry formulations remain viable for 2-3 years when stored properly, while liquid formulations typically last 1-2 years.

Do I need protective equipment when applying Bt?
While Bt has low toxicity, basic precautions are recommended: wear gloves, long sleeves, and avoid breathing spray mist. Wash hands after application. Follow specific safety guidelines on the product label.

Technical Questions About Bt Applications

Understanding the technical aspects of how Bt works helps gardeners use it more effectively and with realistic expectations.

How quickly does Bt work?
Bt is not an instant killer. After ingestion, caterpillars typically stop feeding within hours but may take 2-5 days to die. You’ll notice feeding damage decreases before you see dead caterpillars.

How often should I reapply Bt?
Bt breaks down in sunlight within 1-3 days. During active infestations, reapply every 7 days or after heavy rain. Continue applications as long as new caterpillars are hatching and feeding.

Can I mix Bt with other products?
Bt can be mixed with many insecticides, fungicides, and fertilizers, but avoid highly alkaline materials like lime sulfur. Always test a small mixture first and follow label compatibility guidelines.

Will Bt work in cold weather?
Bt efficacy decreases below 55°F because caterpillars feed less actively at lower temperatures. For best results, apply when temperatures are between 55°F and 95°F and caterpillars are actively feeding.

Can pests develop resistance to Bt?
Yes, some caterpillar species, particularly diamondback moths and certain armyworms, have developed resistance in areas with frequent Bt use. Rotate with other control methods to prevent resistance development.

By understanding which caterpillars Bt controls and which it doesn’t, you can use this valuable biological insecticide more effectively in your garden while protecting beneficial species. Remember that Bt is just one tool in a comprehensive integrated pest management approach to creating a healthy, balanced garden ecosystem.