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Bacillus Thuringiensis

Bacillus thuringiensis

Microbial Pathogens Beneficial 1–1.5 µm (vegetative cell); 0.5–1 µm (spore + crystal)

📖 Description

Bacillus thuringiensis (Bt) is the most widely used microbial biopesticide in the world — over 30,000 tonnes are applied annually on more than 30 million hectares globally. India produces approximately 1,500 tonnes of Bt-based biopesticides per year, used on cotton (against Helicoverpa armigera), vegetables (against Plutella xylostella, Spodoptera litura, Helicoverpa), pulses (against Helicoverpa), and stored grains. The species was first discovered in 1901 by Japanese biologist Shigetane Ishiwata in silkworm larvae, and was independently rediscovered in 1911 by Ernst Berliner in flour moth larvae from Thuringia, Germany (giving the species name). Bt’s insecticidal activity comes from Cry (crystal) and Cyt (cytolytic) proteins that form the parasporal crystal. These proteins are activated in the insect midgut, where they bind to specific receptors on midgut epithelial cells, form pores, and cause cell lysis. Different Bt subspecies produce different Cry proteins with different target spectra — Btk (kurstaki) targets Lepidoptera, Bti (israelensis) targets Diptera (mosquitoes, blackflies), Btt (tenebrionis) targets Coleoptera (some beetles). Bt has an exceptional safety profile — it is essentially non-toxic to mammals, birds, fish, and most beneficial insects, making it the cornerstone of organic pest management worldwide. Bt genes (particularly cry1Ac) have also been engineered into crops — Bt cotton (introduced in India in 2002) now covers over 90 % of Indian cotton area and has dramatically reduced insecticide use, though concerns about resistance development and effects on non-target organisms remain.

⚠️ Symptoms to watch for

In susceptible insects: feeding cessation within 1–2 hours of toxin ingestion; paralysis of the gut within 4–24 hours; larvae become sluggish, stop feeding, and develop a darker colour; septicaemia (bacterial infection of the haemolymph) follows gut wall breakdown; larvae die within 2–5 days from a combination of toxin action and bacterial septicaemia. Dead larvae are flaccid, dark, and may hang from the plant by their prolegs before dropping. In non-target organisms (mammals, birds, fish, most beneficial insects): no effect — the toxin requires activation by specific proteases in the insect midgut and binds to specific receptors on midgut epithelial cells.

🩹 Treatment

Apply Bacillus thuringiensis var. kurstaki (Btk) at 1 g/L for control of lepidopteran caterpillars (Helicoverpa, Spodoptera, Plutella, Pectinophora). Apply Bt var. israelensis (Bti) at 1 g/m² for control of mosquito and blackfly larvae. Apply Bt var. tenebrionis (Btt) for control of coleopteran larvae (Colorado potato beetle). Always apply in the evening (UV degrades Bt within 24–48 hours of sunlight exposure) and use a UV protectant (e.g., molasses at 1 %) to extend field persistence. Use a sticker-spreader to improve coverage on waxy leaf surfaces.

🌿 Organic treatments

Bacillus thuringiensis is itself the primary organic treatment for caterpillar pests. Apply Btk at 1–2 g/L in the evening. Add molasses (1 %) as a UV protectant and feeding stimulant. Add a non-ionic surfactant at 0.1 % to improve coverage. Apply at first sign of caterpillar infestation — early instars (L1–L3) are most susceptible; older instars require higher doses. Repeat at 7–10 day intervals during peak pest activity. Compatible with most other organic inputs (neem, predatory insects, parasitic wasps) — does not harm beneficial insects.

🧪 Chemical treatments

Not applicable — Bacillus thuringiensis is itself a biological insecticide used in organic and conventional IPM programmes. However, in conventional programmes, Bt can be tank-mixed with most chemical insecticides except those with high alkalinity (Bt is inactivated above pH 8). Bt is compatible with most common insecticides — always check the product label for specific compatibility information.

🛡️ Prevention

Not applicable — this is a beneficial microbial biocontrol agent applied as needed for pest management, not prevented. Conservation strategies: integrate Bt with other IPM tools (predatory insects, parasitic wasps, cultural practices). Avoid using Bt continuously on the same crop — over-reliance can select for Bt resistance in pest populations, as has been documented in Plutella xylostella in several countries including India. Rotate with other selective insecticides (spinosad, emamectin benzoate, chlorantraniliprole) to delay resistance development.

🪴 Affected plants

Not a plant pest — microbial pathogen of insects. Applied as a biological insecticide on a wide range of crops including cotton, vegetables (cabbage, cauliflower, tomato, brinjal, okra), pulses (chickpea, pigeonpea, soybean), fruits (apple, grape, mango), rice, maize, and stored grains. Different Bt subspecies target different insect orders.

🦋 Natural predators

Not applicable — Bacillus thuringiensis is itself a pathogen of insects and is not preyed upon by other organisms in the conventional sense. In the environment, Bt spores are degraded by UV radiation (sunlight), high temperature (>40 °C), and extreme pH (<4 or >8). Soil microorganisms can also degrade Bt spores over time. In aquatic environments, Bti spores settle to the bottom and degrade within weeks to months.

🌱 Host plants

Not host-specific to plants — microbial pathogen of insects. Applied as a biological insecticide on cotton, cabbage, cauliflower, broccoli, tomato, brinjal, okra, chickpea, pigeonpea, soybean, apple, grape, mango, rice, maize, stored grains, and many other crops.

🔄 Lifecycle

Bacillus thuringiensis is a soil bacterium — it does not have a 'lifecycle' in the conventional entomological sense.
In the environment: vegetative cells multiply in insect cadavers, form spores and parasporal crystals when nutrients are exhausted, and persist in soil as spores for years to decades.
Spores germinate when ingested by susceptible insects, completing the cycle.
In commercial production: Bt is mass-produced by fermentation in large bioreactors (50,000–100,000 L), harvested when sporulation is complete, formulated with UV protectants and stickers, and packaged for application.

📅 Season

Applied throughout the year as needed for pest management.
Most effective when target insect larvae are in early instars (L1–L3) and when weather conditions are favourable (moderate temperature, low UV, no rain for 24 hours after application).
Evening applications reduce UV degradation.
Field persistence is 3–7 days in tropical India, longer in cool temperate conditions.

🔎 Appearance

Bacillus thuringiensis (Bt) is a Gram-positive, spore-forming, facultative anaerobic soil bacterium.
Vegetative cells are rod-shaped, 1–1.5 µm × 3–5 µm, and occur singly or in chains.
The most diagnostic feature is the parasporal crystal — a diamond-shaped or bipyramidal protein crystal formed alongside the spore during sporulation.
This crystal (Cry protein or 'delta-endotoxin') is the active insecticidal component.
Commercial Bt products are formulated as wettable powders, water-dispersible granules, or liquid suspensions containing 5,000–32,000 International Units per mg of spores and crystals.

💥 Damage level

Beneficial

🔁 Reproduction rate

Fast

🗺️ Geographic regions

Cosmopolitan — Bacillus thuringiensis is naturally present in soils worldwide. In India
Bt-based biopesticides are produced by NBAIR Bangalore
several agricultural universities
and private companies including Bio-green
Pest Control India
and International Panaacea. Applied across all states where target pests occur.

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