Caterpillar

Bt + predators.

Caterpillar control uses Bacillus thuringiensis in sensitive crops…

About Caterpillars

Caterpillars are the larval stage of moths and butterflies, and they are among the most destructive chewing pests in UK horticulture. Species such as cabbage moth, diamondback moth and tortrix moths feed aggressively on soft tissue, leaving large holes, frass (insect droppings) and structural damage to leaves and fruit. In protected crops, populations can build rapidly due to stable temperatures and sheltered growing conditions.

Symptoms of Caterpillar Damage

• Large irregular holes in leaves and growing tips
• Frass (black granular droppings) on foliage or trays
• Skeletonised leaves where tissue has been removed
• Silken webbing (Tortrix and leaf-roller species)
• Damaged buds, flowers or young fruit
• Presence of green or brown larvae hiding along midribs

Why Biological Control Works

Caterpillars are highly susceptible to microbial insecticides such as Bacillus thuringiensis (Bt), which disrupts gut function and kills larvae that feed on treated leaves. Bt is perfectly suited to edible and ornamental crops because it leaves no residues and is completely safe for beneficial insects, pollinators and mammals. Preventative monitoring and early intervention with Bt drastically reduce damage.

Common UK Caterpillar Species

• Plutella xylostella (Diamondback moth)
• Mamestra brassicae (Cabbage moth)
• Autographa gamma (Silver Y moth)
• Adoxophyes orana (Summer fruit tortrix)
• Cnephasia spp. (Leaf-rollers)

IPM Recommendations

• Use pheromone traps to monitor adult flight activity
• Apply Bt early when larvae are small for maximum effect
• Ensure full leaf coverage for consistent ingestion
• Remove heavily damaged plant material to reduce hotspots
• Encourage natural predators such as lacewings and ground beetles
• Maintain hygiene in propagation areas to prevent moth laying

Preventative vs Curative Strategy

Preventative:

• Weekly monitoring with pheromone traps to detect adult moth flights.
• Light applications of Bt where historical pressure is high.
• Early removal of webbed leaves (tortrix) before larvae spread.

Curative:

• Full-rate Bt applications when small larvae are visible on foliage.
• Double-check undersides of leaves and the midrib for rolled tissue.
• Consider repeating Bt applications within 5–7 days in high pressure environments.

Environmental Notes

• Bt is most effective on small, actively feeding larvae (1st–2nd instar)
• Bright light and warm temperatures accelerate larval feeding
• High humidity can increase disease pressure on larvae
• Tortrix species hide in leaf folds — inspect carefully