Box Blight vs Box Tree Caterpillar: How to Tell the Difference

Both box blight and box tree caterpillar are devastating box plants across the UK. Many gardeners confuse the two because both cause browning and defoliation. But the causes are completely different—one is a fungal disease, the other an insect pest—and crucially the treatments are entirely different. Getting the wrong diagnosis means applying the wrong treatment and wasting time while damage continues.

Box blight is caused by fungi that thrive in warm, humid conditions. Box tree caterpillar is an invasive moth pest that feeds voraciously on foliage. They require opposite strategies to control. This guide explains how to tell them apart with absolute certainty, why each is so damaging, and what to do if you have one—or both.

Quick Identification Checklist

Scroll the diagram sideways to see all of it

If you're still unsure after reading this section, look for two specific signs: webbing and frass (droppings). These are exclusively caused by caterpillars. Blight never produces webbing or droppings. If you see either, you have caterpillars. If you see brown leaf spots and black stem streaks without webbing or droppings, you have blight.

The Key Differences — Side by Side Comparison

This detailed comparison table shows how box blight and box tree caterpillar differ across 13 key characteristics:

Box Blight — What It Is and How to Identify It

Box blight is a fungal disease caused by two species: Calonectria pseudonaviculata (formerly Cylindrocladium buxicola) and Volutella buxi. It has been present in the UK since 1994 and is responsible for destroying thousands of box hedges.

Key Symptoms of Box Blight

• Dark brown or black spots on leaves - often with a distinctive purple/red halo

• Black or dark brown streaks and lesions on stems - these are diagnostic for blight

• Rapid leaf drop once spots appear - leaves turn brown, then fall off

• White or pink fungal spore masses on the undersides of affected leaves (visible in humid conditions)

• Patchy die-back that can spread across a hedge over weeks or months

• Sometimes a musty or fungal smell around affected plants

Box blight thrives in warm (12-25°C), humid conditions. It spreads via rain splash from leaf to leaf, contaminated tools, and infected plant material. Poorly drained soil, overcrowded planting, and dense foliage all increase risk. Once established, blight can severely weaken or kill box plants if left untreated.

Box Tree Caterpillar — What It Is and How to Identify It

Box tree caterpillar (Cydalima perspectalis) is an invasive moth pest from Asia that arrived in the UK in 2012. The caterpillars feed voraciously on box foliage, leaving behind distinctive signs that make them relatively easy to identify once you know what to look for.

Key Symptoms of Box Tree Caterpillar

• Webbing - fine white silken threads stretched between leaves and branches

• Frass (droppings) - small green or brown pellets, especially visible on leaves and beneath the plant

• Skeletonised leaves - the caterpillars eat away the leaf tissue, leaving behind only the veins, creating a lace-like appearance

• The caterpillars themselves - green or yellow with black stripes running along their bodies, typically 1-3 cm long

• In severe cases, the green bark of young stems may be stripped, exposing lighter wood beneath

• No fungal spots or stem streaks - caterpillar damage affects leaf tissue, not the leaf spots characteristic of blight

Box tree caterpillar is an insect pest, not a disease. The damage it causes—defoliation and bark stripping—is physical feeding damage. Once caterpillars are controlled, affected leaves do not recover, but new growth will be healthy. Blight-affected leaves do not recover either, but for a different reason (the tissue is infected and necrotic).

Can You Have Both at the Same Time?

Yes, increasingly. Box blight weakens plants by killing foliage and stem tissue. A weakened plant is less able to recover from the physical defoliation caused by caterpillar feeding. The combined effect is devastating: a plant attacked by both diseases may be entirely killed within a single growing season, whereas either disease alone might be survivable with prompt treatment.

If you suspect both problems, check carefully for both webbing/frass AND brown spots/stem streaks. If present, treat both conditions simultaneously but with different products: apply fungicide for blight and XenTari (or nematodes) for caterpillars at the same time, following product instructions carefully.

Treatment Comparison

The treatments for the two problems are fundamentally different:

For Box Tree Caterpillar:

• XenTari (Bacillus thuringiensis) - an organic insecticide that kills caterpillars by disrupting their digestive systems. Most effective on small caterpillars (under 2 cm). Apply when you first notice caterpillars or webbing.

• Beneficial nematodes (Steinernema feltiae) - microscopic organisms that parasitise caterpillar larvae in the soil. Most useful for autumn/winter control of overwintering caterpillars.

• Pheromone traps - monitor adult moths to time your treatments. Traps do not control populations but give early warning.

• Manual removal - pull off webbed sections and crush caterpillars by hand (wear gloves). Only practical for small infestations.

For Box Blight:

• Fungicide (e.g. Fungus Clear Ultra) - applied as a preventative or at first sign of symptoms. Follow product instructions for timing and frequency.

• Improve air circulation - thin out dense foliage to allow air to move through the plant. Blight thrives in still, humid air.

• Avoid overhead watering - water at the base only. Overhead watering spreads spores.

• Disinfect tools after pruning - wipe secateurs with 10% bleach solution or 70% isopropyl alcohol between cuts. Blight spreads easily via contaminated tools.

• Remove and destroy fallen infected leaves - do not compost. Spores overwinter in dead leaves.

• Consider resistant alternatives - some box varieties have partial resistance (e.g. Buxus sinica var. insularis). Check availability from specialist nurseries.

Prevention Tips for Both Problems

These practices reduce the risk of both problems:

• Inspect plants every 2-3 weeks - early detection is critical. Check both sides of leaves and look deep into the foliage.

• Keep tools clean and disinfected - especially important for blight control, but good practice for all plants.

• Avoid overhead watering - water at the base or use drip irrigation. Blight spreads via water splash.

• Promote air circulation - prune to thin out dense areas, especially in the centre of hedges. Good air movement reduces humidity and blight risk.

• Buy from reputable nurseries - avoid buying box plants from discount chains with poor quality control. Inspect new plants carefully before planting.

• Inspect new purchases before planting - check undersides of leaves and stems for any signs of either problem. Contaminated plants spread both issues rapidly.

• Consider resistant alternatives - no box variety is 100% resistant, but some are more tolerant. Buxus sinica var. insularis and Buxus microphylla 'Faulkner' show better resilience in some trials.

• Quarantine new plants - if possible, keep new box plants separate for 2-3 weeks to watch for signs of disease or pests before planting.

Frequently asked questions