How to Identify Box Tree Caterpillar: 7 Signs to Look For

Bleached, papery leaves

Webbing across the foliage

A damaged patch in a green hedge

Bronzing — a different problem

Catching box tree caterpillar early is the single most important thing you can do to protect your box hedges. By the time most gardeners notice the damage — browned, defoliated patches and bare stems — the caterpillars have been feeding for weeks, and the damage is severe. This guide shows you exactly what to look for, when to look for it, and how to tell the difference between box tree caterpillar and the other common box disease: box blight.

The Seven Signs of Box Tree Caterpillar

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These are the signs to look for, listed in the order they typically appear. The first three are the earliest indicators and may be present before any visible leaf damage occurs.

1. Frass (Caterpillar Droppings)

Frass is often the very first sign of an infestation. It appears as tiny pellets — roughly 1-2 mm across — that are initially bright green (the colour of digested box leaves) and gradually turn brown as they dry. You will find frass on the soil surface beneath the box plant, caught in the webbing between leaves, and on the lower foliage of the plant.

To check for frass, crouch down and look at the ground directly beneath your box plant. If you see a scattering of small green or dark pellets that were not there before, suspect box tree caterpillar and inspect the plant immediately.

2. Webbing

Box tree caterpillars spin white, silken webbing amongst the leaves and inner shoots of the box plant. In early infestations this webbing is fine and difficult to spot without parting the outer foliage and looking inside. In severe infestations, thick webbing can cover entire sections of the plant, often matted with frass and shed caterpillar skins.

The webbing is distinctive and unlike anything produced by spiders. It forms a fine gauze or curtain between leaves and shoots, rather than the radial patterns of spider webs. If you see webbing inside your box plant, you almost certainly have box tree caterpillar.

3. Skeletonised Leaves

As caterpillars feed, they eat the green leaf tissue from the surface of box leaves, leaving only the transparent outer membrane. This creates a characteristic skeletonised or lace-like appearance — the leaves look papery, translucent, and pale rather than the normal deep green.

Skeletonised leaves are usually found on the inner branches first (where the caterpillars shelter) before the damage spreads outward. By the time skeletonisation is visible on the outer surface of a hedge, a significant amount of damage has already been done inside.

4. Complete Defoliation

In severe infestations, the caterpillars eat not just the surface of the leaves but the entire leaf. This results in bare, brown branches stripped of all foliage. Defoliation typically progresses from the interior of the plant outward and can happen shockingly quickly — a healthy-looking box hedge can be reduced to bare stems within two to three weeks during peak feeding.

5. Bark Stripping

This is the most serious sign and occurs only in severe infestations where the caterpillars have exhausted the leaf supply. Desperate caterpillars will strip the thin green bark from the stems of the box plant. When the bark is stripped all the way around a stem (girdling), that section of the plant dies permanently and cannot regenerate. If you see bark stripping, the infestation is severe and treatment is urgent.

6. The Caterpillars Themselves

Box tree caterpillars grow from approximately 1 mm at hatching to around 4 cm when fully grown. Their appearance changes as they mature:

• Early stage (1-10 mm): Pale greenish-yellow, almost translucent. Very difficult to spot. They tend to shelter deep inside the plant.

• Mid stage (10-25 mm): Distinctly greenish-yellow with emerging black head and developing black and white longitudinal stripes along the body.

• Late stage (25-40 mm): Clearly green-yellow with bold black and white stripes, black head, and visible body segments. Easy to identify at this stage.

To find the caterpillars, put on gloves and carefully part the outer branches of the box plant, looking on the undersides of leaves and within the webbing. They are most active in the evening and at night — checking after dark with a torch can be very effective.

7. Adult Moths

The adult box tree moth (Cydalima perspectalis) has a wingspan of approximately 40-45 mm. The most common form has white or very pale wings with a distinctive iridescent brown border around the edges. A less common dark form exists, with entirely brown wings and a small white mark. The moths are most active at dusk and are attracted to light — you may see them near outdoor lights or windows in the evening during the flight season (May to October).

Seeing adult moths near your box plants means eggs have already been laid or will be imminently. Begin checking for small caterpillars within one to two weeks of seeing the moths.

The Box Tree Caterpillar Life Cycle in the UK

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Understanding the life cycle helps you time your monitoring and treatment for maximum effectiveness.

• Eggs: Laid by female moths in overlapping clusters of 5-20 on the undersides of box leaves. They are pale yellow-green and flat, making them extremely difficult to spot. Each female can lay up to 300 eggs in her lifetime.

• Larvae (caterpillars): Hatch within 2-3 weeks. The caterpillars feed for several weeks, growing through multiple moults (instars) before spinning a cocoon to pupate. In the final generation of the year, small caterpillars spin a silken hibernaculum (overwintering cocoon) between two leaves and enter dormancy until the following spring.

• Pupae: The caterpillar spins a white silken cocoon within the box foliage and pupates inside for approximately 7-14 days before emerging as an adult moth.

• Adult moths: Emerge, mate, and lay eggs. Adult moths live for approximately 2 weeks. They are strong fliers and can travel considerable distances.

In the UK, there are typically two complete generations per year, with a possible third generation in warm years. This means you may face three separate waves of caterpillar damage between March and October.

Month-by-Month Monitoring Calendar

Use this calendar to know exactly when and what to look for throughout the year:

Box Tree Caterpillar vs Box Blight: Quick Comparison

One of the most common mistakes gardeners make is confusing box tree caterpillar damage with box blight (a fungal disease). The treatments are completely different, so correct identification is critical. Here is a quick comparison:

If you are unsure, the simplest test is to look for frass and webbing. If either is present, you have box tree caterpillar (you may also have blight alongside it). If there is no frass and no webbing but leaves are brown-spotted and dropping, it is more likely box blight. For a full detailed comparison, see our dedicated guide: Box Blight vs Box Tree Caterpillar — How to Tell the Difference.

What to Do If You Find Box Tree Caterpillar

If your inspection reveals any of the signs described above, take action immediately:

• Small/early infestation: Begin manual removal of caterpillars (pick them off into soapy water) and apply XenTari as soon as possible.

• Moderate infestation: Apply XenTari immediately, ensuring thorough coverage of all leaf surfaces including the interior. Re-apply after 7-10 days.

• Severe infestation: Combine manual removal of the largest caterpillars with XenTari application. You may need three applications at 7-10 day intervals.

For detailed treatment instructions, see our complete Box Tree Caterpillar Treatment Guide.

Frequently asked questions