← All guides · Diagnosis · Pests & disease · Updated September 2026

Box Hedge Going Bare or Webbed? Diagnosing Caterpillar vs Blight

You look at the box hedge and something is wrong: sections bare of leaves, or a papery white webbing threaded through the canopy, or leaves browning and dropping onto the soil beneath. Box hedges in Whitstable, Chestfield and the old town have two common enemies right now, and they need completely different treatment. Diagnosing which you're dealing with takes about two minutes.

Quick diagnosis

Symptom: box hedge losing leaves, going bare in sections, showing white webbing, or with leaves browning and falling.

White, silky webbing threaded through sections of the canopy, leaves half-eaten or stripped, tiny green or brown pellets on the soil surface underneath: → Box tree caterpillar. Skip to "Confirming it's caterpillar" below.
No webbing. Leaves turning straw-brown, still visible on the soil beneath the plant. Spreading from a sheltered or damp section of the hedge: → More likely box blight (fungal). Skip to "Confirming it's blight" below.
Bare patches specifically at the base of an otherwise-healthy hedge, no webbing, no brown falling leaves: → Different cause: probably cutting angle or shading, not caterpillar or blight. See the bare patches guide instead.
Yellowing spreading outward from one plant along the hedge row, and stems lift from the soil with almost no root ball attached: → Possible honey fungus (Armillaria). See "Honey fungus: the warning signs" below before treating for caterpillar or blight.
Both webbing and brown falling leaves in the same hedge: → Both problems can be present simultaneously. Treat the caterpillar infestation first, then reassess the remaining blight once the defoliation has stopped.

Confirming it's caterpillar

Box tree moth (Cydalima perspectalis) arrived in the south-east of England in 2011 and spread quickly across Kent. By 2024 the Royal Horticultural Society confirmed it in every English county, and in the first four months of that year the RHS received almost five times as many enquiries about it as in the same period the previous year. If you have box hedging in Whitstable or Chestfield and it's looking thin, caterpillar is now the first thing to check, not the last.

The adult moth is not the problem; the larvae are. Look for:

Webbing. Fine white silk threads draped between twigs, often enclosing small pockets of stripped stem. It looks like a very fine spider web but denser and more irregular in shape.
Frass. Small cylindrical pellets, roughly 1-2mm across, initially bright green, turning brown as they dry. They collect on the soil surface under the hedge, caught in the webbing between leaves, and on lower foliage. If you shake a branch and green specks fall out, that is frass.
Caterpillars. Young caterpillars are pale greenish-yellow with black heads. As they grow they develop bold green and black stripes along their bodies and reach up to 4cm. They hide deep inside the canopy and are easier to spot by parting the branches than by looking from outside.

The moth produces two to three generations a year in UK conditions, active roughly April through to October. A warm autumn can extend this, so infestations found in September or October are still active and worth treating. The sheltered walled gardens behind Harbour Street and South Street in the old town provide the warm, still conditions that moth flight prefers on summer evenings.

What actually works against box tree caterpillar

There is a reasonably short list of genuinely effective options:

Remove caterpillars and webbing by hand

For a small infestation, this works. Part the branches and pick out caterpillars directly; remove sections of webbing with secateurs and dispose of them in household waste (not the compost heap). Do this early morning or late afternoon when caterpillars are feeding rather than sheltering deep in the canopy. Check again two to three days later, as eggs hatch in batches.

Prune out heavily infested sections

Where an entire section of the hedge is heavily webbed and defoliated, prune it back hard enough to remove the infestation, provided the hedge species will tolerate it (box Buxus sempervirens regenerates well if cut back to living wood). Burn or bin the prunings; do not compost them. Time this cut outside the nesting season if possible - see "Timing and the nesting season" below.

Biological control (professional application)

Bacillus thuringiensis var. kurstaki (Btk) is an effective biological treatment that targets caterpillar larvae while leaving other insects unharmed. In the UK it is not licensed for home use on ornamental hedges and must be applied by a professional. It requires multiple applications across the season whenever the temperature is at least 15°C, which in Kent that typically means April through September. We use this approach on commercial and residential box hedges we maintain regularly.

Pheromone traps (monitoring, not control)

Sticky pheromone traps baited with a synthetic female-moth scent catch male moths before they mate. They are useful as an early-warning system, telling you when moths are flying in your garden so you know when to check closely for eggs and young larvae. They are not a standalone control measure for an existing infestation, but they are a good early indicator and worth setting from April onward if you had a problem the previous season.

Confirming it's blight

Box blight is a fungal disease caused by Calonectria pseudonaviculata (previously known as Cylindrocladium buxicola). It favours humid, poorly ventilated conditions and spreads readily on contaminated tools, hands, clothing and infected plant material. The sheltered walled gardens near the Whitstable Conservation Area, and along the older terraced streets around Island Wall, tend to have the enclosed microclimates where blight spreads most aggressively.

To confirm blight rather than caterpillar damage:

No webbing or frass. Blight does not produce webbing or pellets. If either is present, you have caterpillar (though blight can follow a caterpillar infestation).
Leaves still on the soil. Caterpillars eat the leaves completely. Blight turns them brown and then they fall, so they are usually visible on the soil surface beneath the plant, or tangled in the inner branch structure. If the leaves have simply vanished, caterpillar is more likely.
Black stem streaks. Infected stems often show a distinctive blackening or dark brown streaking below the leaves on the affected sections.
White spore masses. In damp weather, you may see pale white or cream-coloured spore masses on the undersides of infected leaves. These are the fungal fruiting bodies and confirm blight positively, though they are not always visible.
Pattern of spread. Blight tends to move outward from a point of first infection, typically where there is least air movement: the interior of a dense hedge, a corner with little air circulation, or sections nearest a wall or fence. This is a less directional spread than caterpillar damage, which tends to cluster where moths laid eggs.

Treating box blight

There is no chemical cure available to home gardeners that eliminates box blight once established, and the fungicides that were licensed for professional use have become increasingly restricted. The practical approach is management rather than elimination:

Prune out infected material. Cut back affected stems to visibly clean tissue, checking the cut end is pale-coloured with no brown streaking. Sterilise secateurs between cuts (a 10% bleach solution or proprietary tool disinfectant) to avoid spreading spores to clean sections.
Destroy prunings. Bin or burn them. The spores are viable for a long time in composted material and will re-infect the hedge or spread to neighbouring box.
Improve airflow. Blight loves still, damp conditions. A slight reduction in overall hedge density, and keeping the centre open when clipping, reduces the microclimate the fungus needs. Avoid wetting the foliage when watering, and do not clip in wet weather.
Do not spread infected material on tools or clothing. One of the most common ways blight jumps to a clean hedge is on unsterilised blades. We sterilise between every hedge on a commercial round.

If blight has killed the majority of the box in a hedge run, replacement planting with a resistant or tolerant alternative (such as Ilex crenata, holly trimmed small, or Euonymus japonicus) is often the most practical long-term route. Both perform well in the climate along this coast.

Honey fungus: the warning signs

If the yellowing is spreading from one plant progressively to its neighbour along the row, and affected plants lift out of the soil surprisingly easily because their roots have rotted, the cause may be honey fungus (Armillaria species). This is a soil-borne root disease rather than a pest or foliar fungus, and it is both serious and persistent. In Whitstable gardens it tends to appear in older plots with established woody root systems from previous planting.

Firm indicators: a distinctive honey-scented white fungal tissue (mycelium) visible under the bark at the base of the stem when you peel back the outer layer; black, shoelace-like rhizomorphs (strands) in the surrounding soil; and clusters of tan-coloured honey fungus toadstools appearing at the base in autumn. If you see these signs, the root system is already compromised and treating the foliage above it will not reverse the damage. This needs a separate assessment rather than the caterpillar or blight treatments described above.

Timing and the nesting season

Box hedges in Whitstable gardens, including the dense old-town parterre-style plantings, are used as nest sites by blackbirds, robins, dunnocks and wren in particular. If you are planning to prune out heavily infested or blighted sections, check the hedge interior for active nests first, especially between 1 March and 31 August. Under the Wildlife and Countryside Act 1981, section 1, it is a criminal offence to intentionally or recklessly damage or destroy a nest that is in use or being built. Recklessly counts, so carry out a careful check before cutting and do not rely on not having noticed a nest. Outside the nesting season, or where you have confirmed no active nest is present, the pruning can go ahead. We check before every cut on maintained hedges regardless of time of year.

Not sure which problem you're dealing with?

Send a couple of close-up photos (including any webbing, pellets or brown fallen leaves visible under the plant) and your postcode to hello@whitstablehedges.co.uk, or call 07763 100 477. We'll give you a straight answer on whether it's caterpillar, blight or something else, and what the realistic treatment options are. For maintained box hedges we can also apply professional biological control where appropriate.

For related problems, see also: bare patches at the base of an established hedge, and the one-side dying guide if the problem is more broadly distributed across the hedge rather than specific to a section.

Sources: Royal Horticultural Society advice on box tree caterpillar (Cydalima perspectalis) and box blight (Calonectria pseudonaviculata), including the RHS 2024 bumper-year report; RHS YouTube, "How To Tackle Box Tree Caterpillars" (14 May 2024); Wildlife and Countryside Act 1981, section 1 (nest protection); UK Health and Safety Executive pesticide registration status for Btk on ornamental hedges.