If your box hedge is being eaten alive, you are not alone. Box tree caterpillar (Cydalima perspectalis) has become the most destructive garden pest to hit UK box plants in living memory, and the problem is getting worse every year. The Royal Horticultural Society reported a fivefold increase in sightings in early 2024 compared to the previous year, and the pest is now established across virtually every region of the United Kingdom.
The good news is that box tree caterpillar can be treated effectively. This guide walks you through every treatment option available to UK gardeners, from the most effective biological controls to manual removal techniques, with clear step-by-step instructions you can follow in your garden today.
| Which Treatment Should I Use? If you want the single most effective treatment: use Xentari (Bacillus thuringiensis) — see Section 2 below. If you prefer a hands-on approach with no chemicals at all: use manual removal — see Section 1. If you want early warning of moth activity: use pheromone traps — see Section 4. If you want a fully biological spray alternative: try nematodes — see Section 3. If the infestation is severe and nothing else has worked: consider chemical insecticide as a last resort — see Section 5. |
Treatment Comparison at a Glance
| Treatment Method | Effectiveness | Eco-Friendly? | Cost | Best For |
| Bacillus thuringiensis (Xentari) | Very High | Yes | Medium | All infestations — the gold standard treatment |
| Manual Removal | Medium-High | Yes | Free | Small plants and early-stage infestations |
| Pheromone Traps | Low-Medium | Yes | Low | Monitoring and early detection, not primary control |
| Nematodes | Medium | Yes | Medium | Gardeners wanting a fully biological approach |
| Chemical Insecticide | High | No | Low-Medium | Severe infestations where Bt has failed — last resort |
1. Manual Removal: The Hands-On Approach
Manual removal is the simplest, cheapest, and most environmentally friendly way to deal with box tree caterpillar. It is most effective for small plants (individual box balls, topiary, or small sections of hedge) and for early-stage infestations where caterpillar numbers are still relatively low.
It is also an essential complement to any spray treatment — even when using Xentari or nematodes, manually removing the largest caterpillars first significantly improves the effectiveness of the spray.
What You Will Need
- A pair of gardening gloves
- A bucket or bowl of warm soapy water (washing-up liquid is fine)
- A garden hose with a jet nozzle (optional but very helpful)
- Secateurs for removing heavily webbed sections
Step-by-Step Instructions
- Put on your gloves and carefully part the outer foliage of the box plant. Box tree caterpillars shelter deep inside the plant where they are hidden from view, so you need to look beyond the outer layer of leaves.
- Look for the caterpillars themselves (greenish-yellow with black and white stripes, up to 4 cm long), their webbing (white silken threads woven between leaves), and frass (small green or brown pellets of caterpillar droppings).
- Pick off every caterpillar you can find and drop them into the bucket of soapy water. The soap breaks the surface tension and prevents them from climbing out. They will drown within minutes.
- If the plant has heavy webbing, use secateurs to cut out the worst-affected sections and dispose of them in a sealed bag in your household waste bin. Do not compost them — the caterpillars and eggs can survive composting.
- Use a strong jet of water from a hose to blast the inside of the plant. This dislodges caterpillars and webbing you may have missed by hand. Place an old sheet or tarpaulin under the plant first to catch dislodged caterpillars, then dispose of them in the soapy water.
- Repeat this process every 2-3 days for at least two weeks. Caterpillars you missed on the first pass, or any that have since hatched from eggs, will be caught on subsequent passes.
| Tip Check your box plants after dark with a torch. Box tree caterpillars are more active in the evening and may be easier to spot feeding on the outer leaves at night. |
2. Bacillus thuringiensis (Xentari): The Most Effective Treatment
If you want the single most effective treatment for box tree caterpillar, this is it. Bacillus thuringiensis var. kurstaki (Bt) is a naturally occurring soil bacterium that produces proteins toxic specifically to caterpillars when ingested. It has no harmful effect on bees, beneficial insects, birds, mammals, pets, or humans. It is the treatment recommended by the RHS and by professional gardeners across the UK.
The most widely available Bt product for box tree caterpillar treatment is Xentari WG, manufactured by Valent BioSciences. It is sold as a water-dispersible granule that you mix with water and spray onto your box plants.
| RECOMMENDED PRODUCT: Xentari WG Insecticide The gold-standard biological treatment for box tree caterpillar. A naturally occurring bacterium that kills caterpillars within days while being completely safe for bees, birds, pets, and children. Visit Amazon to buy Xentari |
How Xentari Works
When caterpillars eat box leaves coated with Bt, the bacterial proteins dissolve in the caterpillar’s alkaline gut and destroy the gut lining. The caterpillars stop feeding within hours and die within 2-5 days. Because the toxin only activates in the specific gut chemistry of caterpillar species, it is harmless to all other organisms.
How to Mix and Apply Xentari
- Check the weather forecast. Xentari works best when applied in dry conditions with no rain forecast for at least 6 hours (ideally 24 hours). Apply in the evening to avoid UV degradation — direct sunlight breaks down the Bt proteins.
- Mix Xentari at a rate of 1 gram per litre of water in a pressure sprayer. For most domestic use, mixing 5 grams in a 5-litre sprayer is the standard dose. Shake or stir thoroughly to dissolve the granules completely.
- Add a few drops of washing-up liquid to the spray solution. This acts as a surfactant (wetting agent) and helps the spray stick to the waxy box leaves rather than beading off.
- Spray the entire box plant thoroughly. The key to success with Xentari is coverage — the caterpillars must eat treated leaves to be affected. Spray the tops of leaves, the undersides of leaves, and deep into the interior of the plant. Part the branches and spray inside. If you cannot see spray dripping from the inner branches, you have not applied enough.
- Pay particular attention to areas with visible webbing — this is where the caterpillars are most concentrated.
- Re-apply after 7-10 days. Xentari does not persist on leaves indefinitely — UV light and rain degrade it. A second application catches any caterpillars that were eggs or very early-stage larvae during the first application and have since begun feeding.
- For severe infestations, apply a third time 7-10 days after the second application.
| RECOMMENDED PRODUCT: 5-Litre Pressure Sprayer A good pressure sprayer is essential for thorough Xentari application. Look for one with an adjustable nozzle that can produce both a fine mist and a targeted jet for reaching deep inside hedges. Visit Amazon to buy a sprayer here |
When to Apply Xentari — The Treatment Calendar
Timing is critical. In the UK, box tree caterpillar typically has two to three generations per year. The key treatment windows are:
- First generation (March-May): Overwintered caterpillars resume feeding in spring. Start monitoring in March and apply Xentari as soon as small caterpillars or fresh frass are spotted. This is the most important treatment window — catching the first generation early prevents the population explosion that follows.
- Second generation (July-August): The second wave of caterpillars hatches from eggs laid by the first-generation moths. Apply Xentari again when new caterpillar activity is detected.
- Third generation (September-October): In warm years (like 2024), a third generation may occur. Monitor into autumn and treat if necessary.
| Important Note on UK Regulations Xentari is technically classified for professional use only in the UK. However, it is widely available for purchase online and is used by thousands of amateur gardeners. There are currently no known cases of enforcement against domestic gardeners using Bt products on their own plants. Always follow the label instructions and use appropriate personal protective equipment (gloves, eye protection) when mixing and spraying. |
3. Nematode Biological Controls
Nematodes are microscopic roundworms that can be sprayed onto box plants to infect and kill caterpillar larvae. Several products are now available in the UK specifically targeting box tree caterpillar, including formulations from Nemasys and other biological control suppliers.
How Nematodes Work
The nematodes enter the caterpillars through natural body openings, release bacteria that kill the host within 48-72 hours, and then reproduce inside the dead caterpillar before moving on to find new hosts. This provides a degree of ongoing, self-sustaining control that spray-and-forget products do not.
How to Apply Nematodes
- Mix the nematode sachet with water in a sprayer according to the packet instructions. The water must be at room temperature — cold water reduces nematode activity.
- Apply in the evening or on an overcast day. Nematodes are killed by UV light, so timing is critical.
- Spray thoroughly, covering all leaf surfaces and the inner plant structure.
- Keep the plant moist for at least 2 hours after application. Nematodes need a film of moisture to survive on the leaf surfaces and locate the caterpillars. Mist the plant with water if conditions are dry.
- Re-apply after 14 days.
| RECOMMENDED PRODUCT: Box Tree Caterpillar Nematodes A fully biological treatment using beneficial nematodes. Apply in the evening, keep plants moist, and let the nematodes do the work. Visit Amazon to buy Xentari |
Nematodes vs Xentari: Which Should I Use?
Xentari (Bt) is generally more effective and more reliable than nematodes for box tree caterpillar. Nematodes require specific conditions (moisture, moderate temperatures, evening application) and their effectiveness varies more widely. However, nematodes can be a good complementary treatment alongside Bt, or a suitable choice for gardeners who prefer a fully living biological control agent.
4. Pheromone Traps: Early Warning and Monitoring
Pheromone traps use a synthetic version of the female box tree moth’s sex pheromone to lure and trap male moths. They are not a standalone treatment — they will not eliminate an infestation on their own — but they are extremely valuable as a monitoring and early warning tool.
Why Use Pheromone Traps?
- They tell you exactly when the moths are active in your area, so you know when to start looking for caterpillars and when to time your spray treatments.
- They give you early warning before damage becomes visible — by the time you see the damage, the caterpillars have been feeding for weeks.
- In small gardens, trapping significant numbers of male moths can reduce the local breeding population, though this effect is limited in larger or open gardens.
How to Use Pheromone Traps
- Purchase a box tree moth pheromone trap kit. These include a trap (typically a triangular or delta-shaped unit) and pheromone lure capsules.
- Hang the trap at head height near your box plants, ideally in a sheltered spot. Position it within 2-3 metres of the box but not directly on it.
- Deploy from April through to October to cover the full moth flight season.
- Replace the pheromone lure capsule every 4-6 weeks (they gradually lose potency).
- Check the trap weekly. When you start catching moths, begin inspecting your box plants closely for eggs and small caterpillars, and prepare to apply treatment.
| RECOMMENDED PRODUCT: Box Tree Moth Pheromone Trap An essential monitoring tool. Hang near your box plants from April onwards to detect moth activity before the caterpillars cause visible damage. Visit Amazon to buy a pheromone trap here |
5. Chemical Insecticides: The Last Resort
Chemical insecticides containing pyrethrin, cypermethrin, or deltamethrin are effective against box tree caterpillar but should be considered a last resort. They are broad-spectrum, meaning they kill beneficial insects (including bees, hoverflies, ladybirds, and predatory wasps) alongside the target pest. They also kill the natural enemies of box tree caterpillar that are gradually building up in the UK ecosystem.
If You Must Use Chemical Insecticide
- Choose products containing natural pyrethrins (derived from chrysanthemum flowers) over synthetic pyrethroids. They are less persistent in the environment.
- Apply in the evening when pollinators are not active.
- Never spray when the box plant or any nearby plants are in flower.
- Spray inside the plant as well as the outer surfaces — caterpillars shelter deep within the foliage.
- Follow all label instructions regarding dilution rates, personal protective equipment, and re-entry intervals.
We recommend trying Xentari (Section 2) first. It is equally effective at killing caterpillars while being completely harmless to bees and other beneficial insects. Chemical insecticides should only be considered if Bt treatment has been tried and has genuinely failed — which is rare if applied correctly with good coverage.
6. After Treatment: Helping Your Box Recover
Once you have dealt with the caterpillar infestation, your box plants will need help to recover, particularly if they have suffered significant defoliation.
- Prune any dead or severely damaged branches back to healthy wood. Box is resilient and can regenerate from bare stems if the wood is still alive. Use sharp, clean secateurs and make cuts just above a leaf node or bud.
- Apply a specialist box fertiliser to encourage vigorous new growth. Buxus-specific feeds contain the right balance of nutrients for box plants and often include magnesium to promote the deep green leaf colour.
- Water the plants regularly, particularly during dry spells. Stressed plants recover more slowly.
- Mulch around the base of the plant with well-rotted compost or bark chips to retain moisture and suppress weeds.
- Be patient. A severely defoliated box plant can take 1-2 full growing seasons to fill out again. Continue fertilising through this period.
- Continue monitoring for re-infestation. Box tree caterpillar will return — it is now a permanent part of UK garden life. Maintain your pheromone traps, check plants regularly from March onwards, and be ready to treat again.
| RECOMMENDED PRODUCT: Buxus Specialist Fertiliser Help your box recover after caterpillar damage with a specialist Buxus feed containing the right nutrients for vigorous regrowth.Visit Amazon to buy Buxus fertiliser here |
7. Preventing Future Infestations
Complete prevention is not realistic — box tree moths can fly considerable distances and will find your box plants. However, you can significantly reduce the severity of infestations with these ongoing practices:
- Deploy pheromone traps every year from April to October.
- Inspect box plants closely every two weeks from March through October. Look inside the plant, not just at the outer leaves.
- Apply a preventative Xentari spray in April and again in July-August, even before you see caterpillars. This catches the earliest larvae before they can cause visible damage.
- When buying new box plants, inspect them extremely carefully before planting. The most common way box tree caterpillar spreads to new areas is through infested nursery stock.
- Encourage natural predators in your garden: birds (install nest boxes), wasps (avoid killing them), and ground beetles (maintain areas of ground cover and leaf litter).
- Consider whether some of your box could be replaced with resistant alternatives — see our guide to alternatives to box hedges.
What You Will Need: Products Summary
| Product | Type | When to Use | Link |
| Xentari WG | Biological insecticide (Bt) | Primary treatment — spray at first sign of caterpillars | Visit Amazon |
| Pheromone Trap | Monitoring trap | Deploy April-October to detect moth activity | Visit Amazon |
| Nematode Treatment | Biological control | Alternative/supplement to Bt in moist conditions | Visit Amazon |
| Pressure Sprayer (5L) | Application tool | Essential for thorough Bt application | Visit Amazon |
| Buxus Feed | Fertiliser | After treatment to aid plant recovery | Visit Amazon |
Frequently Asked Questions
What is the best treatment for box tree caterpillar in the UK?
The most effective treatment is Bacillus thuringiensis var. kurstaki, sold as Xentari WG. It is a biological insecticide that specifically targets caterpillars while being completely safe for bees, beneficial insects, birds, pets, and children. It is the treatment recommended by the RHS and by professional gardeners. Apply it as a spray, ensuring thorough coverage of all leaf surfaces including the interior of the plant.
When should I spray for box tree caterpillar?
The key treatment windows are: first generation in March-May (the most important — treat as soon as small caterpillars or fresh frass are spotted), second generation in July-August, and potentially a third generation in September-October during warm years. Apply Xentari in the evening when UV levels are low, and when no rain is forecast for at least 6 hours. For preventative treatment, spray in April and again in late July even before you see caterpillars.
Is Xentari safe for bees?
Yes. Bacillus thuringiensis var. kurstaki (the active ingredient in Xentari) is toxic only to Lepidopteran larvae (caterpillars and moth/butterfly larvae) when ingested. It has no effect on bees, wasps, beetles, spiders, birds, mammals, or any other organisms. This is because the Bt protein only activates in the highly alkaline gut of caterpillar species — it passes harmlessly through other organisms.
Can box recover from caterpillar damage?
Yes, in most cases. Box (Buxus) is a remarkably resilient plant and can recover from even complete defoliation, provided the stems remain alive. After treatment, prune dead wood, apply specialist Buxus fertiliser, water regularly, and be patient — full recovery can take 1-2 growing seasons. The exception is when caterpillars have stripped the green bark from stems, which kills those branches permanently. Remove any dead wood and the plant will regenerate from the remaining live stems.
How do I tell the difference between box blight and box tree caterpillar?
Box blight (caused by the fungi Cylindrocladium buxicola or Volutella buxi) causes brown leaf spots, black streaks on stems, and leaf drop — but no webbing, no frass, and no caterpillars. Box tree caterpillar damage is characterised by webbing between leaves, green/brown frass pellets beneath the plant, skeletonised leaves, and the visible caterpillars themselves. It is possible to have both problems simultaneously. For a detailed comparison, see our guide: Box Blight vs Box Tree Caterpillar — How to Tell the Difference.
Will box tree caterpillar spread to other plants in my garden?
Box tree caterpillar feeds almost exclusively on Buxus (box) species. Under conditions of extreme food scarcity — for example, if the box plant is already completely defoliated — there are rare reports of the caterpillars feeding on Euonymus and other related plants. However, in practice, your other garden plants are not at risk. The caterpillars will not spread to roses, lavender, shrubs, or any other common garden plants.
Should I just replace my box with something else?
That is a personal decision that depends on how much you value your box plants and how much time you are willing to invest in ongoing management. Box tree caterpillar is now a permanent part of UK garden life — it will not disappear. However, with a good treatment routine (monitoring, timely Xentari application, and regular checking), it is entirely manageable. Many gardeners successfully maintain beautiful box hedges and topiary alongside the caterpillar. If you do want to explore alternatives, Ilex crenata (Japanese holly) is the closest visual substitute. See our full guide to box hedge alternatives.
Are pheromone traps enough to control box tree caterpillar?
No. Pheromone traps catch male moths only and will not eliminate an infestation. They are best used as a monitoring tool — they tell you when the moths are active in your area so you can time your inspections and spray treatments. In a small, enclosed garden, trapping large numbers of males can reduce the local breeding population somewhat, but this should be seen as a bonus rather than a primary control strategy.