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If you are looking for the single most effective treatment for box tree caterpillar, Xentari is the answer. It is the product recommended by professional gardeners, the RHS, and garden writers across the UK, and it is the treatment we use ourselves. In this review, we cover exactly what Xentari is, how it works, how to use it step by step, what results to expect, and the important questions about safety and legality in the UK.
Xentari at a Glance
| Product Name | Xentari WG |
| Manufacturer | Valent BioSciences |
| Active Ingredient | Bacillus thuringiensis var. kurstaki (Bt) |
| Type | Biological insecticide (water-dispersible granule) |
| Targets | Caterpillars (Lepidoptera larvae) only |
| Safe for Bees? | Yes — completely non-toxic to bees and beneficial insects |
| Safe for Pets/Children? | Yes — non-toxic to mammals, birds, fish |
| Mixing Rate | 1 gram per litre of water (5g per 5L sprayer) |
| Application Method | Foliar spray using pressure sprayer |
| Re-application | Every 7-14 days during active infestation |
| Speed of Action | Caterpillars stop feeding within hours; die within 2-5 days |
| UK Availability | Widely available online (Amazon, eBay, specialist retailers) |
| UK Legal Status | Approved for professional use; widely used by amateur gardeners |
| Our Rating | ★★★★★ (5/5) — The best treatment available |
What is Xentari?
Xentari WG is a biological insecticide manufactured by Valent BioSciences. Its active ingredient is Bacillus thuringiensis var. kurstaki (commonly abbreviated to Bt or Btk), a naturally occurring soil bacterium that has been used in agriculture and horticulture for decades as a safe, targeted pest control agent.
Xentari is sold as a water-dispersible granule (the ‘WG’ in the name). You mix the granules with water in a sprayer and apply it to the leaves of your box plants. When caterpillars eat the treated leaves, the Bt proteins destroy their gut lining, causing them to stop feeding within hours and die within 2-5 days.
What makes Bt remarkable is its specificity. The toxic proteins only activate in the highly alkaline gut of Lepidopteran larvae (caterpillars). They pass harmlessly through every other type of organism — bees, wasps, ladybirds, beetles, spiders, birds, fish, mammals, and humans. This makes Xentari one of the safest pest control products available anywhere.
How Does Xentari Work?
The science behind Xentari is elegantly simple:
- You spray Xentari solution onto the leaves of your box plant, coating both upper and lower leaf surfaces.
- Box tree caterpillars eat the treated leaves as part of their normal feeding behaviour.
- Inside the caterpillar’s gut — which is highly alkaline (pH 10-11), unlike the acidic stomachs of most other organisms — the Bt protein crystals dissolve.
- The dissolved proteins bind to specific receptors in the caterpillar’s gut wall, creating pores that destroy the gut lining.
- The caterpillar stops feeding within 1-4 hours as its gut is destroyed.
- Bacteria from the gut leak into the body cavity, causing a fatal septicaemia. The caterpillar dies within 2-5 days.
Because the Bt proteins require the specific alkaline gut chemistry and receptor sites found only in Lepidopteran larvae, they have absolutely no effect on any other organisms. A bee landing on a treated leaf is completely unaffected. A bird eating a dead caterpillar is completely unaffected. This specificity is why Bt products are approved for organic farming worldwide.
How to Use Xentari: Step-by-Step Guide
Follow these instructions carefully for the best results. The most common reason Xentari appears not to work is insufficient coverage — the caterpillars sheltering deep inside the plant did not eat enough treated leaf material.
What You Will Need
- Xentari WG (one sachet treats a large number of box plants)
- A 5-litre (or larger) pressure sprayer with an adjustable nozzle
- A few drops of washing-up liquid (as a surfactant/wetting agent)
- Gloves and eye protection (standard good practice when spraying)
- A dry evening with no rain forecast for at least 6 hours (ideally 24 hours)
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Mixing Instructions
- Fill your sprayer with 5 litres of clean water at room temperature. Cold water can reduce mixing efficiency.
- Measure 5 grams of Xentari WG (approximately one level teaspoon). This gives the standard dilution rate of 1 gram per litre.
- Add the Xentari granules to the sprayer. Shake or stir vigorously for 2-3 minutes until fully dissolved. The solution should be slightly milky.
- Add 3-4 drops of ordinary washing-up liquid. This breaks the surface tension of the water and helps the spray stick to the waxy box leaves rather than beading off. Do not add too much — a few drops is sufficient.
- Pressurise the sprayer and you are ready to go.
Application Instructions
- Start spraying in the late afternoon or evening. UV light from direct sunlight degrades the Bt proteins, so evening application gives the longest effective life on the leaves.
- Spray the entire plant thoroughly. This is the critical step. You must coat all leaf surfaces — tops, undersides, and the inner foliage that is hidden by the outer growth. Part the branches with your free hand and spray directly into the interior of the plant.
- Keep spraying until you can see the solution dripping from the inner branches. If the inner foliage is not visibly wet, you have not applied enough.
- Pay special attention to any areas where you can see webbing, frass, or visible caterpillars — these are the areas of highest caterpillar concentration.
- Move around the plant and spray from multiple angles to ensure complete coverage.
- If you have a long box hedge, work systematically from one end to the other so you do not miss any sections.
Re-Application Schedule
A single application of Xentari is rarely sufficient on its own. Re-apply according to this schedule:
- Second application: 7-10 days after the first. This catches any caterpillars that were eggs or very early-stage larvae during the first spray and have since hatched and begun feeding.
- Third application (if needed): 7-10 days after the second, for severe infestations only.
- After heavy rain: If heavy rain occurs within 6 hours of application, re-apply on the next dry evening. Light rain after 6+ hours is usually acceptable.
- Subsequent generations: When pheromone traps or visual inspection indicate new caterpillar activity (typically July-August for the second generation), begin the application cycle again.
What Results to Expect
Based on our experience and the consistent reports from UK gardeners:
- Within 1-4 hours: Caterpillars that have eaten treated leaves stop feeding. You may notice them becoming sluggish and less active.
- Within 2-3 days: Affected caterpillars begin dying. You may find dead or dying caterpillars in the webbing or on the ground beneath the plant.
- Within 5-7 days: The majority of caterpillars that ate treated leaves are dead. The plant should show no further signs of new feeding damage.
- Within 2-4 weeks: Dead leaves and webbing can be washed out with a hose or will gradually be displaced by new growth. The plant begins to show signs of recovery.
- Within 1-2 growing seasons: With continued protection and feeding, a defoliated box plant will fill out and return to health.
| Tip: After Treatment Once the caterpillars are dealt with, feed your box plants with a specialist Buxus fertiliser to encourage vigorous regrowth. Healthy, well-fed plants recover much faster than neglected ones. |
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Troubleshooting: Xentari Not Working?
If you have applied Xentari but the caterpillars are still alive and feeding, the issue is almost always one of the following:
| Problem | Likely Cause | Solution |
| Caterpillars still alive after 3 days | Insufficient coverage — the caterpillars did not eat treated leaves | Re-spray with better coverage, especially deep inside the plant. Add washing-up liquid as surfactant. |
| Caterpillars return 2-3 weeks later | New generation hatched from eggs that were present during treatment | Normal — apply a second round of Xentari. Bt does not kill eggs, only feeding caterpillars. |
| Rain within hours of spraying | Xentari washed off before caterpillars could ingest it | Re-apply on the next dry evening. Always check the forecast before spraying. |
| Xentari does not dissolve properly | Water too cold or insufficient mixing | Use room-temperature water. Shake the sprayer vigorously for 2-3 minutes. Pre-mix in a small amount of warm water first. |
| Plant still looks bad after treatment | Dead leaves remain on plant; new growth takes time | Be patient. Remove dead leaves by hand or with a jet of water. Feed the plant. Recovery takes weeks to months. |
Is Xentari Safe?
Safe for Bees and Pollinators
Yes, completely. Bt proteins only affect organisms with the specific alkaline gut chemistry and receptor sites found in Lepidopteran larvae (caterpillars and moth/butterfly larvae). Bees have acidic guts and completely different gut receptors — the Bt proteins pass through their systems without any effect. This has been confirmed by decades of scientific research and real-world use in organic agriculture worldwide.
Safe for Pets and Children
Yes. Bt is non-toxic to mammals. The proteins are digested harmlessly in the acidic mammalian stomach. Xentari is classified by the World Health Organisation as a Group III substance (unlikely to cause harm). However, as with any garden product, use common-sense precautions: wear gloves when mixing, do not spray directly into your eyes, and keep children and pets away from the wet spray until it has dried on the leaves (typically 1-2 hours).
Safe for Birds and Wildlife
Yes. Birds that eat caterpillars killed by Bt are completely unaffected. Ground beetles, spiders, frogs, hedgehogs, and other garden wildlife are also unaffected. In fact, by eliminating box tree caterpillar without harming beneficial insects, Xentari actively supports garden biodiversity.
What About Other Caterpillars and Butterflies?
This is the one area where caution is warranted. Because Bt targets all Lepidopteran larvae, it can potentially harm caterpillars of native moths and butterflies if they are directly sprayed or eat treated leaves. In practice, this risk is very low when treating box plants, because very few native caterpillar species feed on box. However, avoid spraying other plants in your garden that may host native caterpillars, and do not spray wildflowers or areas where you know native butterfly species are present.
Is Xentari Legal in the UK?
This is a question many gardeners ask. Xentari is fully approved and legal for use in the UK as a plant protection product. However, it is technically authorised for professional use only — meaning it is licensed for use by professional gardeners, landscapers, and pest controllers rather than amateur domestic gardeners.
In practice, Xentari is widely available for purchase by anyone from Amazon, eBay, and specialist garden retailers. Thousands of amateur gardeners across the UK use it every year with no issues. There are no known cases of any enforcement action against a domestic gardener for using Bt products on their own garden plants.
If you prefer to use a product specifically licensed for amateur use, alternatives containing Bt include some products marketed under different brand names. However, Xentari remains the most widely available and well-regarded formulation.
Xentari vs Other Treatments
- Xentari vs Nematodes: Xentari is more effective, more reliable, and less dependent on weather conditions. Nematodes require moist conditions and moderate temperatures to work. We recommend Xentari as the primary treatment and nematodes as a supplementary option.
- Xentari vs Chemical Insecticide: Xentari is equally effective at killing caterpillars but is completely safe for bees and beneficial insects. Chemical insecticides are broad-spectrum and kill everything. There is no good reason to choose chemical insecticide over Xentari unless Bt treatment has genuinely failed (which is rare with proper application).
- Xentari vs Manual Removal: Manual removal is free and chemical-free, but is very time-consuming and cannot reach every caterpillar deep inside a hedge. The best approach is to combine both: manually remove the largest caterpillars first, then spray with Xentari.
- Xentari vs Pheromone Traps: Pheromone traps are a monitoring tool, not a treatment. They tell you when to spray Xentari. Use both together for the best results.
Where to Buy Xentari in the UK
Xentari is available from several sources in the UK:
- Amazon UK — the most convenient option for most gardeners. Available in various pack sizes. Prime delivery available.
- eBay UK — often available from specialist horticultural sellers.
- Specialist online retailers such as Andermatt Garden (andermattgarden.co.uk).
- Some garden centres and independent nurseries stock Bt products, though availability varies.
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Frequently Asked Questions About Xentari
How quickly does Xentari kill box tree caterpillars?
Caterpillars stop feeding within 1-4 hours of eating treated leaves. They die within 2-5 days. You should see a significant reduction in caterpillar activity within the first week.
How often should I spray Xentari?
Apply twice per generation at 7-10 day intervals. For the first generation (March-May), spray when caterpillars are first spotted and again 7-10 days later. Repeat for the second generation (July-August). In warm years with a third generation (September-October), spray again if active caterpillars are found.
Can I use Xentari in the rain?
Do not spray if rain is expected within 6 hours. Light rain after 6+ hours is usually acceptable, though 24 hours of dry weather after application is ideal. If heavy rain falls within the first few hours, re-apply on the next dry evening.
How much Xentari do I need?
Mix at 1 gram per litre of water. A 5-litre sprayer dose (5 grams) will treat approximately 4-6 medium box balls or 2-3 metres of box hedge, depending on hedge height and density. For a large garden with extensive box hedging, you may need 15-20 litres of spray solution per application.
Does Xentari have a shelf life?
Yes. Store Xentari in a cool, dry place away from direct sunlight. Unopened, it has a shelf life of approximately 2-3 years. Once opened, use within one season for best results. Mixed spray solution should be used immediately and not stored.
Can I use Xentari on edible plants?
Bt products are approved for use on edible crops and are widely used in organic farming. Xentari is effective against caterpillar pests on brassicas (cabbage white butterfly), fruit trees, and other edible plants. However, always check the product label for specific crop guidance and observe any stated harvest intervals.
Is Xentari the same as Dipel?
Both Xentari and Dipel contain Bacillus thuringiensis as the active ingredient, but they use different strains and formulations. Xentari uses Bt var. kurstaki strain ABTS-1857 and is generally considered more effective against box tree caterpillar. Dipel uses Bt var. kurstaki strain HD-1. Both work on the same principle, but Xentari is the preferred choice for box tree caterpillar treatment in the UK.
Will Xentari kill all the caterpillars on my box?
Xentari will kill all caterpillars that eat a sufficient quantity of treated leaves. It will not kill eggs (which have not yet hatched and are not feeding) or pupae (which are not feeding). This is why a second application 7-10 days later is important — it catches newly hatched caterpillars that were eggs during the first spray. With two properly applied treatments per generation, you can expect to eliminate virtually all active caterpillars.
Our Verdict
Xentari is, in our assessment, the best treatment available for box tree caterpillar in the UK. It is highly effective, completely safe for bees and beneficial insects, non-toxic to pets and children, and straightforward to use. The only real limitation is that it must be applied with good coverage and re-applied for each new generation of caterpillars — it is not a one-and-done solution. But then, no treatment for box tree caterpillar is. For any UK gardener serious about keeping their box hedges alive, Xentari should be the first product in the toolbox.
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