XenTari vs Nematodes for Box Tree Caterpillar: Which Works Better?
How do they actually differ?
Bt is a bacterium that has to be eaten. You spray it onto leaf, the caterpillar eats the leaf, and a protein in the bacterium destroys its gut lining. It stops feeding within about an hour and dies within three to five days. The spray does not need to hit the caterpillar — it needs to coat the leaf the caterpillar will eat next.
Nematodes are living roundworms that hunt. They enter the caterpillar through its natural openings and release bacteria that kill it within 48 to 72 hours. Because they are alive, they need to survive on the leaf long enough to find a host — which is where the conditions come in.
| Bt (TOPBUXUS ABC) | Nematodes | |
|---|---|---|
| What it is | A bacterium | Living roundworms |
| Reliability | High — works in most conditions | Variable — conditions decide |
| Temperature needed | Any UK spraying weather | 12–25 °C |
| Moisture needed | Dry for ~6 hours after spraying | Foliage damp for 2+ hours after |
| Time of day | Evening (UV degrades it) | Evening or overcast only |
| Speed | Feeding stops in ~1 hour | Death in 48–72 hours |
| Shelf life | About 3 years | Weeks — ships chilled, use immediately |
| Safe for bees | Yes | Yes |
| Re-apply | Every 7–10 days | Every 14 days |
Recommended
TOPBUXUS ABC — Anti Box Tree Caterpillar — 15 g treats 150 m² · 5 × 3 g sachets
Why do nematodes fail?
Almost always because of moisture. Nematodes need a film of water on the leaf to move through, and box leaves are small, waxy and shed water fast. On a breezy evening the foliage can be dry within twenty minutes, and at that point the nematodes are finished.
The other common failure is the supply chain. They are live organisms with a short shelf life, shipped chilled. A pack that sat in a warm depot over a weekend can arrive dead, and there is no way to tell by looking. Buy from a supplier who ships chilled with a clear use-by date, and order only when you are ready to apply within a few days.
What I got wrong the first time
I applied nematodes at seven in the evening in late May on a warm, breezy day, and misted once. The hedge was dry within half an hour and I saw no effect at all. The second attempt — overcast, still, misted three times over two hours — worked. Same product, same hedge, entirely different result.
When are nematodes actually worth using?
Three situations. If you want a living control agent on principle and are willing to work around the conditions. If you are already watering in the evening and the foliage will stay damp anyway. And as a second line alongside Bt during a heavy generation, where the two work on different timescales and catch different larvae.
What they are not is an easier option. They are harder to use correctly than Bt, and the failure mode is invisible until you have lost two weeks.
Recommended
TOPBUXUS ABC — Anti Box Tree Caterpillar — 15 g treats 150 m² · 5 × 3 g sachets
Can I use both?
Yes, and it is a reasonable strategy in a bad year. Do not tank-mix them — Bt wants a dry evening and nematodes want a damp one, so the conditions fight each other. Spray Bt first, then apply nematodes seven days later on a suitably overcast, still evening. Between them you cover both the fast kill and the slower, self-sustaining one.
What I use
Bt, every time, as the primary treatment. It is cheaper per treatment, it keeps for three years so I can buy it in February, and it does not care whether the evening is damp. I keep nematodes for the occasional heavy second generation, and I treat any result from them as a bonus rather than a plan. Our full nematode review covers application in detail.
If you buy one product, buy this. It is the same Bacillus thuringiensis sachet that was sold as TOPBUXUS XenTari for years — same active, same 3 g per 30 m² dose, new name. It stopped the feeding in my hedge within a day, and it is harmless to bees, birds, pets and children.
Check price on AmazonFrequently asked questions
Are nematodes better than XenTari for box tree caterpillar?
No. Both are biological and both are safe for pollinators, but Bt is far more reliable. Nematodes need damp foliage, temperatures between 12 and 25 °C, no direct sunlight and application within days of delivery. Bt works in almost any conditions a UK evening will give you and keeps for around three years.
Can I use nematodes and Bt together?
Yes, but not in the same tank. Bt needs a dry evening with no rain for about six hours; nematodes need the foliage to stay damp for at least two. Spray Bt first, then apply nematodes about a week later on an overcast, still evening.
Why did my nematodes not work?
Usually moisture. Box leaves are small and waxy and shed water quickly, so on a breezy evening the foliage can be dry within twenty minutes — and dry foliage kills nematodes. The second most common cause is a pack that was too warm in transit; they are live organisms and arrive dead if the cold chain breaks.
Are nematodes safe for bees and pets?
Yes. The species used for caterpillar control target insect larvae and are harmless to bees, birds, pets, wildlife and people. They occur naturally in soil worldwide.
How long do nematodes take to work?
48 to 72 hours from application, compared with about an hour for Bt to stop feeding. That delay matters during a heavy generation: caterpillars keep eating for two to three more days while the nematodes do their work.
Which is cheaper, Bt or nematodes?
Bt, on any realistic measure. A pack of Bt sachets treats 150 m² and keeps for about three years, so you can buy once and use it across several seasons. Nematodes have to be bought fresh for each application and cannot be stored.
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