When to Spray for Box Tree Caterpillar (UK Timing Guide)
Why does timing matter so much?
Bt has to be eaten to work, and it does not persist. Sunlight breaks it down within a couple of days and rain washes it off. So a spray only protects you for as long as there is treated leaf and a caterpillar actively feeding on it — which means spraying when the caterpillars are small and eating, not before they hatch and not after they have pupated.
Get the first generation early and you remove most of the moths that would have laid the second generation. Miss it, and you spend the rest of the summer fighting a population that doubled while you were not looking.
How do I know a generation has started?
Three signals, in the order they appear. Moths in your pheromone trap come first — that means eggs are being laid, and caterpillars are two to three weeks out. Then fresh frass: tiny green-brown pellets on the leaves below the plant or on paving underneath it. Then webbing, fine white silk binding leaves together deep inside the plant.
By the time you see brown patches from across the garden you are weeks late. Frass is the signal to act on.
Recommended
TOPBUXUS Box Tree Moth Trap — Delta trap + 2 pheromone lures — a full season
What time of day should I spray?
Evening, after the sun is off the hedge. Bt proteins are degraded by ultraviolet light, so a spray applied at midday in June can be substantially spent before the caterpillars have eaten enough of it. Evening application gives it the whole night to sit on the leaf.
It also matters for a reason that has nothing to do with chemistry: box tree caterpillars feed more actively in the evening, so more of them are out on the leaf surface where the spray is.
The six-hour rule
Check the forecast before you mix. Bt needs about six hours on the leaf before rain to be effective, and ideally a full day. If it is going to rain overnight, wait — a washed-off spray is a wasted sachet and, worse, a wasted week.
Recommended
TOPBUXUS ABC — Anti Box Tree Caterpillar — 15 g treats 150 m² · 5 × 3 g sachets
How often should I re-apply?
Every 7 to 10 days while a generation is active, for two applications minimum and three in a heavy year. This is not about topping up the dose. Eggs hatch over a period of days, so a single spray only catches the larvae that were already feeding. The second application catches the ones that hatched after the first had degraded.
Stop when you stop finding fresh frass. Then start looking again about six weeks later, because the next generation is coming.
The UK spraying calendar, month by month
| Month | What is happening | What to do |
|---|---|---|
| February | Overwintering larvae dormant inside the plant | Buy your Bt and trap now, before stock tightens |
| March | Larvae become active as temperatures rise | Start inspecting fortnightly. Look inside the plant |
| April | First-generation feeding begins; hang traps mid-month | Spray at first frass. This is the critical window |
| May | Peak first-generation feeding | Second spray 7–10 days after the first |
| June | Pupation; first moths on the wing | Watch the trap. Little to spray |
| July | Second generation hatches | Spray again at first frass |
| August | Peak second-generation feeding | Second spray 7–10 days later |
| September | Possible third generation in a warm year | Inspect; spray only if you find fresh frass |
| October | Larvae spin hibernacula to overwinter | Last chance to treat. Clear fallen debris |
| November–January | Dormant | Feed the plant. Plan next year |
Does this change where I live?
Yes — by about two to three weeks. Southern England runs earliest and warmest, and reliably gets two generations with a third in a hot year. Scotland, the north east and Northern Ireland run later and often see only one full generation. The calendar above is written for southern England; add a fortnight if you are in the north.
Our regional guide breaks down severity and first-recorded dates by area.
What if I have already missed it?
Spray anyway. A hedge that has been stripped can still be saved if the stems are alive, and killing the current generation stops them pupating into the moths that lay the next one. Then read the rescue guide — scratch a stem, and if there is green under the bark the plant will come back.
What you need before the season starts:
- TOPBUXUS ABC — The one thing you must buyCheck price
- TOPBUXUS moth trap — Tells you when to sprayCheck price
Frequently asked questions
When should I spray for box tree caterpillar in the UK?
Spray at the first sign of fresh frass or webbing, which in most of the UK means late April to May for the first generation and mid-July to August for the second. Apply in the evening when no rain is forecast for about six hours, and repeat 7 to 10 days later. The first generation is the most important to catch.
How often should I spray for box tree caterpillar?
Every 7 to 10 days while a generation is active, with at least two applications per generation. Eggs hatch over several days, so one spray only catches the larvae already feeding; the second catches those that hatched afterwards. Stop when you stop finding fresh frass.
Can I spray box tree caterpillar in the rain?
No. Bt needs about six hours on the leaf before rain to work, and ideally a full day. Rain within that window washes it off and the treatment is wasted. Check the forecast before mixing — waiting a day costs you nothing, a washed-off spray costs you a week.
Should I spray box tree caterpillar in the morning or evening?
Evening. Ultraviolet light degrades Bt, so a spray applied in the middle of a bright day can be largely spent before the caterpillars eat enough of it. Box tree caterpillars also feed more actively in the evening, so more of them are on the leaf surface where the spray is.
Is it too late to spray in September?
No. If you find fresh frass in September you have an active third generation and it is worth treating — both to stop the damage and to reduce the number of larvae that successfully overwinter inside your plant. Treating late in the season makes the following spring easier.
How do I know the caterpillars have gone?
No fresh frass. Frass is the reliable signal because it appears daily while they are feeding. Clear the old pellets off the paving or leaves beneath the plant, then check again after two days — if nothing new has appeared, feeding has stopped.
When should I hang my pheromone trap?
Mid-April, before the first moths fly, and replace the lure around mid-July. Hanging it late is the most common mistake: the trap's value is the early warning it gives you, and a trap hung in June has already missed the signal that mattered most.
Do I need to spray every year?
In practice, yes. Box tree caterpillar is now permanently established across the UK and moths fly considerable distances, so a hedge that is clear one year will be found again the next. A monitoring trap plus two well-timed sprays a year is far less work than rescuing a stripped hedge.
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