Box Tree Caterpillar Stopped on a Buxus sempervirens Border with XenTari Spray

Webbing first, then the skeletonised leaves. That is the order most people notice a box tree caterpillar (Cydalima perspectivalis) infestation on a Buxus sempervirens border, and by then the larvae have usually been feeding for a week inside the bush where the spray never reaches. The moth arrived in Europe from East Asia around 2007 and spread across most of the continent within a decade. XenTari, sold in sachets of around 25 grams, contains Bacillus thuringiensis subspecies aizawai, a soil bacterium whose crystal proteins destroy the gut lining of the feeding larvae. Adult moths, bees, and ladybirds are untouched because they never eat the treated leaf.

Why the spray works on the caterpillar and nothing else

Bt aizawai produces protein crystals that only become toxic in the alkaline midgut of certain lepidopteran larvae. A caterpillar bites a treated leaf, the crystal dissolves, enzymes activate it, and it punches holes in the gut wall. The insect stops feeding within hours and dies over the following two or three days. A honeybee gathering nothing but nectar and pollen has an acidic gut and the wrong receptors, so the same protein passes through inert.

That selectivity is the whole reason XenTari sits in a different category from a broad pyrethroid. It does nothing to the moth, nothing to hoverfly larvae, nothing to the birds that later eat the dying caterpillars. The trade-off is that it only works while larvae are actively chewing. Spray a bush with no feeding larvae and you have wetted the leaves for nothing. The product also degrades under ultraviolet light, so a batch mixed and left in sun for a day loses most of its punch. This is why the label pushes evening application and a fresh mix every time.

Reading the two or three generations per season

In a temperate maritime climate the box tree moth runs two full generations most years and often starts a partial third in a warm autumn. The overwintering larvae, tucked in silk cocoons between two box leaves, resume feeding as soon as spring temperatures climb past roughly 7 Celsius. That first wave, usually April into May, is the one gardeners miss because the caterpillars are small, green, and hidden deep in the bush.

A pheromone trap baited for Cydalima perspectivalis tells you when the adults are flying. Once you catch moths, the eggs and the next larval wave follow within one to three weeks depending on temperature. Peak flights in many regions land around late June and again in late August. The practical window for XenTari opens when you see the first young larvae and fresh webbing, not when you see moths. A moth in the trap is a forecast; a chewing larva is the target.

What happens if you spray only once, at the August peak, and skip the spring wave? The spring larvae pupate, the summer moths lay heavily, and the border that looked fine in June is stripped by September. Two treatments per generation, spaced seven to ten days, cover the staggered hatch far better than a single heavy dose.

Getting the mix onto the inside of the bush

Coverage is where most XenTari applications fail. The caterpillars feed from the centre outward, so the leaves they eat first are the shaded interior ones, exactly where a casual pass with a knapsack sprayer never lands. A dose around 0.5 to 1 gram per litre of water, wetting to run-off, is standard, but the number matters less than where the droplets go.

On a low clipped border you can part the top with one hand and drive the lance down into the bush. On anything taller, a lance with a swirl nozzle set to a fine cone, pushed in from the side at several points along the run, gets the interior wet. Add the caterpillar to the reachable ones by hand-picking the obvious clusters first; every larva you remove is one that will not lay 300 eggs as a moth.

Mix only what you will use that evening. Bt aizawai spores stay viable dry in the sachet but lose activity fast once suspended in water, and sunlight finishes the job. Spraying after 6pm, or on an overcast still day, keeps the protein active long enough for the larvae to feed on treated foliage overnight. Rain within a few hours washes it off, so a dry forecast for the night matters more than the exact concentration.

The blight question that spraying does not answer

A border chewed bare by caterpillars looks, from a distance, much like one collapsing from Cylindrocladium buxicola, the fungus behind box blight. They are separate problems with separate fixes, and confusing them wastes a season. Caterpillar damage leaves skeletonised leaves, green frass pellets, and fine white webbing. Blight leaves dark leaf spots, black streaks on the stems, and bare patches with no webbing and no droppings.

XenTari does nothing against the fungus. If the bare patches are blight, cutting out affected growth, improving airflow, and clearing fallen leaf litter matters more than any spray, because the spores splash up from the ground and survive in debris for years. Topbuxus Health-Mix is often applied to blight-weakened box; it is a foliar feed with a small fungicidal component that pushes fresh regrowth and firms up the leaf. On a plant recovering from caterpillar stripping, the same feed helps the bush refoliate faster once the larvae are dead. It does not kill caterpillars and should not be relied on to.

Feeding the border back after a stripping

A box hedge defoliated in July still has live stems and dormant buds if the bark is green when you scratch it. Kill the remaining larvae, then feed. Nitrogen drives the flush of new leaves, and a box that has just lost its canopy is running on stored reserves it needs to rebuild before autumn.

Topbuxus Health-Mix dissolved at the labelled rate and watered or sprayed over the foliage every few weeks through the growing season is the common route on recovering buxus. Any balanced feed with adequate nitrogen does the underlying work. Keep the root zone watered during the refoliation, because a stressed shallow-rooted box under drought will drop the new leaves as fast as it makes them.

A worked example on a thirty metre run

Take a clipped Buxus sempervirens border thirty metres long, knee high, roughly 0.4 metres deep. That is about twelve square metres of surface, and getting the interior wet to run-off on a bush that dense takes more liquid than the surface area suggests, call it eight to ten litres per pass.

At 0.75 grams per litre that is around seven grams of XenTari per treatment, so a single 25 gram sachet covers three passes with a little left over. Two treatments per generation, two clear generations, means roughly four passes across the season, about 28 grams, so a second sachet gets you through with margin for the partial autumn wave. Spray each pass in the evening, part the hedge and drive the lance in every metre or so, and hand-pick the visible clusters before you start. The cost of the Bt is trivial next to the two or three years it takes to regrow a mature box edge from bare stems.

The part no spray schedule resolves is what you do the year the moth arrives before you have a trap out, when the first you know of it is the webbing. By then the spring generation has already laid the summer brood, and the question is whether a stripped-but-living box is worth defending at all, or whether the run should come out and a caterpillar-proof species take its place.