Skip to main content

Rise of the Box Moth Caterpillar – An Invasive Pest of Boxwood

Rise of the Box Moth Caterpillar – An Invasive Pest of Boxwood

In this article, we explore the origins of this pest, the damage it causes, how to identify an infestation, and effective control measures – including when and what to spray (such as XenTari biological insecticide). We’ll also suggest some alternative plants to use instead of Buxus sempervirens if you decide the battle against this pest isn’t worth it.

Origins and Spread:

The box tree moth is native to East Asia and was first observed in Europe in the mid-2000s. It likely arrived inadvertently via imported plant material. The moth was detected in southern England around 2007, and the first caterpillars in private gardens were recorded in 2011. Since then, it has spread widely across England (especially the southeast) and beyond. By 2018 it was considered a common resident pest in much of the UK. Unfortunately, our climate (especially in the milder south) allows the moth to produce multiple generations per year, so populations have exploded. Without natural predators to keep it in check, the box moth has become a major problem for anyone growing boxwood hedges or topiary.

Damage and Symptoms:

Box tree caterpillars feed exclusively on plants in the genus Buxus (boxwood). A heavy infestation can strip a box plant of most of its leaves, leaving behind a skeleton of twigs. The key signs and symptoms of box moth caterpillar damage include:

  1. Severe defoliation: Foliage chewed and eaten, often leaving only the leaf midrib. Patches of the hedge appear thinned or bare, which may be mistaken for box blight at first.
  2. Silken webbing: Caterpillars produce webbing that clumps leaves together in the areas where they feed. Look for tangled messes of dry leaves held by white silk – this is a telltale sign.
  3. Green-black frass (droppings): You’ll notice small black pellets collecting on lower leaves or on the ground under the plant. This frass (insect excrement) is a sure indicator that caterpillars are actively feeding above.
  4. Visible caterpillars: On close inspection, you may find the larvae themselves, especially inside the webbed sections. Box tree caterpillars are greenish-yellow with thick black and thin white stripes, about 25–40 mm long when mature. They also have shiny black heads. Younger larvae are lighter colored and hard to spot.
  5. Pupae and eggs: You might find chrysalis cocoons among the leaves – slender white cocoon webs where caterpillars pupate into moths. The eggs are tiny, flat and yellow, laid in clusters on leaf undersides, but these are very difficult to see without magnification.
  6. Presence of moths: The adult box tree moths are active flyers (mostly from late spring through summer). They have white wings with a iridescent brown border (wingspan ~4 cm), although a darker brown form exists. You might notice these moths resting on walls or near lights at night. Seeing them around your garden is an early warning that caterpillars may soon follow.

Feeding damage is usually observed from April through October, as there are typically two or more generations per year in the UK’s climate. Caterpillars that hatch in late summer can even overwinter hidden among box leaves, resuming feeding in early spring when temperatures rise. Repeated defoliation weakens the plant significantly – even if it doesn’t outright kill the box, it loses its vigor and ornamental value. In fact, after stripping foliage, the larvae will even gnaw on the green bark of boxwood twigs, causing girdling (ring-barking) of branches. This can lead to die-back of those stems and compound the damage.

Control Measures – Monitoring and Spraying:

Vigilance is critical in managing this pest. Monitor your box plants regularly, especially during spring and summer. At first sign of caterpillars or damage, prompt action can save your hedges. Here are the recommended control approaches:

  • Hand Picking: If you have only a few or manageable-sized plants, inspect them thoroughly. You can manually remove caterpillars and squish them (wear gloves). Similarly, prune out the worst affected webbed shoots and dispose of them. This labor-intensive method is feasible for light infestations on small topiaries.
  • Pheromone Traps: These traps lure male moths using a female sex pheromone. While they won’t catch all the moths, they are useful as a monitoring tool – if your trap starts catching moths, you know the pest is active in your area. Traps hung in spring can give early warning of incoming moths, so you can time interventions before caterpillar damage peaks.
  • Biological Insecticide (Bt XenTari): The most effective and garden-friendly treatment is Bacillus thuringiensis var. kurstaki, a bacteria that specifically kills caterpillars. It’s sold under brands like Topbuxus XenTari and Buxus Health Mix. When sprayed on box foliage, the bacteria are eaten by the caterpillars and release toxins in their gut, causing them to stop feeding within hours and die within a day or two. This does not harm birds, bees, or other wildlife, as it targets only caterpillars. Timing is important: spray at the first sign of caterpillars, and repeat treatments several times in the season (typically late April, mid-summer, and early autumn generations). Thorough coverage of the inner foliage is needed, as the larvae often hide deep inside the bushes. Note: XenTari is not yet fully licensed for amateur use in the UK (as of writing), but it is available to purchase online and widely used by gardeners. Always follow instructions – usually one sachet per 3 liters water, applied with a garden sprayer.
  • Chemical Insecticides: Traditional insecticides (pyrethroids, etc.) have limited effectiveness against box caterpillar, partly because the larvae are hidden in foliage and also due to resistance issues. Moreover, broad-spectrum chemicals can harm beneficial insects. Many experts now recommend sticking to the biological option (Bt) for safe and targeted control.
  • Nematodes: An organic control sometimes used is a nematode solution (e.g. Steinernema species) applied to the foliage. Nematodes can infect and kill caterpillars, but results have been mixed and they may also affect non-target species. If using, apply in cooler evening hours (they degrade in sunlight) and keep foliage moist.
  • Natural Predators: In its native range, box moth has predators and parasites that keep it in check. In the UK, some generalist predators like birds (e.g. tits, starlings) and wasps have been observed eating the caterpillars. However, this natural predation has not been sufficient to control the pest in gardens. Encouraging birds (with feeders) or chickens in the garden may provide slight help, but you will likely need additional measures.

When using XenTari or any spray, aim for calm weather and spray in the evening or early morning to avoid direct sunlight (Bt works best out of strong UV light). Late April to early May is a critical time for a first treatment, as overwintered caterpillars become active then. A second wave in late July/August often requires treatment again as a new generation of moths lays eggs then. Continue monitoring through early autumn. Gardeners who have fought this battle often say you must be relentless – missing one breeding cycle can lead to a population boom.

Aside from direct plant treatments, if an infestation is severe you might decide to remove the affected box plants entirely. Dispose of them (do not compost, as cocoons or eggs might survive). This can halt the local life cycle if you remove all box, but obviously it leaves a gap in your garden design. Many are now asking: What can I plant instead of boxwood?

Alternatives to Buxus:

With the combined threat of box blight disease and box tree caterpillar, it’s understandable that gardeners are seeking substitute plants for evergreen hedging and topiary. Fortunately, several other shrubs can provide a similar small-leaved, neat look without the susceptibility to these problems. Here are a few excellent alternative plants to consider in place of Buxus sempervirens:

  • Japanese Holly (Ilex crenata): One of the top recommended alternatives, this holly has tiny, oval, boxwood-like leaves (but with no spines). It forms dense evergreen growth that can be clipped into hedges or shapes. Ilex crenata tolerates clipping well and is generally resistant to the pests and diseases that plague box. Several cultivars are available (e.g. ‘Dark Green’, ‘Convexa’). Do note it prefers slightly acidic soil and may yellow on chalk – soil test and amend if needed.
  • Lonicera nitida (Shrubby Honeysuckle): A fast-growing, fine-leaved evergreen shrub. It has small glossy leaves and can be easily trimmed into low hedges, balls or other topiary forms. Lonicera nitida grows quickly, so it needs more frequent trimming to keep tidy, but it fills in rapidly and is very tough. It has the bonus of being inexpensive and widely available.
  • Euonymus japonicus ‘Microphyllus’ (Small-leaved Euonymus): Euonymus is a versatile evergreen, and the tiny-leaved varieties can mimic boxwood in hedges. They are slow-growing and dense. Variegated forms exist too (green and white or yellow), which can add interest. Euonymus generally is low-maintenance and untroubled by serious pests.
  • Teucrium x lucidrys (Germander): An old-fashioned choice for knot gardens, germander is an aromatic sub-shrub with small dark green leaves. It has been used historically in parterres as a edging in lieu of box. It prefers sunny, well-drained sites and produces pretty purple flowers if left untrimmed. Very drought-tolerant as well.
  • Taxus baccata (Yew): For taller hedges or topiary, yew is a superb alternative. Yew can be clipped to any shape and has a fine texture. It is slower-growing and, of course, needs care with its poisonous berries/leaves (if livestock or pets are a concern). Dwarf varieties like Taxus baccata ‘Repandens’ or ‘Renke’s Kleiner Grüner’ can serve for lower shapes. Yew hedges are very long-lived and resist most pests – just avoid waterlogged soil.
  • Other options: Osmanthus (e.g. O. delavayi) is an evergreen with small holly-like leaves that can be clipped; Pittosporum tenuifolium (dwarf forms like ‘Golf Ball’ or variegated ‘Irene Paterson’) can form lovely mounds – though pittosporum may not tolerate harsh frost below -10°C. Privet (Ligustrum) including the dwarf Chinese privet Ligustrum sinense ‘Sunshine’ (golden leaves) can be used for edging and low hedges. Cotoneaster and Corokia are also trialed as box alternatives, offering small leaves and twiggy form (some Corokia have a trendy wispy look with silvered foliage). The RHS has been running trials on many such species to evaluate their performance as “new box”.

No single alternative is a perfect match for all of boxwood’s desirable traits. However, by choosing one suited to your garden’s conditions, you can achieve a similar structural effect. For low hedges in formal designs, Japanese holly or Lonicera nitida are top choices mentioned by experts. They are low-maintenance, pest-resistant, and will keep your garden design intact even as boxwood problems rise.

In Conclusion

The rise of the box moth caterpillar has been a serious blow to traditional box hedging. If you catch infestations early and use biological controls like XenTari, you can still preserve established box plants in many cases. Ensure you stay vigilant each growing season, and don’t hesitate to prune out damage and even replace plants if necessary. By diversifying your garden with some alternative evergreens, you’ll not only mitigate risk but also discover new textures and colors. Gardening is always evolving – and with good planning, our beloved green hedges and shapes will continue to thrive, whether made of box or not.