Choosing planting

The silkworm moth, a formidable pest for trees.

Identification, life cycle and control of this butterfly and its caterpillars

Contents

Modified the 18 January 2026 by Pascale 6 min. Difficulty: beginner

Let’s get straight to the point! There isn’t a single bombyx, but bombyxes. Indeed, the term “bombyx” denotes several species of moths, among which the mulberry bombyx, whose caterpillar is used in the production of bristle. We will instead focus on bombyx that occur in our gardens, feared for the damage they can cause. Disparate bombyx, bramble bombyx, brown-butted and gold-bellied bombyx, liveried bombyx… all these moths share a common trait: they produce defoliating caterpillars that attack the foliage of many trees and shrubs.

Let us discover together how to distinguish these different bombyx, the damage they cause, but above all how to combat them effectively and naturally to protect plants in our gardens.

Difficulty

The Different Bombyx Species: Identification and Life Cycle

The word “bombyx” is a vernacular term that designates various species of Lepidoptera. These are moths whose caterpillars are polyphagous, meaning they feed on a very wide range of different plants. Needless to say, the damage can quickly become substantial… as these moths and caterpillars, all somewhat hairy, are widespread in France.

Among bombyx that cause the most damage, the gypsy moth (Lymantria dispar) ranks first. The adult female is recognisable by her white wings speckled with black zigzags; the male is brown and has antennae that make him resemble a rabbit. As for the caterpillar, black at hatching, it becomes covered with red warts, which in some turn blue, then grey at the last larval stage. It also bears two black bands on the head. The adults fly in July–August; the females lay the eggs, covered with a felt-like coating of hairs, on the bark of the trees on which they sit. The caterpillars hatch there quickly and stay there through autumn and winter. In spring, from mid‑May to July, they disperse, carried by the wind thanks to their silk thread which can reach seven metres in length. The caterpillar selects the host plant where later the cycle will continue.

The brown-tail moth (Euproctis chrysorrhoea) is another dreaded bombyx, especially since its caterpillar is classified as urticating. This latter is distinguished by two red warts on the dorsal surface of the segments and a row of white striations. In July, the female lays her eggs on a branch of a tree and then covers them, too, with a coat of hairs. Once hatched, the caterpillars choose a leaf to which they cling with their silk threads all winter. In March, they emerge, hungry for buds and young shoots.

The yellow-tail moth (Euproctis similis) is rarer than the previous one, but just as defoliating and urticating. The caterpillar has a black body, with a red-orange dorsal line, white markings and other red-orange markings. The female lays her eggs on the underside of a leaf, which hatch very quickly. The caterpillars disperse immediately, weaving a small shelter for the winter from which they emerge in spring to continue their development.

The disparate bombyx (top left), the bramble bombyx (top right), the brown-tail and yellow-tail bombyxes and the livrée bombyx (large photo on the right)

The bramble bombyx (Macrothylacia rubi) is certainly the largest caterpillar commonly found in France. With a black and orange body, it is very hairy and slightly urticating. Between May and June, the female lays her eggs on the stem of a plant, but also on a windowsill! They are fairly large and beige in colour. The caterpillars that hatch disperse in search of food. Discreet in the early stages of their development, they are seen mainly in late summer when they seek shelter for the winter, then for pupation. For example along a road, basking in the sun!

The livrée bombyx (Malacosoma neustria), the last bombyx most commonly observed in gardens, also called the ringed bombyx or livery of trees. This non-urticating caterpillar is easily recognisable by its blue head, adorned with two black spots, and by its body striped with orange, blue and white lines. In August, the female lays her eggs in a spiralled pattern around a stem. After autumn and winter spent in warmth, the caterpillars emerge in early April and weave a communal web. At the final larval stage, they disperse and weave a small cocoon between two leaves.

Which plants do silkworms prefer?

Polyphagous, the various bombyx caterpillars all have their preferences! Thus, the spongious, i.e. the caterpillars of Lymantria dispar, are fond of leafy trees, but also of resinous ones. In France, they have a ferocious appetite for the oaks (Quercus), the beech (Fagus sylvatica), the chestnut trees (Castanea sativa), the hornbeams (Carpinus) or even the fruit trees such as the apple trees, the pear trees, wild and cultivated Prunus. They also attack the birches (Betula), the alders (Alnus), the willows (Salix), the lindens (Tilia) and even the pines (Pinus) and the spruces (Abies), the spruces (Picea), the larches (Larix)… And, in the event of heavy populations, if food becomes scarce, the spongy moths turn to herbaceous and shrubby vegetation.

Les chenilles du bombyx de la ronce jettent leur dévolu sur les plantes de la famille des Rosacées, à commencer par les brambles (Rubus), but also the hawthorns (Rosa canina) and the roses (Rosa), the blackthorn (Prunus spinosa), the potentilles (Potentilla), the heathers (Erica and Calluna), the blueberry (Vaccinium), the creeping willow (Salix repens)…

Les chenilles du bombyx cul-brun et cul-doré se rencontrent sur les arbres fruitiers, les aubépines (Crateagnus), les ormes (Ulmus), les saules (Salix), le noisetier (Cotinus), l’arbousier (Arbutus unedo), le sorbier des oiseleurs (Sorbus aucuparia)…

Enfin, le bombyx à livrée a plus ou moins les mêmes cibles, avec une prédilection pour les arbres fruitiers où l’on rencontre des nids sur les branches.

Different cocoons of several bombyx species

La plupart de ces bombyx (à part le bombyx disparate) ne causent que des dégâts relativement peu importants aux arbres. Souvent, la croissance est retardée, car après le passage des chenilles, l’arbre doit reconstituer son feuillage. La fructification des arbres fruitiers n’est guère impactée. En revanche, le bombyx disparate est à l’origine des défoliations totales de milliers d’hectares de chênaies, hêtraies et charmaies dans certaines régions de France dans les années 2020. Ces pullulations ont été favorisées par les épisodes de sécheresse. D’après les observations, les pullulations seraient en régression depuis 2022 dans certaines régions, en augmentation ailleurs, par exemple en Corse, dans le Gers ou en Haute-Garonne au printemps et en été 2024.

How to fight effectively?

As each of these bombyx has slightly different life cycles, control adapts. Nevertheless, natural regulation is fairly effective. Indeed, most bombyx have their own predators and parasites. Thus, they are parasitized by hymenoptera, tachinid flies, nematodes, some insects (beetles, bugs, ants…), viruses, fungi and bacteria.

In terms of predators, bombyx also pay a heavy toll. Although they are cloaked in hair, the bombyx caterpillars attract the predatory attention of birds. Thus, blue tits and goldcrests feast on gypsy moth caterpillars, but also the cuckoo or the great spotted woodpecker. By contrast, the urticating properties of the brown-tail and gold-tail bombyx deter insectivorous birds. As for the caterpillars of the bramble bombyx, they are mainly crushed on roads when they seek a winter refuge. Encouraging biodiversity in your garden helps prevent these bombyx outbreaks.

In private gardens, it is mainly the gypsy moth that is the most to fear. And the best way to combat it is the destruction of the year’s egg masses, very visible on the trunks of trees. With gloves on, simply scrape these masses off and burn them. The other method of biological control involves using a specific treatment based on Bacillus thuringiensis, which is effective across the various larval stages.