The gypsy moth, a formidable pest for trees

The different species of silkworm moths: identification and life cycle

The word “bombyx” is a common name used for various species of moth. These are moths whose caterpillars are polyphagous, meaning that they feed on a very large number of different plants. Suffice it to say that the damage can quickly become very extensive… as these moths and caterpillars, all more or less covered in hairs, are very common in France.

The gypsy moth (Lymantria dispar) is the most destructive of the bombyx moths. The adult female can be recognised by its white wings covered in black zigzags, while the male is brown and has antennae that make it look like a rabbit. The caterpillar is black when it hatches, then develops red warts, some of which turn blue and then grey during the final larval stage. It also has two black bands on its head. The adults fly in July-August, and the females lay their eggs, covered in a felt-like layer of hairs, on the bark of the trees on which they are found. The caterpillars soon hatch there and remain there throughout autumn and winter. In spring, from mid-May to July, they disperse, carried by the wind on their silk thread, which can reach 7 m in length. The caterpillar chooses the host plant on which the cycle will later continue.

The brown-tail moth (Euproctis chrysorrhoea) is another feared variety of bombyx moth, especially as its caterpillar is classified as urticating. It can be identified by the two red warts on the upper side of its segments and a row of white striate markings. In July, the female lays her eggs on a tree branch and then covers them, too, with a blanket of hairs. Once hatched, the caterpillars choose a leaf to which they cling with their silk threads throughout the winter. In March, they emerge, hungry for buds and young shoots.

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

The gypsy moth (top left), the fox moth (top right), the brown-tail and yellow-tail moths, and the lackey moth (large photo on the right)

The fox moth (Macrothylacia rubi) is certainly the largest common caterpillar in France. It has a black and orange body and is very hairy and slightly urticating. Between May and June, the female lays her eggs wherever she can, on a plant stem, but also on a windowsill! They are quite large and beige in colour. The caterpillars that emerge from them disperse in search of food. Discreet during the early stages of their development, they are seen mainly at the end of summer, when they look for shelter for the winter and then for pupation. For example, on a road, basking in the sunshine!

The final bombyx moth most often observed in gardens is the lackey moth (Malacosoma neustria), also called the ringed lackey moth or the tree lackey moth. This non-urticating caterpillar is easily recognised by its blue head, marked with two black spots, and its body striated with orange, blue and white lines. In August, the female lays her eggs in a spiral around a stem. After spending autumn and winter in the warmth, the caterpillars emerge at the beginning of April and spin a communal web. During the final larval stage, they disperse and spin a small cocoon between two leaves.

Which plants are favoured by silkworm moths?

Polyphagous, the various silkworm moth caterpillars all have their preferences! Thus, spongy moth caterpillars, that is to say the caterpillars of Lymantria dispar, are fond of leafy trees, but also conifers. In France, they have a voracious appetite for oaks (Quercus), beech (Fagus sylvatica), sweet chestnut (Castanea sativa), hornbeams (Carpinus) and fruit trees such as apple trees, pear trees, and wild and cultivated Prunus. They also attack birches (Betula), alders (Alnus), willows (Salix), limes (Tilia) and even pines (Pinus), firs (Abies), spruces (Picea) and larches (Larix)… And, when populations are high and food becomes scarce, Lymantria dispar caterpillars turn to herbaceous and shrubby vegetation.

Bramble moth caterpillars favour plants in the rose family, starting with brambles (Rubus), but also dog roses (Rosa canina) and roses (Rosa), blackthorns (Prunus spinosa), cinquefoils (Potentilla), heathers (Erica and Calluna), blueberries (Vaccinium) and creeping willow (Salix repens)…

Brown-tail and gold-tail moth caterpillars are found on fruit trees, hawthorns (Crataegus), elms (Ulmus), willows (Salix), hazel (Corylus), strawberry tree (Arbutus unedo) and rowan (Sorbus aucuparia)…

Finally, lackey moths have more or less the same targets, with a preference for fruit trees, where nests can be found on the branches.

Different cocoons from several silkworm moth species

Most of these silkworm moths, apart from the spongy moth, cause only relatively minor damage to trees. Growth is often delayed because, after the caterpillars have passed through, the tree has to rebuild its foliage. Fruit production is barely affected. However, the spongy moth is responsible for the complete defoliation of thousands of hectares of oak, beech and hornbeam woodland in certain regions of France during the 2020s. These population outbreaks were encouraged by periods of drought. According to observations, outbreaks have been declining since 2022 in some regions, but increasing elsewhere, for example in Corsica, Gers and Haute-Garonne in spring and summer 2024.

How can you tackle it effectively?

As each of these moths has a slightly different life cycle, control methods need to be adapted. Even so, natural regulation is fairly effective. In fact, most moths have their own predators and parasites. They are parasitised by hymenopterans, tachinid flies, nematodes, certain insects (beetles, bugs, ants…), viruses, fungi and bacteria.

As far as predators are concerned, moths also pay a heavy price. Although they are covered in hairs, moth caterpillars attract the attention of birds. Blue tits and goldcrests, for example, feast on caterpillars of the gypsy moth, as do the cuckoo and the great spotted woodpecker. By contrast, the urticating hairs of the brown-tail and golden tail moths deter insectivorous birds. As for bramble moth caterpillars, they are mainly crushed on roads when they set off in search of a refugium for the winter. Encouraging biodiversity in your garden can therefore help prevent these moth attacks.

In private gardens, it is above all the gypsy moth that is most worrying. The best way to control it is to destroy the egg masses laid that year, which are clearly visible on tree trunks. Wearing gloves, simply scrape off these masses and burn them. The other biological control method involves using a specific treatment based on Bacillus thuriengiensis, which is effective against the different larval stages.

Female and male adult gypsy moths, whose flight can be seen from mid-July to mid-August

Finally, sexual pheromone trapping systems are available for placing during the adults’ flight period, between mid-July and mid-August.

Further reading:

• Bacillus thuringiensis: a natural insecticide
• Pheromone traps: why and how to use them in the garden?