Horse-chestnut leaf miner: a pest to fear?

Horse-chestnut leaf miner: profile of a particularly invasive insect

Cameraria ohridella. Behind this Latin name hides the horse-chestnut leaf miner, sometimes called the horse-chestnut moth. It is a lepidopteran of the family Gracillariidae. Begin with the moth: 3–5 mm long, with an ochre-brown body. Its forewings are silvery-striate while the hindwings are narrower and fringed. Because of its size it often goes unnoticed, which makes it difficult to spot.

Yet it is well established across the territory, moving rapidly from tree to tree, in urban and rural environments alike. Studies indicate the spread of the horse-chestnut leaf miner is around 50 to 58 km per year. This spread appears slower in regions with lower population density or mountainous areas. Conversely, in urban areas infestations are far more rapid.

Observations across Europe suggest the horse-chestnut leaf miner may be a vector of horse-chestnut bacterial canker (Pseudomonas syringae).

Adult and larva of Cameraria ohridella

What is the life cycle of the horse-chestnut leaf miner?

Under our country’s climatic conditions, horse-chestnut leaf miner has about three generations a year, with a life cycle of around 7 to 11 weeks, which explains its rapid spread. Depending on region, as soon as temperatures reach 12 °C for at least 48 hours, first adults emerge in spring, between late March and mid-April. Born to reproduce, adults of the first generation live about thirty days, later generations a little less.

Mating takes place on tree trunks and is repeated throughout the adults’ short lives. Once fertilised, the female lays between 20 and 100 tiny eggs directly on leaves of its host plant, horse-chestnut. Eggs concentrate along leaf veins. Larvae hatch after three weeks’ incubation. From that point, infestation is underway.

Larvae then burrow into the cuticle, between the leaf epidermises, to tunnel galleries called mines, 1 to 2 mm long. They feed on the parenchyma. As they develop, mines widen and may join together to cover whole leaf. Larvae grow in their mines for 20 to 45 days, depending on generation, before pupating inside the mine. Second generation generally emerges in July, third in August. And the cycle repeats with each generation.

Last pupa remains in the leaf that falls to ground. It will overwinter there before emerging in spring.

Different stages of horse-chestnut foliage infestation by leaf miner

What damage does it cause?

Specifically, a leaf‑miner attack is detected by the reddish‑brown mines that appear on the leaves. Narrow at first, these mines lengthen and widen and may even join together if several larvae share a leaf. Ultimately, the leaves turn completely brown and the canopy browns in mid‑summer: photosynthesis is then no longer effective, and horse chestnuts tend to weaken and become more susceptible to pathogens. Yellowed leaves drop prematurely in mid‑summer. This leaf fall may seem severe, but leaf‑miner attacks appear to have only aesthetic consequences. The vigour of horse chestnuts does not seem affected, no horse chestnut having died from a leaf‑miner attack.

However, there are still repercussions for flowering and fruiting. In the longer term, scientists do not yet know what lasting effects repeated infestations over several consecutive years may cause.

As for the trees targeted, these are mainly horse chestnuts (Aesculus hippocastanum), the most common horse chestnuts in our area. All cultivars can be affected, namely Aesculus hippocastanum ‘Hampton Court Gold’ with young shoots yellow turning green, Aesculus hippocastanum ‘Memmingeri’ with a very regular habit, and Aesculus hippocastanum ‘Umbraculifera’, smaller in size.

Specimens of Aesculus pavia or Aesculus flava can also be affected. Thus, the palmate foliage of the ornamental red‑flowering horse chestnut ( Aesculus pavia ‘Humilis’) can be impacted by leaf miners, as can the ‘Koehnei’ variety with upright pink panicles, and the yellow buckeye (Aesculus flava).

By contrast, horse chestnuts with red flowers (Aesculus x carnea) are much less affected.

Two other tree species can also suffer horse‑chestnut leaf‑miner attacks. The sycamore maple (Acer pseudoplatanus) or the Norway maple (Acer platanoides) may be occasionally attacked if growing near horse chestnuts.

Three horse chestnut species are particularly targeted by leaf miners: the horse chestnut, the ornamental red‑flowering horse chestnut and the yellow buckeye

How to control this pest naturally?

Controlling the horse-chestnut leaf miner is considered difficult. Above all because action by natural parasites, particularly hymenopterans, is insufficient to regulate populations. In addition, this leaf miner spreads very rapidly, especially in areas densely planted with horse chestnuts or in urban areas.

There are chemical insecticides, however many local authorities refuse to use them because of their environmental impact. Preventive control therefore remains the option to limit damage without eradicating the insect.

In your garden, if you have a horse chestnut, one measure that really reduces populations is raking up fallen leaves in autumn. Indeed, the last generation, which enters diapause over winter, overwinters in the leaves fallen on the ground. Raking must be thorough, not only beneath the tree but also in surrounding areas. This will greatly reduce infestation the following spring.

Otherwise, at garden scale, a pheromone trap specifically for the horse-chestnut leaf miner proves relatively effective at catching males, thereby limiting reproduction. These pheromone capsules dedicated to horse-chestnut leaf miners are placed in a specific trap during the various nuptial flight periods, between late March and August–September.

Phéromones spéciales Mineuse du marronnier Cameraria ohridella Biotop - 2 capsules

Pheromone traps capture males, limiting mating

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Finally, it is always essential to maintain good diversity within your garden. Indeed, horse-chestnut leaf miners have several predators among birds, in particular great tits and blue tits. Earwigs (Forficula auricula) also occasionally consume larvae. Another predator, the grasshopper Meconeme fragile (Meconemus meridionalis), is fairly active in controlling the horse-chestnut leaf miner. It is therefore essential to grow melliferous plants, plant native hedgerows composed of flowering, berry-bearing shrubs, and install feeding stations and nest boxes for birds, as well as insect shelters.