By leaving the dead leaves at the foot of their horse-chestnut trees, many French people have literally housed the next generation of a tiny pest all winter long, Cameraria ohridella. A portion of its chrysalids remains intact in these leaves until spring, when the adults emerge. Their caterpillars will then bore galleries that, in the event of a heavy infestation, cover the entire leaf and turn the crown brown in midsummer.
Leaving leaves on the ground is often portrayed as the savvy gardener’s reflex: under many trees, that litter feeds the soil and shelters a small fauna. Under an Indian horse-chestnut, the calculation changes, since the leaf miner spends the winter there.
- The horse-chestnut leaf miner overwinters in leaves fallen to the ground.
- Collection should be carried out from leaf fall until the end of February.
- A pile of compost only destroys the chrysalids at 40 °C for more than a week.
A 3-mm butterfly, unknown to science before 1984
Until 1984, no one knew this insect.
That year it was observed for the first time on a horse-chestnut tree, in Macedonia near Lake Ohrid, and was described as a new species. Its origin remains unknown, notes INRA. For France, dates vary by source. FREDON and the Ministry of Agriculture place the arrival around 2000, with a first appearance in Alsace, then Nord-Pas-de-Calais and Île-de-France in 2001, while an INRA article reports the insect as early as 1998.
In 2003, the leaf miner was present in almost all departments, with the exception of part of the southwest Massif Central and western Brittany. Futura Sciences mentions the territory as being fully colonized within four years. The Agriculture Ministry’s dispersion study links this spread to human population densities, without naming a precise mechanism.
What happens in the fallen leaf
Once hatched, the larva bores a gallery between the two epidermises of the leaf, hence the name leaf miner. It then spins a cocoon to metamorphose. In autumn, some of the chrysalids remain intact in the fallen leaves, and the adults emerge only in spring. The butterfly itself measures about 3 mm, with brown forewings striped with white.
According to Futura Sciences, the butterflies emerge from their cocoons in near-synchrony, timed with magnolia blooming. The female lays between 20 and 1,000 eggs, then dies quickly. The caterpillars, half a millimeter long, enter the leaf almost immediately after hatching.
The species can have up to three generations in a year. A Swiss site dedicated to biological control notes that a strong summer generation can mine 100% of the leaves. There, leaf fall begins as early as August and the tree is bare by late September. In France, a severe attack manifests as mines covering the entire leaf, a brown canopy, and premature defoliation in midsummer.
Not all species are affected in the same way. The white horse-chestnut, Aesculus hippocastanum, is the most impacted, while the red horse-chestnut, Aesculus × carnea, shows greater resistance.
The damage is not only cosmetic. According to the Swiss site, the surface that captures light is drastically reduced, the tree’s regulation of the urban climate weakens, and it fixes less dust. The early leaf drop also increases the costs of city cleaning.
Collect before the end of February, and not alone
The FREDON sets the action window: from leaf fall to the end of February.
For FREDON Nord-Pas-de-Calais, systematic collection and destruction of fallen leaves is the first control measure. It must be started as early as possible, before the wind disperses them.
The action must be collective, at the level of the municipality, on both public and private lands. If not, uncleaned sites recontaminate others the following year. A meticulous gardener whose neighbor leaves the leaf litter in place can find butterflies again in the spring. Futura Sciences notes that a thorough clean is very effective, but it only concerns urban spaces, not large parks or forests such as Bois de Boulogne.
Concretely, this yields a simple timetable: rake as soon as the leaves fall, use sturdy bags, haul to the dump, then a final pass before the end of February for leaves stuck behind a low wall or under a hedge. A note to neighbors or the town hall helps coordinate the operation by street or by neighborhood.
Where to put the leaves remains a question. According to FREDON, composting is only allowed if the pile is tightly covered and reaches at least 40 °C for more than a week, or 65 °C for one day. A normal, weather-exposed pile would have little chance of achieving this.
Burning of green waste is illegal for private individuals.
Horse-chestnut leaves therefore go to the dump or to green-waste collection when the municipality offers one. Checking with the town hall provides the dates and local guidelines.
What the collection promises, and what it leaves open
None of the consulted sources quantify the decrease achieved through collection, nor the number of chrysalids hidden in the leaves of a tree. Futura Sciences nevertheless ranks it among the most effective techniques. According to the Swiss site, it reduces the infestation pressure of the first generation, the spring generation.
Last, reassuring note: according to the same Swiss site, the leaf miner does not kill the tree. However, repeated heavy infestation year after year does weaken it, leading to smaller fruit and seeds.
Sources: biocontrol.ch | planete-agrobio.com
