The insect’s German name “Kastanienminiermotte” is misleading as C. ohridella cannot develop on the sweet chestnut, Castanea sativa L. The most important host of the organism is the white-flowering horse chestnut, Aesculus hippocastanum L. This tree species is not endemic to our region, but is hugely popular thanks to its glorious flowers in spring. In general, it is assumed that the first horse chestnut trees were introduced to Austria by the Imperial Court botanist Carolus Clusius, who is thought to have brought them here from the Ottoman Empire towards the end of the 16th century. Meanwhile, this species is one of the most important trees found in parks and on avenues across Central Europe. It is estimated that there are 30,000 horse chestnut trees in Vienna alone.
There is reasonable doubt that A. hippocastanum is the original host plant for the horse chestnut leaf miner. One of the reasons is, for example, that it is rather surprising in that the tiny butterfly has only spread across Europe over the past 20 – 30 years, although its host has been around in Europe for the better part of 300 years. Moreover, the decade-long mass infestation of horse chestnut trees is also quite noticeable. Such epidemic population densities of this leaf miner species seem to point to a food source that has been tapped into very recently and not to a working trophic relationship between the plant and herbivore insect that has developed over evolutionary periods. However, this question on the original host plant of the horse chestnut leaf miner can only be answered once its geographical origin has been clarified (see Spread).
Minor C. ohridella infestations have also been found on other horse chestnut species, in addition to the white-flowering horse chestnut. One of these species is Aesculus x carnea, a red-flowering hybrid of the common horse chestnut and the American red buckeye or firecracker plant (Aesculus pavia L.), grown frequently in Europe. However, the mortality rate of horse chestnut leaf miners on these hosts is quite high due to both mechanical barriers and toxic substances in the leaves. Of the about 20 known horse chestnut species worldwide, most American species seem to be largely resistant to C. ohridella infestations, while some species from the “Old World” appear suitable hosts. The horse chestnut leaf miner’s development has also been observed on maple tree species. However, these were only very few individual cases.
The horse chestnut leaf miner’s first appearance was rather unspectacular and was underestimated in significance. Assumedly, the insect first appeared on artificially cultured horse chestnut plantations in the region around Lake Ohrid (Macedonia) in the early 1970s. Scientists Simova-Tošić and Filev (1985) found that this was an insect that had not been classified up to then. The publication mentions the year of the publication and the year of the study, 1984, mistakenly as the year of the insect’s first appearance. The publication (written in Serbo-Croatian) points out that local observers had already noticed the symptoms for 10 years.
However, it was only when the horse chestnut leaf miner was brought into Central Europe that the public became aware of this organism. The first sporadic mines of the horse chestnut leaf miner were detected in the region around Linz in autumn 1989 and cases of mass propagation were already found by 1990/91. The moth was then discovered in the St. Pölten area in 1992. Since then, the pest has spread rapidly across all of Austria. This second infestation hotspot in Austria helped the moth to spread to the northwest, but also to our eastern and southern neighbours. Additionally, the insect’s population spread from Macedonia, where it was discovered, to the remaining Balkan countries and further into Eastern Europe. The two expansion waves met in southern Hungary, Slovenia, Austria’s southern provinces and South Tyrol in Italy. Meanwhile, the horse chestnut leaf miner is found across all of Europe, from Greece, the Dalmatian coast, northern Italy and southern France in the South to southern Scandinavia, northern Germany, Denmark, the Benelux countries and the UK in the north. Its western spread boundary is currently in France and Spain, the most easterly reports have come from Ukraine.
The origin of the horse chestnut leaf miner is still unknown, despite a worldwide search. At present, various possibilities are being considered. The most likely is that the moth has been imported from the East Asian region, where there are closely related leaf miners. It is possible that the small leaf miner grows on a different host plant, maybe another deciduous tree from the maple family. The population density of this leaf moth could be so low in its original habitat that it has not been noticed up to now. Furthermore, there is a possibility that the horse chestnut leaf miner has not been imported, but actually originated from the Balkans or a neighbouring area of origin in Asia Minor. In this case, it would be more than likely that the rapid spread across Europe has taken place, following host-plant replacement in recent years.