Horse Chestnut Leaf Miner

Cameraria ohridella

Warnservice
Changed on: 07.06.2019
Icon

No other insect that is only a few millimetres long and completely harmless to humans has received so much public attention as the horse chestnut leaf miner, Cameraria ohridella. This small Lepidoptera species came into the spotlight as a result of the immense damage caused by the caterpillar to the leaves of the popular horse chestnut tree. This causes entire trees in parks, avenues and squares to turn brown prematurely, leaving behind a depressing picture to the inhabitants of cities and towns, who are stressed out by the heat, looking for shade. Meanwhile, it has been found that the impact of this moth infestation affects the health of the trees to a much less severe extent  than the optical impression given would lead us to believe. However, the epidemic spread of this pest that has been going on for over 30 years has had a considerable negative impact on the aesthetics and recreational function of public parks in Europe’s urban settlements.

No other insect that is only a few millimetres long and completely harmless to humans has received so much public attention as the horse chestnut leaf miner, Cameraria ohridella. This small Lepidoptera species came into the spotlight as a result of the immense damage caused by the caterpillar to the leaves of the popular horse chestnut tree. This causes entire trees in parks, avenues and squares to turn brown prematurely, leaving behind a depressing picture to the inhabitants of cities and towns, who are stressed out by the heat, looking for shade. Meanwhile, it has been found that the impact of this moth infestation affects the health of the trees to a much less severe extent  than the optical impression given would lead us to believe. However, the epidemic spread of this pest that has been going on for over 30 years has had a considerable negative impact on the aesthetics and recreational function of public parks in Europe’s urban settlements.

Pest

caption
Feeding larvae instar of the horse chestnut leaf miner in opened mine
Fressendes Larvenstadium der Kastanienminiermotte in einer geöffneten Mine
caption
The larvae are visible against the light
Im Durchlicht sind die Larven in den Minen gut sichtbar
caption
Horse chestnut leaf miner egg
caption
Horse chestnut leaf miner
Kastanienminiermotte auf Blatt sitzend

The horse chestnut leaf miner, Cameraria ohidella Deschka & Dimić, is a butterfly of the Gracillariidae family (leaf miners) that is up to 5 mm in length. It’s copper-coloured forewings have white stripes with black rims. The insect’s hindwings have fringes, suggesting an adaptation to floating and drifting in the air as so-called aeroplankton. The name “leaf miner” describes the life of the larvae, which bore into the leaf after hatching from the egg, feeding on the upper leaf-tissue layers, but without damaging the leaf’s epidermis. Thus, they create cavities, so-called “mines”, inside the leaf where they can continue their development protected from most external hazards.

The annual development cycle of the horse chestnut leaf miner starts with the moth hatching after its hibernation period around mid-April. The females begin to lay up to 70 eggs on the top side of horse chestnut leaves by the end of April. The oval eggs measuring less than 0.5 mm are almost translucent and are usually deposited in the surface depressions found at the leaf veins, which makes them almost invisible to the naked eye. The larvae hatch after 10 days of embryonic development and begin to bore into the leaf immediately. From now on, their entire development happens within the mines inside the leaf. The larvae are perfectly adapted to life in the mine with their flat bodies, short legs and their triangular, flat heads with strong mandibles. They mainly eat the upper part of the leaf tissue, the so-called palisade parenchyma, which is rich in chlorophyll and is responsible for the green colour of the leaf. The larvae go through four to five feeding instars, before reaching their final size of about 5 mm.

This is followed by two spinning phases, the first of which covers the mine floor with silk, creating a small bulge on the bottom side of the leaf. The second spinning instar then creates a lenticular cocoon in this cavity, where the larvae pupates. Their entire prepupal development takes about one month, depending on weather conditions and is followed by a two-week pupal phase. In our regions, the first moths of the spring generation hatch from mid-June onwards. This is followed by two more generations at least, with the moths of the summer generation swarming from mid-August to far into autumn. Third generation moths usually do not hatch in the same year, but hibernate as pupae in their winter cocoons inside the mines of the fallen leaves. They hatch from the fallen leaves in the following spring after a hibernation period of about six months with the development cycle starting all over again.

 

 

Symptoms and damage

The upper epidermis above the affected areas is cut off from the water supply by the mines created by the feeding larvae and begins to wilt. This is what causes the typical brown spots visible only on the upper side of the leaf. In severe infestations, the mines of individuals can merge causing irregular brown patches. In extreme cases, almost the entire leaf area may be mined by dozens to hundreds of pupae. As a result, heavily infested leaves dry out and begin to curl up from the rim. This will cause the leaves of the horse chestnut trees turn completely brown in June and they might already begin to drop off in August.


At a first glance, the symptoms caused by the horse chestnut leaf miner could be mistaken for necrotic leaf margins brought about by drying stress. Furthermore, infestations of the plant-pathogenic fungus, Guignardia aesculi, also lead to brown patches on horse chestnut leaves. However, when holding up the leaves against the light, tiny caterpillars and the traces of their excrement become visible in the moth’s mines. Additionally, patches caused by drying stress or fungal infection on the upper and bottom part of the leaf can be recognised rather easily and are often outlined by a yellow band.

Trees defoliated in this way will blossom for a second time and might even flower in the late summer and autumn as an emergency stress response, in individual instances. However, this phenomenon is not only caused by leaf miner infestations, but is a general reaction by the horse chestnut tree to stress-related situations.  The Viennese song about the “narrische Kastanienbaum” (“crazy chestnut tree”) is evidence that horse chestnut trees were known to flower in autumn, even before the first arrival of the horse chestnut leaf miners in Austria.
More recent research found that the physiological damage to the horse chestnut tree is quite small even during severe leaf miner infestations. The growth of the trees and the depositing of reserves are done mainly within the first half of the year. The damage to the leaf surface is not severe enough by the end of June to affect the tree’s photosynthetic cycle, even when severely infested. The loss on functioning leaf area only becomes noticeable in the tree’s water and nutrition balance in the second half of the year. At this time, the majority of assimilates are invested in seed production.

Thus, severe consequences to the trees’ health resulting from moth infestations are not to be expected, although the quality of the seeds produced depends on the severity of the infestation. However, the latter bears no significance for trees planted in urban parks. Horse chestnut trees are only exposed to an indirect risk if they are replaced increasingly with other tree species with lower pest pressure.  

caption
Stress-induced flowering in autumn
herbstliche Notblüte
caption
Blossom of the white-flowering horse chestnut
Blüte  der (weissblühenden) Rosskastanie
caption
Pupa of the horse chestnut leaf miner in a cocoon
Puppe der Kastanienminiermotte in ihrem Kokon

Host plants and spread

Host plants and spread

caption
Spread map of the horse chestnut leaf miner
Ausbreitungskarte der Kastanienminiermotte
caption
Spread map of the horse chestnut leaf miner
Ausbreitungskarte der Kastanienminiermotte

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. 

Spread

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.

 

 

Prevention and control

Prevention and control

The simplest way to control the pest has also been the most environmentally-friendly of all the methods so far. Thorough removal of the fallen autumn leaves in isolated, manageable horse chestnut tree populations helps reduce the infestation in the following spring considerably. Given that leaf miners overwinter in the mines of fallen leaves, removing the leaves equals the destruction of the insect’s hibernation instars. The leaves should be disposed of with normal household waste or in industrial-sized composting plants. This will affect the first spring generation of the moth severely and the trees will remain green to the end of June and there will also be considerably lighter infestations in the summer. However, the leaf miner population will have recovered by autumn at the latest and the third generation can reach an epidemic extent once again.

A pheromone specific to horse chestnut leaf miners can be used before the flight of the first and second generation, in addition to removing the fallen leaves. The pheromone attracts the male moths, reducing the laying of eggs and, thus, limiting damage intensity. This measure is applied to control infestation or combat the insects directly, depending on the type of trap used. The pheromone funnel trap (container filled with water and some washing up liquid with a funnel and lid) works well for trapping male moths in large numbers. The funnel traps should be placed in the crown of the tree and near the trunk from beginning to mid-April, depending on the weather. The pheromones should be changed every 4-6 weeks and the traps should remain in the tree up to September. The pheromone delta trap (with sticky bottom) is ideally suited to determine the beginning of the flight period and estimate the moths’ population density. A glue ring could also be attached to the tree trunk, in addition to the other traps. This is most effective when done in April, following the hatching or mating of the moths.

The current list of authorised plant protection products to control horse chestnut leaf miners can be found in the Austrian plant protection product register (see Register of authorised plant protection products in Austria, pest: horse chestnut leaf miner, leaf miners, leaf mining insects, application; ornamental plant cultivation). The active substance methoxyfenzoide is only licenced to be used in tree nurseries and should be applied following the height of the flight and the depositing of the eggs. However, the necessity to use this substance should be examined thoroughly for environmental and cost reasons. The substance azadirachtin affects the larvae of the horse chestnut leaf miner. The optimal application time after the leaves of the horse chestnut tree have fully unfurled (i.e. when the first chestnut blossoms appear), the hatching of the larvae or when the first mines are visible, will decide the success of the control method used. The AGES Department for Plant Health in Agriculture and Horticulture provides an alert service on this issue.

 

 


x