FRANKFURT (dpa) - The horse chestnut leaf miner, Cameraria ohridella, a moth of unknown origin, is tiny, silent and inconspicuous and yet it leaves a trail of destruction in its wake.
The moths relentlessly eat their way through the leaves of chestnut trees, threatening parks and gardens with autumnal leaf shedding in the middle of summer. Entire boulevards in the eastern German states of Brandenburg and Saxony-Anhalt are affected, and the summer’s second generation of moths is already at work.
The leaf miner, still unknown in Germany 10 years ago, has spread from Macedonia throughout the whole of Central Europe. The insects make their way north or west by truck or rail and attack chestnut leaves as larvae. Fighting them is impossible.
The moths are only a few millimetres long and are real specialists. They attack only the white-flowered horse chestnut that is widespread as a popular urban tree in gardens, parks and along streets. Red- or yellow-flowered horse chestnut trees are spared.
From the tiny eggs that the females lay on the leaves in spring, the larvae wander directly into the leaves and build food channels light-coloured, transparent channels that dry out and turn brown.
The leaves are literally eaten. The larvae shed their skins several times and pupate in a cocoon out of which the moth later hatches and provides for a new generation. Up to four generations develop in one summer, the larvae of the last generation hibernate in the leaves. The foliage becomes dry and falls off long before autumn. The trees weaken but do not die.
Horst Bathon of the Federal Biological Office’s (BBA) Institute for Biological Plant Protection in Darmstadt sees no immediate possibility of stopping the insects.
“We probably have to live with them,” he says.
There is no approved agent against the new plague. Spraying the trees with chemicals is impossible in the city. According to Bathon, such agents can be applied only when a safe method is found, for example injecting the agent into the tree-trunk.
The experts are pinning their hopes on pheromone traps. However, there is only one such trap and this attracts male insects only with a single scent. Although the trap does reduce the population, Bathon says that it cannot reduce the moth-infestation permanently because the males which do not enter the trap continue to propagate.
Besides, using ladders or hydraulic lifts to place the traps in the treetops costs much effort and money. Municipal authorities in Frankfurt fitted 70 trees with traps in a test last year. But, as the city has about 5,000 horse chestnut trees, so the trial was just a drop in the bucket.
