Small wasps and leaf litter for the protection of horse chestnut trees

press release of November 16, 2004

A group of researchers from the National Centre of Competence in Research (NCCR) Plant Survival, a scientific network steered by the University of Neuchâtel, has committed itself in the fight against the horse chestnut leaf miner. This totally ecological method that the researchers at the University of Bern are developing could even be used for the protection of stored cereals.

In autumn, the horse chestnut season is in full swing. During the last few years though, horse chestnut trees in European cities are characterized by a premature loss of their leaves during the summer. The horse chestnut leaf miner, Cameraria ohridella, an insect pest that has spread across Europe in less than two decades, is responsible for the damage caused. However, as shown by the research of Sven Bacher, 'Privat docent' at the University of Bern, and his colleagues Patrik Kehrli and Michael Lehmann, there exists a natural way of controlling this insect. The method could also be applied towards the protection of other plant resources such as wheat, which once stored in silos is also a target of insect pests.

The preservation of horse chestnut trees is very important for the ecological equilibrium of cities. These trees help in improving the climate by acting as windbreakers, by purifying the air or by providing much needed shade during the hot summer periods. Replanting horse chestnut trees is a very long and costly process: it takes fifty years for the tree to reach the adult stage and a study in 2003 in Berlin has shown that to replace 80% of the current horse chestnut trees it would cost more than 200 million euros.

The researchers of the NCCR Plant Survival, which will have their results published in the scientific journal Biological Control, have identified several species of wasps that are known to lay their eggs inside the leaf miner larvae thus alleviating the contamination of the horse chestnut tree. The problem, however, is that these wasps overwinter in the same leaf litter as the leaf miners do.

The group from Bern has ingeniously used the difference in size between the wasp and leaf miner to favour the beneficial insect to the detriment of the pest. The process consists of collecting the dead leaves in special containers and placing them under the horse chestnut trees during fall and winter. These sealed containers have openings on the sides that are covered with gauze nets. When the insects emerge in the spring, the mesh size of the gauze is just big enough to let the wasps pass through, but too small for the leaf miner to escape from the containers. Trapped, they then die in their prison. This system has proven to be effective. Favourable for the beneficials: one kilogram of leaf litter generates about one hundred parasitic wasps of which 78% easily penetrate the filter. Deadly for the pest: 99% of the leaf miners that emerge remain trapped in the container. While waiting for the large-scale application of this method, gardeners can help to stop the proliferation of leaf miners in compost by simply covering their compost piles with a layer of non-infested leaf litter.

The work of Sven Bacher and his colleagues is part of the project CONTROCAM for the biological control of the leaf miner that is registered with the 5th EU Framework Programme. CONTROCAM not only aims to protect horse chestnut trees in cities, but also to preserve the last known sites in the Balkan forests where this tree grows in the wild. Especially since Cameraria ohridella is also present there.

for more information

Dr. Sven Bacher
Zoologisches Institut
Universität Bern
Baltzerstr. 6
3012 Bern

Editor: Igor Chlebny