The horse-chestnut leaf miner, Cameraria ohridella Deschka and Dimic, is a moth of unknown origin that was first observed in Macedonia in 1984 and has now invaded most of Europe. It is now attacked by several native leaf miners' parasitoids. Despite the low parasitism rates observed in C. ohridella, populations are so high that an unusual amount of polyphagous parasitoids are produced in the vicinity of infested horse-chestnut trees two or three times per year. In spring, the bulk of the parasitoids emerge at least six weeks before the first suitable C. ohridella larvae or pupae are available. These parasitoids have the possibility to massively attack the first indigenous leaf miners developing in spring. Predators may also increase in numbers in the vicinity of C. ohridella outbreaks. ln the present study, carried out in Switzerland, France and Bulgaria, we test the hypothesis that the presence of C. ohridella has an influence on the populations of native leaf miners though shared naturai enemies. This indirect interaction between two or more species via their natural enemy complex is called "apparent competition". The first results show a general decrease in leaf miner richness in the presence of C. ohridella, particularly in spring. Population densities of some leaf miners sharing their parasitoid complex with C. ohridella, such as the beech and oak leaf mining weevils Orchestes fagi and Orchestes quercus in Switzerland and France, and several leaf mining moths in Bulgaria, significantly decreased in the vicinity of horse-chestnut trees infested by C. ohridella. Studies are presently carried out to investigate whether these decreases are due to parasitoids or predators associated with C. ohridella.