Abstract • The horse-chestnut leaf miner, Cameraria ohridella, is a moth of unknown origin that has recently invaded Europe and severely defoliates the European horse-chestnut, an important ornamental tree. • Several indigenous parasitoids have colonized this new host, but parasitism remains low. One of the hypotheses suggested to explain the low parasitism is that candidate parasitoids emerge too early in spring to attack the first host generation and, thus, need early-occurring leaf miners as alternate hosts. This hypothesis was tested by observing the synchronization between the phenology of the moth and that of its main parasitoids, and by comparing parasitism rates and parasitoid richness in different environments with various levels of biological diversity. • In spring, the bulk of the parasitoids emerge at least 5 weeks before the occurrence of the first suitable larvae of C. ohridella whereas most parasitoid adults reared outdoors die within 5 weeks after emergence. • Parasitism rates and parasitoid richness do not increase with biological diversity, suggesting that most parasitoids attacking the first generation of C. ohridella do not come from alternate hosts. Parasitism does not increase later in the year in the subsequent generations, when host-parasitoid synchronization becomes less critical. • We conclude that, although the spring emergence of parasitoids is not synchronized with the phenology of C. ohridella, the parasitoids attacking the first generation are probably old or late-emerging adults of the overwintering generation. The lack of synchronization is probably not the only reason for the poor recruitment of native parasitoids by C. ohridella.