We present the first review of Hymenoptera alien to Europe. Our study revealed that
nearly 300 species of Hymenoptera belonging to 30 families have been introduced to
Europe. In terms of alien species diversity within invertebrate orders, this result
ranks Hymenoptera third following Coleoptera and Hemiptera. Two third of alien Hymenoptera
are parasitoids or hyperparasitoids that were mostly introduced for biological control
purposes. Only 35 phytophagous species, 47 predator species and 3 species of pollinators
have been introduced. Six families of wasps (Aphelinidae, Encyrtidae, Eulophidae,
Braconidae, Torymidae, Pteromalidae) represent together with ants (Formicidae) about
80% of the alien Hymenoptera introduced to Europe. The three most diverse families
are Aphelinidae (60 species representing 32% of the Aphelinid European fauna), Encyrtidae
(55) and Formicidae (42) while the Chalcidoidea together represents 2/3 of the total
Hymenoptera species introduced to Europe. The first two families are associated with
mealybugs, a group that also included numerous aliens to Europe. In addition, they
are numerous cases of Hymenoptera introduced from one part of Europe to another, especially
from continental Europe to British Islands. These introductions mostly concerned phytophagous
or gall-maker species (76 %), less frequently parasitoids. The number of new records
of alien Hymenoptera per year has shown an exponential increase during the last 200
years. The number of alien species introduced by year reached a maximum of 5 species
per year between 1975 and 2000. North America provided the greatest part of the hymenopteran
species alien to Europe (96 species, 35.3%), followed by Asia (84 species, 30.9%)
and Africa (49 species, 18%). Three Mediterranean countries (only continental parts)
hosted the largest number of alien Hymenoptera: Italy (144 spp.), France (111 spp.)
and Spain (90 spp.) but no correlation was found with the area of countries. Intentional
introduction, mostly for biological control, has been the main pathway of introduction
for Hymenoptera. Consequently, the most invaded habitats are agricultural and horticultural
as well as greenhouses. To the contrary, Hymenoptera alien in Europe are mostly associated
with woodland and forest habitats. Ecological and economic impacts of alien Hymenoptera
have been poorly studied. Ants have probably displaced native species and this is
also true for introduced parasitoids that are suspected to displace native parasitoids
by competition, but reliable examples are still scarce. The cost of these impacts
has never been estimated.