Temporal and spatial variations in the parasitoid complex of the horse chestnut leafminer during its invasion of Europe

The enemy release hypothesis posits that the initial success of invasive species depends on the scarcity and poor adaptation of native natural enemies such as predators and parasitoids. As for parasitoids, invading hosts are first attacked at low rates by a species-poor complex of mainly generalist species. Over the years, however, parasitoid richness may increase either because the invading host continuously encounters new parasitoid species during its spread (geographic spread-hypothesis) or because local parasitoids need different periods of time to adapt to the novel host (adjustment-hypothesis). Both scenarios should result in a continuous increase of parasitoid richness over time. In this study, we reconstructed the development of the hymenopteran parasitoid complex of the invasive leafminer Cameraria ohridella (Lepidoptera, Gracillariidae). Our results show that the overall parasitism rate increases as a function of host residence time as well as geographic and climatic factors, altogether reflecting the historic spread of C. ohridella. The same variables also explain the individual parasitism rates of several species in the parasitoid complex, but fail to explain the abundance of others. Evidence supporting the “geographic spread-hypothesis” was found in the parasitism pattern of Cirrospilus talitzkii (Hymenoptera, Eulophidae), while that of Pediobius saulius, another eulophid, indicated an increase of parasitism rates by behavioral, phenological or biological adjustments. Compared to fully integrated host-parasitoid associations, however, parasitism rates of C. ohridella are still very low. In addition, the parasitoid complex lacks specialists, provided that the species determined are valid and not complexes of cryptic (and presumably more specialized) species. Probably, the adjustment of specialist parasitoids requires more than a few decades, particularly to invaders which establish in ecological niches free of native hosts, thus eliminating any possibility of recruitment of pre-adapted parasitoids

Bruchidius terrenus

Albizia julibrissin Durazz and Cercis siliquastrum L. (Fabales: Fabaceae) are native to Turkey and used as ornamentals. We studied the seed beetles Bruchidius terrenus (Sharp) and B. siliquastri Delobel (Coleoptera: Chrysomelidae: Bruchinae), which infest the seeds of A. julibrissin and C. siliquastrum, respectively, and their parasitoids. We recorded both bruchines from Turkey for the first time. We also found Dinarmus acutus (Hymenoptera: Pteromalidae) as a parasitoid of both B. terrenus and B. siliquastri

Invasive leafminers on woody plants: a global review of pathways, impact, and management

Leafminers are a taxonomically diverse group of endophagous insects. A number of them are pests in forestry, horticulture and agriculture, and some of them have become important invasive species. Here, we discuss the characteristics of invasive leafminers of woody plants. We first present 12 cases of invasive leaf-mining species belonging to four different insect orders. For each of them, we briefly describe their invasion, including pathways of introduction, their impact and management methods and their ecology. We then discuss various aspects of these invasions. Leafminers are introduced to new continents and spread through various pathways such as horticultural trade and accidental transport of adults and pre-imaginal stages in containers and vehicles. They may also spread long distances with air currents. A few species have serious economic impacts as orchard pests, such as the citrus leafminer, Phyllocnistis citrella, or as pests of ornamental plants, such as the horse-chestnut leafminer, Cameraria ohridella. The ecological impact of these species should be better studied, especially those killing native trees, such as the birch leaf-mining weevil, Orchestes fagi, in Canada. Compared to other insect groups, invasive leafminers are usually recruited by a range of native parasitoids, which may or may not succeed in controlling the invasive species. Biological control by introduction of parasitoids from the native range has often been successful to control invasive leafminers. The review ends by short discussions on taxonomic issues and on the use of leafminers as models to study invasion ecology