The Asiatic fruit fly Drosophila suzukii has recently invaded Europe and North and South America, causing severe damage to fruit production systems. Although agronomic host plants of that fly are now well documented, little is known about the suitability of wild and ornamental hosts in its exotic area. In order to study the potential trophic niche of D. suzukii with relation to fruit characteristics, fleshy fruits from 67 plant species were sampled in natural and anthropic ecosystems (forests, hedgerows, grasslands, coastal areas, gardens and urban areas) of the north of France and submitted to experimental infestations. A set of fruit traits (structure, colour, shape, skin texture, diameter and weight, phenology) potentially interacting with oviposition choices and development success of D. suzukii was measured. Almost half of the tested plant species belonging to 17 plant families allowed the full development of D. suzukii. This suggests that the extreme polyphagy of the fly and the very large reservoir of hosts producing fruits all year round ensure temporal continuity in resource availability and contribute to the persistence and the exceptional invasion success of D. suzukii in natural habitats and neighbouring cultivated systems. Nevertheless, this very plastic trophic niche is not systematically beneficial to the fly. Some of the tested plants attractive to D. suzukii gravid females stimulate oviposition but do not allow full larval development. Planted near sensitive crops, these “trap plants” may attract and lure D. suzukii, therefore contributing to the control of the invasive fly.

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Unlike other Drosophila species, the invasive Drosophila suzukii Matsumura (Diptera: Drosophilidae) shows a remarkable pest status. Among the physiological traits that may explain the high level of resistance to parasitoids of Drosophila larvae, the haemocyte load is shown repeatedly to play an important role. To determine whether haemocyte load can explain immunity resistance of D. suzukii to parasitoids, the haemocytes of parasitized and healthy larvae are quantified in two Japanese and three French populations of D. suzukii. Parasitization tests are conducted with two larval parasitoids: the paleartic Leptopilina heterotoma Thomson (Hymenoptera: Figitidae) and the Asian Asobara japonica Belokobylskij (Hymenoptera: Braconidae). Based on morphological and functional criteria, D. suzukii has classes of haemocytes similar to those described in Drosophila melanogaster. However, healthy larvae of the five populations tested possess particularly large numbers of haemocytes compared with D. melanogaster. Haemocyte load is also higher in larvae from the French populations than in the Japanese strains. The ability of D. suzukii larvae to encapsulate eggs of L. heterotoma is associated with a particularly high load of circulating haemocytes. However, it is notable that A. japonica induces a strong depression of the haemocyte population in this resistant host associated with an inability to encapsulate parasitoid eggs. The results show that the cellular immune system plays a major role in the failure of larval parasitoids to develop in most instances in larvae of D. suzukii, possibly contributing to the success of this species as an invader.

Wolbachia is a maternally inherited bacterium that is widely distributed among arthropods, in which it manipulates the reproduction of its hosts. Although generally facultative for its hosts, Wolbachia has recently become obligatory in Asobara tabida (Hymenoptera: Braconidae) in which it is required for the completion of oogenesis. Here, we describe a new Wolbachia strain (wAjap) that is associated with the genus Asobara and infects Asobara japonica. wAjap was detected in all female-biased populations of A. japonica found in the main islands of Japan, but not in the arrhenotokous populations from the southern islands. Using phylogenetic analyses based on multi-locus sequence typing (MLST), we show that this strain is closely related to wAtab3 (the strain required for oogenesis in A. tabida), even though they differ on Wolbachia surface protein (WSP) and WO phage sequences. Using antibiotic treatments, we show that cured thelytokous females are not dependent on Wolbachia for oogenesis. However, they produced only sons, showing that wAjap induces thelytokous parthenogenesis. Analyses of mating behavior and offspring production of individuals from Wolbachia-infected populations showed that while males were still sexually functional, females no longer attract males, making Wolbachia an obligate partner for daughter production in thelytokous populations. The fact that Wolbachia has become independently obligatory in two species of the same genus tends to show that dependence evolution can be common and swift, although no clear benefit for the parasitoid can be attributed to this dependence. Although dependence should lead to co-divergence between Wolbachia and its hosts, the very few cases of co-speciation observed in host-Wolbachia associations question the stability of these obligatory associations.