Mg-enriched ovipositors as a possible adaptation to hard-skinned fruit oviposition in Drosophila suzukii and D. subpulchrella

The globally spreading pest Drosophila suzukii and its relative D. subpulchrella (Diptera: Drosophilidae) possess an elongated ovipositor with enlarged bristles which is associated with their ability to oviposit into hard-skinned fruits. Other species of the genus ovipositing in damaged fruit and decaying material have blunt ovipositors with small bristles. In insects, the ability to cut or penetrate hard substrates may, apart from the intrusion organ's shape, also depend on the incorporation of metals in the cuticle. Here, we hypothesized that D. suzukii and D. subpulchrella have more metal-enriched ovipositors than the closely related D. melanogaster, D. biarmipes and D. mimetica (all unable to attack hard-skinned fruit). Energy-dispersive X-ray spectrometry showed that two alkaline earth metals, magnesium (Mg) and calcium (Ca), occur in the ovipositors of Drosophila, providing the first evidence of metal-enriched ovipositors in Diptera. Mg occurred in all individuals of D. suzukii and D. subpulchrella but in no individuals of the other species. Mg was only detected in the ovipositor bristles, with up to about 0.3%wt, suggesting that these structures play an important role in drilling fruit skin. Ca, on the other hand, occurred rarely and in traces across most species, and was detected both in and out of bristles. We suggest that Mg-enrichment may represent an adaptation to hard-skinned fruit oviposition, and, together with its modified ovipositor morphology, may further explain why D. suzukii is so successful as an invader.