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Drosophila suzukii preferentiallylays eggs on spherical surfaces
with a smaller radius
Junichi Akutsu & Takashi Matsuo*

Drosophila suzukii is an agricultural pest that predominantly harms small fruits, having a serrated
ovipositor that is able to pierce the skin of ripening fruits. Its oviposition preference has beenstudied from various aspects including chemical and physical properties of oviposition substrates.
However, its preference for certain shapes or sizes of substrates has not been explored. In this study,
we tested the oviposition preference of D. suzukii for artificial oviposition substrates with different
surface curvatures using 27 strains recently established from wild populations collected in Japan. We
found that D. suzukii laid more eggs on a surface with smaller radii (4.8 and 5.7 mm) compared withlarger radii (7.7 and 9.6 mm). We also found that the most preferred radius differed among strains.
Notably, the preference was independent of the volume of substrates, suggesting that D. suzukii uses
the surface curvature as a cue for its oviposition site selection. These results provide an additional
explanation for why D. suzukii preferentially uses small fruits as its oviposition sites.
Drosophila suzukii, the spotted wing Drosophila, is an invasive agricultural pest expanding its range around theworld1,2. D. suzukii has a serrated ovipositor that is able to penetrate the skin of ripening fruits and preferentiallydamages small fruits3,4. D. suzukii uses various cues in its oviposition site selection, including chemical cues
(odorants, tastants, and acidity) and physical cues (colour, firmness, and texture)4–23. Among them, firmnessof the fruit skin has been considered to play an important role in limiting the host range of D. suzukii to small
fruits2. In agreement with the morphological characteristic in its ovipositor, D. suzukii could lay eggs on relatively
harder artificial substrates compared with other Drosophila species16. In contrast, the much harder skin of large
fruits has been thought to prevent oviposition even by D. suzukii, limiting its hosts to small fruits1–3. Althoughfirmness of the fruit skin explains why D. suzukii cannot harm large fruits, it does not exclude the possibility
that D. suzukii prefers small fruits for other reasons. In fact, its oviposition preference for certain shapes or sizesof substrates has not been tested experimentally.
In preliminary experiments using artificial oviposition substrates, we accidentally found that D. suzukii laid
more eggs on spherical surfaces than on flat surfaces. In this study, we examined whether D. suzukii prefers acertain surface curvature, using multiple strains collected in Japan. We also tried to disentangle the effects ofsubstrate size and surface curvature controlling the volume of substrates independently from the curvature.

Material and methods

D. suzukii strains. D. suzukii strains were established from wild populations collected at various locationsin Japan during 2019–2021 (Table 1). In most cases, single pairs that emerged from collected host fruits wereused to establish the strains to minimize the influence of possible genetic drift during the laboratory culture by
reducing the initial genetic variation within a strain. Some strains were established using other methods (trapcollection as a source, and a single mated female or multiple pairs as an origin). The strains were cultured in thesame way as for D. melanogaster. Newly eclosed adults were transferred into vials containing standard Drosophila culture medium made of corn meal and glucose and maintained at 20 °C with the 16L:8D light cycle until
the next generation emerged. In this condition, one generation took 3 weeks (17 generations/year). Individuals
used for experiments were reared at 25 °C from the larval stage.
Oviposition substrate was made of 2% agar solution (Seakem® LE Agarose, Lonza,Basel, Switzerland) cast in silicone moulds for UV resin-crafting hobbies, which have hemispherical holes ofvarious sizes. Oviposition substrates with different surface curvatures of the same volume were prepared bypouring 200 μl of agar solution into each hole and allowed to solidify (shapes of the oviposition substrates areFour‑choice assay.

Laboratory of Applied Entomology, Department of Agricultural and Environmental Biology, The University of Tokyo,
1-1-1 Yayoi, Bunkyo-ku, Tokyo, Japan. *email: matsuo@utlae.org
Scientific Reports |(2022) 12:15792| https://doi.org/10.1038/s41598-022-20022-z

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