Prolonged diapause and its possible adaptive significance in the gall wasp Synergus itoensis (Hymenoptera: Cynipidae)

Here, we report the bimodal adult emergence in a synergine gall‐inducing wasp. On December 29, 2019, 150 Quercus glauca acorns containing Synergus itoensis galls were obtained from the Ito Campus of Kyushu University, Fukuoka, Japan. The galls were kept in a test tube for each acorn in the shade under field conditions until October 24, 2022. The emergence of adult wasps from the galls was recorded. More adults emerged in 2020 (151 females and 181 males) than in 2021 (120 females and 93 males); however, no emergence occurred in 2022. The sex ratio was not significantly different from 1:1 in either 2020 or 2021. The 50% emergence date was early September in both years. Acorn production in Q. glauca fluctuates greatly from year to year. Hence, oviposition resources for adult female S. itoensis are abundant in years with high acorn production, but scarce in years with low acorn production. The bimodal emergence of adult S. itoensis wasps may have evolved to adapt to the fluctuations in acorn production in Q. glauca.