Host and oak species impact clutch and adult size of Oobius rudnevi, the only known egg parasitoid of Cerambyx beetles

Oobius rudnevi (Nowicki) (Hymenoptera: Encyrtidae) is the only known egg
parasitoid of the longhorn beetles Cerambyx cerdo L. and Cerambyx welensii
(Küster) (Coleoptera: Cerambycidae). This micro-wasp is a gregarious and
predominantly univoltine species, active during the summer when host eggs
are present in the wild. Both longhorn beetles occur sympatrically in holm
oak, cork oak, and Pyrenean oak forests in SW Iberia, where they can cause
significant damage to trees by boring large galleries into the
wood. Despite the potential of O. rudnevi as a native
biocontrol agent, our understanding of its life history and parasitic
capacity remains limited. This study aimed to investigate whether host egg
species and oak species interact to influence O. rudnevi‘s reproductive
allocation, adult size, and sex ratio. To achieve this, we compared
sentinel eggs of C. cerdo and C. welensii, which were field-exposed in
holm oak, cork oak, and Pyrenean oak forests over a two-year study
(2017–2018). Our main findings were: 1) both host egg species and
oak tree species significantly affected wasp clutch size and adult size;
2) larger host eggs were parasitised more frequently, regardless of host
or oak species; and 3) host eggs with larger clutch sizes produced smaller
adult parasitoids and more females than males. These results advance our
knowledge of the life history of O. rudnevi as a native
biocontrol agent of C. cerdo and C. welensii in oak woodlands and are
discussed from an evolutionary perspective.

鲁德尼沃氏卵跳小蜂（Oobius rudnevi，Nowicki）（膜翅目（Hymenoptera）: 跳小蜂科（Encyrtidae））是目前已知的唯一一种寄生大栎天牛（Cerambyx cerdo L.）与韦氏栎天牛（Cerambyx welensii，Küster）（鞘翅目（Coleoptera）: 天牛科（Cerambycidae））卵的寄生性天敌。该小型寄生蜂为群集寄生性昆虫，多数为一化性物种，野外活动期与寄主卵存在期一致，均为夏季。这两种天牛在伊比利亚半岛西南部的圣栎、栓皮栎与比利牛斯栎林中同域分布，它们可通过在木质部钻凿大型坑道对树木造成严重危害。尽管鲁德尼沃氏卵跳小蜂具备作为本土生防天敌的应用潜力，但目前学界对其生活史与寄生能力的认知仍较为有限。本研究旨在探究寄主卵种类与栎树种类是否存在交互作用，进而影响该寄生蜂的繁殖分配、成虫体型及性比。为此，本研究于2017至2018年间开展了为期两年的野外实验，将大栎天牛与韦氏栎天牛的哨兵卵块分别暴露于圣栎、栓皮栎及比利牛斯栎林中以开展对比研究。本研究主要得到以下结论：1）寄主卵种类与栎树种类均会显著影响寄生蜂的产卵量与成虫体型；2）无论寄主种类与栎树种类如何，体积更大的寄主卵被寄生的概率更高；3）单块寄主卵内的寄生蜂产卵量越高，羽化出的寄生蜂成虫体型越小，且雌雄性比偏向雌性。本研究结果加深了学界对鲁德尼沃氏卵跳小蜂作为伊比利亚半岛栎林内大栎天牛与韦氏栎天牛本土生防天敌的生活史的认知，且本文将从进化生物学视角对上述结果展开讨论。