Phenological synchrony and geographic stability of a new obligate endoparasitoid targeting stem chestnut gall wasp in China

Abstract The Seguin chestnut, Castanea seguinii Dode (Fagales: Fagaceae), a vital reservoir of disease-resistant germplasm in China, is increasingly threatened by the stem gall wasp Synergus castaneus Bernardo (Hymenoptera: Cynipidae). Effective biological control relies on identifying specialised natural enemies, yet the parasitoid fauna associated with native Chinese chestnuts remains poorly characterised. In this study, we identify and describe a key natural enemy, Aulogymnus liuchunhuensis sp. nov. (Hymenoptera: Eulophidae), using an integrative approach based on morphological examination, mitochondrial cytochrome c oxidase subunit I barcoding, and field surveys with rearing experiments across three provinces in China. Unlike the majority of its ectoparasitic congeners, A. liuchunhuensis is an obligate endoparasitoid. Extensive field surveys across three provinces revealed a bimodal emergence pattern in the gall community. While the autumn community is dominated by generalists with significant geographic variability, A. liuchunhuensis exhibits high phenological synchrony with the host’s spring peak. By targeting host eggs or early-instar larvae, this species appears to circumvent the host’s physiological immune responses, resulting in a stable and geographically consistent host–parasitoid relationship. This contrasts sharply with late-season parasitoids, which are subject to local adaptation and variable efficacy. Our findings suggest that A. liuchunhuensis acts as a primary, rapid-response regulation agent capable of suppressing pest populations before gall lignification protects the larvae. This discovery provides critical insights into the ecological mechanisms governing host–parasitoid synchrony and offers a promising new candidate for the biological control of S. castaneus to conserve native Castanea germplasm.