Prescribed burning and post-fire recovery time shape microhymenopteran communities in high-nature-value grasslands

Semi-natural dry grasslands of Central Europe represent important biodiversity hotspots but are increasingly affected by land-use change and restoration management practices, including prescribed burning. Although fire is applied to maintain open grassland structure, its effects on microhymenopterans remain poorly understood. We investigated the responses of microhymenoptera assemblages to contrasting fire regimes in xerothermic grasslands. We compared abandoned unburned sites with sites exposed to prescribed burning differing in frequency and time since fire. Sampling was conducted using Malaise traps from April to July 2024 across nine localities. In total, 3,975 specimens representing six superfamilies and several families of the Chalcidoidea superfamily were analysed using generalized linear mixed models. Microhymenoptera abundance was generally highest in unburned control sites, indicating that long-term abandonment provides relatively stable and resource-rich conditions. However, sites sampled one year after fire showed a temporary increase in abundance, suggesting a short-term positive response during early post-fire succession. In contrast, sites sampled shortly after the most recent burn, whether burned once or in successive years showed significantly reduced abundance across several dominant groups, particularly Chalcidoidea and within it Mymaridae and Aphelinidae. Seasonality strongly influenced microhymenoptera dynamics, with abundance consistently declining from spring to mid-summer across all management regimes. Our results demonstrate that the ecological effects of prescribed burning on microhymenoptera communities are strongly influenced by fire frequency. While burning followed by at least one full growing season of undisturbed recovery may enhance abundance through opened vegetation structure and post-fire recolonisation, short intervals between successive burns appear detrimental to microhymenopteran populations. These findings highlight the importance of maintaining a heterogeneous mosaic of burned and unburned habitats for biodiversity conservation in species-rich xerothermic grasslands.