Phenology of microhymenoptera and their potential threat by insect decline

Although microhymenoptera are highly abundant in all terrestrial ecosystems, they are overlooked in most of insect monitoring studies due to their small-size and demanding identification linked with lack of taxonomic experts. Until now, it is unclear to what extent microhymenoptera are affected by insect decline, as there is a huge knowledge gap on their abundance. To fill this knowledge gap, we used Malaise trap samples from three study sites of a complete vegetation period (March to November) of an ongoing insect monitoring study in south-western Germany (i) to study the relationship of insect total biomass, and abundance and diversity of microhymenoptera, and (ii) to assess the phenology of microhymenoptera families. Our results show that microhymenoptera abundance and diversity are positively correlated with total insect biomass, indicating that insect biomass is a valuable proxy for insect abundancy trends even for small-sized insects. In total, we counted 90,452 specimens from 26 families belonging to 10 superfamilies of Hymenoptera. Microhymenoptera numbers peaked twice during the year, first between June and July and second between July and August. Interestingly, egg-parasitoids, such as Scelionidae, Mymaridae and Trichogrammatidae had a slightly shifted second activity period in August and September. Our data provides a baseline for the occurrence of microhymenoptera in meadow ecosystems in south-western Germany and underlines the potential of mass samples to study microhymenoptera in the context of insect decline.