Warmer temperatures reinforce negative land-use impacts on bees, but not on higher insect trophic levels

Climate and land-use change are major drivers of insect decline, yet their
interactive effects on insect richness and abundance, especially across
trophic levels, remain poorly understood. Here, we investigate how
temperature and land use shape insect communities across spatial scales
and trophic levels, from flowering plants and cavity-nesting bees to
hunting wasps, their antagonists, and parasitism rates. Using trap nests
and a space-for-time approach, we surveyed 179 plots spanning four habitat
types (forest, grassland, arable land, and settlements) across 60 study
regions in Germany covering semi-natural, agricultural, and urban
landscapes. Bee richness and abundance responded to climate–land-use
interactions across spatial scales, being higher with warmer local daytime
temperatures and overall warmer climates, but only in less intensive land
uses. In contrast, elevated nighttime temperatures negatively affected
bees. Higher trophic levels benefited more consistently from warmer
climates than lower trophic levels and were less affected by high local
daytime and nighttime temperatures. Parasitism rates were lowest in arable
land but similar across habitats within semi-natural regions, suggesting
that landscape-scale processes buffer local effects. Our findings
underscore the importance of considering nighttime temperatures for
diurnal insects and suggest that rising temperatures may exacerbate the
negative impacts of land use on pollinators.