SyrFitSources: An agent-based model to investigate the effects of land use on the population dynamics of aphidophagous hoverflies

Aphidophagous hoverflies are affected by insect decline, which is partly caused by habitat loss due to intensified agriculture. We present SyrFitSources, an agent-based model (ABM) for investigating the effect of land use on the population dynamics of the aphidophagous hoverfly Episyrphus balteatus on landscape scale in Central Europe. SyrFitSources considers 1) the life cycle of E. balteatus, 2) the resources of the larvae, i.e., aphids, 3) the resources of the imagines, i.e., pollen and nectar, and 4) the effect of insecticides on aphids and on syrphid larvae. For model testing, analysis, calibration, and evaluation, we used pattern-oriented modeling to compare the ABM results with observed numbers of active E. balteatus in 13 landscapes in Southern Germany in 2011. SyrFitSources reproduced these activity patterns. In 14 sensitivity experiments, we investigated the effects of stage-dependent mortality, migration, feeding, and reproduction on imaginal density. The effects were generally small, where variation of migration-related parameters had the largest effect on imaginal density. An additional resource-reduction scenario of aphids and floral resources suggests that the reduction of aphids has a stronger effect on the density of E. balteatus than that of floral resources. Two scenarios on the effect of insecticides on hoverfly larvae indicate that hoverfly populations are more strongly influenced by the insecticide-related reduction of aphid populations than by the direct effect of insecticides on hoverfly larvae. We conclude that SyrFitSources is a valuable tool for applied sciences, e.g., to assess ecological effectiveness of biodiversity measures in agricultural landscapes.