Devasting effects of river flooding to the ground-nesting bee, Andrena vaga (Hymenoptera: Andrenidae), and its associated fauna

Large populations of non-social bees are rarely seen in today's cultural landscapes. One of the few exceptions is the fossorial bee Andrena vaga Panzer, (Andrenidae), a typical insect species of river flood plains, which forms sizeable nesting aggregations along the high water dams of the Upper Rhine, each containing up to several tens of thousands of bees. In May 1999, a record flood of the River Rhine led to extreme water levels at the high water dams of the Upper Rhine. Though natural flooding is often viewed as a management practice with considerable conservation perspectives for river flood plain management, we demonstrate a catastrophic effect of the 1999 inundation on the immature offspring of two affected populations of A. vaga. All immatures in brood cells located low on the landward side of a dam died, very likely because saturation of the soil restricted oxygen availability. The two affected aggregations sited on the high water dam declined in the following year to 23 and 56% of pre-flood size, but not at a neighbouring, non-flooded aggregation. Rare parasites associated with A. vaga also decreased in number following flooding. These results highlight a dilemma on the use of natural flooding of river plains for conservation purposes.