Numerous negative ecological effects of urban lighting have been identified during
the last decades. In spite of the development of lighting technologies, the detrimental
effect of this form of light pollution has not declined. Several insect species are
affected including the night-swarming mayflyEphoron virgo:
when encountering bridges during their mass swarming, these mayflies often fall victim
to artificial lighting. We show a simple method for the conservation of these mayflies
exploiting their positive phototaxis. With downstream-facing light-emitting diode
beacon lights above two tributaries of the river Danube, we managed to guide egg-laying
females to the water and prevent them from perishing outside the river near urban
lights. By means of measuring the mayfly outflow from the river as a function of time
and the on/off state of the beacons, we showed that the number of mayflies exiting
the river's area was practically zero when our beacons were operating. Tributaries
could be the sources of mayfly recolonization in case of water quality degradation
of large rivers. The protection of mayfly populations in small rivers and safeguarding
their aggregation and oviposition sites is therefore important.
