All night long: Activity times of moths as observed at artificial light sources

Abstract The positive phototactic behaviour of nocturnal insects can make them particularly vulnerable to light pollution, but can also act as an opportunity to sample nocturnal insect communities in an easy and efficient way via light traps. For both purposes—light pollution research and methodological application—it is very important to understand the activity times of nocturnal insects. By defining the peak activity time, we are able to understand when it is best to turn the lights off so as to not disturb insects. At the same time, we gain insights into how long a light trap has to be operated to accurately and adequately reflect the local nocturnal insect community. At the moment, the operating time of light traps is mostly based on expert opinions that state insects are only active until the dew point is reached, which is around midnight for the temperate European area. Yet, this opinion until today lacks scientific evidence. In our study, we noted down the arrival times of individual moths from dusk to dawn for five sampling nights in the summer of 2022. With our data, we analysed (A) if most of the moths/individuals are caught before midnight, (B) if flight activity can be explained by temperature, and (C) if the moth community sampled before midnight adequately reflects the community of moths sampled through the entire night. Our data showed that only 68% of all species and 57% of all individuals were caught before midnight. A total of 90% of all species and individuals were caught until approximately 02:00 to 03:00, which means that light traps should at least be operated until 02:00–03:00 to get a sufficiently complete sample. Species composition of the communities sampled before midnight significantly differed from the whole night samples. Yet, this effect was minor compared with the variation between sampling nights, underscoring the necessity of conducting multiple sampling nights to accurately mirror the local community.