Effect of bulb type on moth trap catch and composition in UK gardens

Abstract 1. A wide variety of light sources are employed to trap moths, differing in brightness and spectrum. Relatively little is known about how these factors affect the resulting sample. 2. We analyse 7 moth trap bulb types using 10 years of records from the Garden Moth Scheme to provide the largest and most comprehensive comparison of moth trap bulb types to date. 3. 125W Mercury Vapour (MV) bulbs collected the largest samples. The next largest samples were collected by 60W Actinic bulbs (67.80% of 125W MV catch), followed by 15W Actinic (56.66%), 80W MV (55.91%), 40W Actinic (49.12%), 20W Actinic (36.56%) and 6W Actinic (34.33%). 4. We demonstrate that MV bulbs, which emit a larger proportion of long wavelength radiation, collected a distinct fauna to Actinic bulbs, which emit primarily short-wave radiation. Species composition also varied between Actinic bulbs which differ in brightness, with brighter Actinic traps tending to collect a larger proportion of large-winged species. 5. We provide robust support for the ‘mobility hypothesis’ whereby large-winged, strong flying moths are more strongly affected by artificial light (in moth traps or from other sources), suggesting selective pressure against large winged species in landscapes affected by artificial light at night (ALAN). Our findings have significant consequences for survey design, citizen science projects, and for understanding the impact of ALAN on the moth community.