Latitudinal and seasonal changes of nocturnal illumination with a hypothesis about their effect on catches of insects in light-traps

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Abstract
An account is given of changes in nocturnal illumination at the spring and autumn equinoxes and summer and winter solstices at latitudes between 50 and 60°N. A Relative Trap Index (RTI) was devised to describe changes in light-trap effectiveness and used to adjust catches of Noctuidae and Geometridae for change in trap effectiveness caused by seasonal and latitudinal change in nocturnal illumination at latitudes between 50 and 58°N. When so adjusted, catches were frequently larger in northern than in southern latitudes, not smaller as unadjusted catches show. It is clear that unless light-trap catches are adjusted to allow for changes of illumination, comparisons over time and space, particularly those which involve assumptions about sample constancy, are suspect. Because of the large differences in RTI that may exist between northern and southern traps, particularly in summer and between the summer solstice and other times, it may be necessary to use different light sources at different times of year to maintain a consistent trap sample.

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