Bats and Lighting

LEDs Found to Attract Fewer Insects than Conventional Light Types. What Does This Mean for Bats?

In a recent paper written by Wakefield et al. (2016) published in the journal Ecology and Evolution, the attractiveness of domestic lights to insects was investigated with a focus on LEDs which are predicted to constitute 70% of the outdoor and residential lighting markets by 2020.

The domestic lights tested included compact fluorescent, tungsten filament, ‘cool-white’ LEDs and ‘warm-white’ LEDs. Light traps for each were deployed in the same habitat and microclimatic conditions but also far enough away from each other so that insects attracted to one light could not be caught in another. The lights were turned on within 10 minutes of sunset and kept on until sunrise, with new insect catching beakers being introduced after 4 hours to separate the insect samples into ‘evening’ and ‘morning’ insects.

The study found that LEDs attracted significantly fewer insects than other light sources but found that there was no significant difference in the number attracted to the ‘cool-white’ and ‘warm-white’ LEDs. The largest proportion of the insects were caught in the tungsten filament light traps (54%) followed by the compact fluorescent light traps (24%).

The study concluded that ‘the use of LEDs has the potential to mitigate disturbances to wildlife and occurrences of insect-borne diseases compared to alternative light technologies’.

Bats and Lighting

Illuminating the commuting routes of bats can cause them disturbance and illuminating their roost entrances can lead to the complete abandonment of the roost, particularly concerning for our large number of light-sensitive species.

Lighting can also affect their feeding behaviour in two ways:

• By the attraction of a range of insects to certain light sources; and

• By the presence of lit conditions causing them disturbance.

Outdoor lights and light spill from the inside of buildings attracts many night-flying species of insect that are prey for bats. We often see moths being attracted to light sources, but crane flies, midges, lacewings and a range of other species are also affected, thus also affecting a wide range of bat species with different feeding niches.

Blaenau Gwent bat surveyOur more light tolerant species (pipistrelles, noctule, Leisler’s bat and serotine) have been shown to swarm around white mercury street lights to feed on the insects attracted to them.

For them, the use of LEDs for street lighting in urban areas could lead to a reduction in foraging opportunities if insects are not otherwise abundant within reasonable commuting distance from the roost (it is assumed that there would be a higher density of good foraging opportunities in rural areas). Although, our large bats (noctule, Leisler’s bat and serotine) are more likely to commute long distances from the roost to forage, so using LEDs may have a lesser impact on them than the smaller pipistrelle species.

Our slower flying broad-winged bat species such as long-eared bats, Myotis species (Brandt’s, whiskered, Daubenton’s, Natterer’s and Bechstein’s), Barbastelle and horseshoe bats, are much more light sensitive and generally avoid well-lit areas.greater horseshoe bat

For them, the use of LEDs for street lighting may be a positive thing. Studies have shown that insects are attracted to lit areas from further afield, potentially resulting in adjacent habitats supporting reduced numbers of insects. As low light level areas, these would likely be the habitats used for foraging by our light- sensitive species.

Whilst lighting is still best avoided when it comes to bats, the use of LED lighting, instead of more traditional lighting types like tungsten filament or compact fluorescent, may be beneficial for light intolerant species where darker insect-rich adjacent habitats are present.

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