Insects and That
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Insects and That

Why insects don’t love life in the spotlight

The deep attraction of a streetlight to insects. Photo: Nevit Dilmen

Artificial light aims to help humans comfortably navigate their surroundings — but for other species, it can do the opposite.

It’s long been known, for example, that illuminated buildings can be bird killers. Recent research has demonstrated that this is not just because migrating birds collide with them, but because some birds’ calls inadvertently encourage others to do the same.

There’s a wealth of research suggesting that artificial light at night (ALAN) can also mean bad news for insects — a matter Germany is taking more seriously than most.

The country’s environment ministry has just made moves to ban floodlights at night for 10 months of the year and create new rules for outdoor lighting — all on behalf of our favourite invertebrates. Which certainly begs the question: just what is ALAN doing to insects?

Light versus life chances

More than 60% of all invertebrates are nocturnal — and their generally exceptional visual abilities in low light make them the most susceptible to bright, ‘non-natural’ light sources. Moths are of course famous to the point of meme-ready for their attraction to electric lighting, with the scientific consensus suggesting it’s based on misdirection during moon navigation.

At the basest level, attraction to lights can lead to immediate death, by simply hitting the light source, or a more drawn out demise of confusion and exhaustion. Other effects are more subtle but can be no less profound over time and scale.

Bright lights can disrupt almost everything important in an insect’s life: finding food and finding mates. For those insect attracted to light, it creates what are known as ‘demographic traps’, whereby there’s greater incoming to an area than outgoing which in turn affects how species interact.

It has been shown that the mating patterns of two fireflies, Photuris versicolor and Photinus pyralis, are disrupted in areas of light pollution, as they are less likely to engage in light displays, their means of communications during courtship. Effects have also been shown on flies, beetles, parasitoid wasps and, naturally, moths — with studies on the latter demonstrating effects on pheromone production, mating, feeding and development of non-adult life stages.

In agricultural settings, it has been suggested that light pollution can worsen the negative effects of habitat fragmentation on insect populations by reducing insect ability to fly between patches of the habitat which remains available.

Inevitably, a moth pic. Photo: Fir0002/ Wikimedia Commons

Light pollution doesn’t affect only the nocturnal, and not only the many invertebrates that show what’s known as positive phototaxis — put simply, that’s movement towards light. There are others, such as cockroaches, that display the opposite movement (negative phototaxis). For the latter, light pollution can mean them leaving areas where they would otherwise seek food and reproduction opportunities, harming their life chances.

Lighter scenes are not bleak news for all. There is some suggestion that increased windows of available light can help insects predators which rely on visual cues to find prey, reducing pest issues — but it’s fair to say that this, as yet, isn’t backed up with a huge body of evidence, and it’s likely to be a small positive flipside to a more formidable negative narrative.

Please, lay your eggs here. Photo: Andreas Senftleben from Pixabay

The fatal attraction of shiny surfaces

It’s not just electric lighting that poses a problem for bugs: polarized light pollution off man-made surfaces — whether they be buildings, cars or even solar panels — can provide more struggles through polarotaxis — attraction to polarized artificial surfaces.

For insects with aquatic life stages such as mayflies, caddisflies and stoneflies, this can be particularly troublesome. Black cars have been shown to appeal to such insects, and dark grey apparently even more so to mayfly.

Females of various flying insects can prefer to lay their eggs on solar panels than water bodies. This is a classic example of what is known as an evolutionary trap — where an animal is prompted by externalities to prefer a destructive idea over a productive one — and what’s worse, the combination of phototaxis and polarotaxis has a bigger effect on aquatic insects active at night than one or the other alone.

As is often the case in nature, a disaster for one is a bonanza for another. It has been shown that birds visit highly polarizing glass buildings which they use effectively as giant bird-feeders due to the aggregation of flying insects there. Bats and spiders are known to get involved in their own assisted feeding frenzies. The attraction of an easy meal is a great universal.

Technological and evolutionary solutions

So, are there possible remedies to our light-soaked environments? Being more like Germany is a good place to start. There needs to be more acknowledgement by decision-makers that light affects ecosystems, and that it is not unreasonable to modify use of technology accordingly.

Not all light is the same, of course — so which type is worse? There has been some suggestion that the dawn of the age of the LED means greater risk of pulling insects off track due to the common ‘cool white’ light peaking at shorter wavelengths. This is due to most insects having highest light sensitivity to UV, blue and green. Yet some studies have come to entirely the opposite conclusion, namely that old-style incandescent bulbs attract more from across the insect orders than LEDs — albeit with the ‘warm’ LED light attracting even less than the cool.

A research group in Los Angeles took things further to come up with LEDs customised for low attractiveness to insects, which emit more longer-wavelength light. These trapped less moths, flies and other insects over a 32-night trapping period, proving that technological modifications can be made to make our world more insect-friendly.

Reducing the effect of polarized light is not as simple as making surfaces matte rather than shiny. Such a move has proved to have little effect in a few studies, with anti-reflective coatings seeming more like an option worth pursuing should the importance of insect conservation in design be greater acknowledged and acted upon.

Meanwhile, insect evolution may come up with some solutions of its own. A fascinating study looking at moths in Switzerland and France suggests that exposure can reduce flight-to-light behaviour over generations.

A significant difference was noted in this behaviour between moth populations in urban and pristine dark-sky habitats; the reason being, it is surmised, that the ones that didn’t get drawn to the light enjoyed more reproductive success, with those exhibiting more classic moth behaviour not so much. There is a downside, though: the light-resistant urbanites are likely to have reduced mobility, limiting their options in life. Not a totally winning approach, then.

Perhaps energy efficiency concerns will in time chime perfectly with the interests of insects, with reduced outdoor lighting cutting bills, reducing greenhouse gas emissions and saving insect lives. Until then, more needs to be learned about just how big a problem this is on a global scale — which is easier said than done.

While it is abundantly clear that light pollution does affect insect life, just how much it contributes to the decline of individual species or insect diversity as a whole remains unclear. There is more work to be done in disentangling light pollution from the myriad other pressures insects face in the increasingly complex third decade of the twenty first century.

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Gary Hartley

Gary Hartley

Writer of different things. Come for the insects, stay for the odd literary works, or vice versa. @garyfromleeds https://medium.com/insectsandthat