Insects fly to the light
G’day, Dr Karl here.
Are flying insects really attracted to bright artificial night time lights? The ancient Romans thought so, and they used lights at night to pull insects in. Then there’s the well-known phrase, “drawn like a moth to a flame”. Which makes it seem like people over many years have spotted insects coming in to the lights, at night.
I’ve been asked why insects are attracted to light on various radio shows for a few decades – and it turns out I’ve always given an incorrect answer. To be fair, it wasn’t entirely my fault. I was just sharing the best knowledge of that time. Until now, there were four main hypotheses to explain insects loving the lights at night.
Now, the first one (which was both my favourite and wrong) was that the flying insects could navigate by fixing on the Moon. The Moon is very distant, about 400,000 km away, so if our insect started flying in a straight line with the Moon on its (say) right side, then after about 10 or 15 minutes, the Moon would still be on its right side. But down here on planet, Earth, artificial lights are so much closer. Suppose a flying insect is navigating by your barbecue light, which is only 50 metres away, and on its right side. After just a few metres of flying, our little insect sees that the barbecue light is no longer exactly on its right hand side, but a bit behind.
Our insect is clever enough to swing slightly to the right, so that your barbecue light will still be on its right – but which sends it spiralling in towards your barbeque light.
The second incorrect hypothesis was the insects would automatically head for any nearby light, because this was some kind of inbuilt protective escape mechanism. The rationale was the insects were aiming for a gap in the foliage, where some light might be shining through, and so they could escape through a tiny gap, guided by the light.
The third wrong hypothesis was that nocturnal flying insects have eyes that can pick up heat radiation. And sure, before LEDs, incandescent lights gave out much more heat than light.
The fourth erroneous hypothesis was that nocturnal flying insects have eyes that are especially adapted to the lower amounts of light present at night.
So they are potentially blinded by bright artificial lights, and as a result, will fly, erratically or even crash. The authors of a new paper did some neat tests to disprove every single one of these hypotheses. This was because over the last few years, we’ve had enormous advances in vision and computer technology, both hardware and software. Now we can film small, dark insects, moving very quickly, and in very low light levels. And we also have good tracking software, and faster computers, so with their powers combined, we can analyse the movies of the small flying insects, and see what they actually do. For example, we can now measure how much flying animals tilt their bodies to change direction – a very important clue, in understanding why flying insects end up near artificial lights. The new theory was developed after filming insects flying in the dark. It says, very specifically, that flying insects are not attracted to artificial lights.
But artificial lights do interfere with a basic insect automatic flying control system. This mechanism is called the Dorsal Light Reflex. “Dorsal” just means “back”. Flying insects use this automatic system to keep themselves flying straight and true.
The Dorsal Light Reflex is very simple - flying insects, line themselves up to keep the light shining on their back. That’s it. And in both daytime and night time, the brightest part of the natural environment is usually the sky.
But at night, artificial lights are brighter than the natural light of the stars and the Moon, and messes up this automated control system. So insects end up doing one of three behaviours.
The first one is called “inverting”. So if insects are flying above the house or street lights,they’ll flip onto their back, and simply just start plummeting towards the ground. Once they’re no longer in the light, they can recover and then they can fly back up into the light again, and keep on going through this loop of inverting and recovering.
The second behaviour is called “stalling”. In this case they do loop-the-loop circles - and when they’re upside down, they simply fall towards the ground. And again, once they no longer see the light, they revert to their normal behaviour, and then get caught in a loop of stalling and recovering - over and over.
The third behaviour is called “orbiting”. In this case they do successfully keep their backs to the light, but now they are doing circles around the light.
Instead of their wings pushing air mostly downwards, their wings are pushing air mostly off to the side, and only a little bit downwards. But, with a lot of hard work, they can keep on flying.
So our poor nocturnal flying insects are not drawn to the light – nor are they guided by it. Instead, they are are totally blind-sided ....
The ancient Romans thought that insects were attracted to bright light, and then there's the well-known phrase, "drawn like a moth to a flame", so it seems like people over many years have spotted insects coming into the lights at night. But what's the real reason?
Host: Dr Karl Kruszelnicki