Tracking moth migration from the skies
Many insects migrate, sometimes over long distances of hundreds or even thousands of kilometres, yet with extraordinary accuracy. It puts our ability to get from A to B to shame. But when it comes to how they actually go about this, and what techniques they use to find their way, we have very little idea. So, armed with an aeroplane, some stick-on radio tags and a handful of migratory hawkmoths destined for the Alps in Germany, Myles Menz has been trying to discover how something the size of your thumb can achieve amazing feats of navigation…
Myles - And so the idea was to fit these really tiny radio transmitters onto these moths. And then we would release them at night, around autumn, when they were supposedly ready to migrate. And then we waited and hoped that we would actually catch them while they were migrating. And we did.
Chris - I'm intrigued by the idea of putting a radio transmitter on an insect. I mean, how big are these devices and how big are the moths that you're studying?
Myles - So these devices are really quite small. This is more traditional radio tracking technology that's been used for many decades often for tracking birds. It's got to a stage now where the tags are small enough that we can fit them onto some large insects. And so some of the smallest tags that we were using are around 0.2 of a gram. So 200 milligrams. And in this case, the moths were really quite large. So one of the large females can weigh up to about three and a half grams. So actually the radio transmitter is really quite a small proportion of this animal's body weight.
Chris - And how do you track the radio signals?
Myles - We follow the signal by using an antenna and we basically hone in on the signal strength. In this case, the moths were flying at night and probably travelling quite some distance. So we used an aircraft, and the aircraft was fitted with receiver antennas under the wings which allowed us to effectively triangulate the position of the insect from the aeroplane.
Chris - I'm smiling to myself. The idea of you flying along after a bunch of moths in a light aircraft. How far were they going then? And from where to where?
Myles - So in this case, our tracking was undertaken in Southern Germany and we were able to follow the moths as they moved South or Southwest towards the Alps. And so this was over a distance of a little bit over 80 kilometres at any one time. This is one of the longest direct tracks that we have of being able to continuously follow a migrating insect. What we found most surprisingly was that these animals are able to fly in really very straight lines. Effectively, they had a heading that they would select or that they would fly on and they were able to make a beeline for that. They were able to do this in a series of different wind conditions. For example, if they had some nice, favourable tailwinds that might be blowing them in a southwesterly direction towards where they would want to migrate towards, they would fly a bit higher above the ground. So probably around 300 metres. However, if there were winds that were blowing directly towards them, so headwinds or even crosswinds, they would actually adjust their headings to compensate for wind drift, which we found was really quite fascinating. And they do this also quite a bit lower to the ground. So probably within that first 50 metres where they were somewhat buffered from the wind, but able to control their headings better.
Chris - That's an extraordinarily effective mastery of flight for such a simple creature.
Myles - Yeah, absolutely. We've known quite a bit about how insects might respond to winds. And so there's been work showing that, for example, butterflies during the day might fly a bit lower towards the ground. And we certainly see that some insects might just fly down wind, but certainly in this case, what it's really revealed and something that we've been thinking about for quite a long time is that many insects aren't really passive migrants and they're very active migrants. And so they're not just getting blown about by the wind, they're really selecting directions where they want to go and really adjusting their course and adjusting their flight path to actually achieve that.
Chris - When the research was published on the monarch butterfly, which makes stupendously long migrations in the Americas, they also do interesting things where they set their body clock to where the sun is. So they know what time of day it is, and they align that with the magnetic field so that they're always on course using the sun as a guide but, as their body clock ticks, it compensates for the movement of the sun across the sky. Of course your insects are flying at night. So presumably there are going to be some moonless nights. So it can't be a visual thing they're following. So how are they getting those straight lines from A to B where they want to go?
Myles - So we think it might actually be a mix of navigator mechanisms. They're probably using a magnetic compass. So they have some understanding of the Earth's magnetic field, but probably also a visual compass. We know hawk moths have very good vision, but they're probably going to be using cues that are maybe a bit more consistent, such as say a mountain pass - a high Alpine pass or another landscape feature. And certainly we did notice when we were flying that some of the directions the moths were heading were actually matching up to what appeared to possibly be visual cues. And there has been some work, from the Australian Bogong moths showing that the moths can actually use a mix of the Earth's magnetic field and visual cues to navigate.