DEEP in a wood in southern England, tucked under a pile of dead leaves, a neatly folded “silver Y” moth is struggling to emerge from its chrysalis. Watching this delicate creature take its first faltering steps, you would never think it was about to set off on a journey that could take it as far south as north Africa. That’s a long way for a moth whose adult life lasts only a week or two.
With their blink-and-they’re-gone lifespans, migratory butterflies and moths like the silver Y need some pretty smart strategies to cover distances rivalling those of many migratory birds. So how do they manage to cover such large distances? And how do they know which direction to fly when it’s a journey they make only once in a lifetime? The answers are of more than academic interest as many of these creatures’ larvae are crop pests, and in a changing global climate we need to know where they will next turn up.
Migrating lepidoptera routinely reach altitudes of over a kilometre, so studying them in mid-flight is far from straightforward. Samples trapped by aircraft or tethered helium balloons have provided a glimpse of their migrations, but this is expensive, not suited to long-term studies and not easy in the dark – which is when moths prefer to fly.
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Fortunately, a UK team of entomologists led by Jason Chapman at Rothamsted Research in Hertfordshire have found a way to sidestep these problems. What’s more, they can make their observations without leaving the ground. Since 2000, the entomologists have been using two radar scanners, one at Rothamsted and the other at Chilbolton in Hampshire, to sweep the skies day and night.
Insects flying through the radar beam bounce back signals that reveal much more than can be gleaned by simply trapping them. Each blip on the radar readout records an insect’s altitude, the speed and direction of travel, body alignment, shape, size and wing-beat frequency. The radar can detect insects weighing as little as 15 milligrams – far smaller than most migrating moths, which can weigh all of 500 milligrams – flying at altitudes up to 1200 metres. An insect’s species can be deduced from its size, shape and wing beat.
This year, Chapman published the results of seven years’ worth of radar data, providing information on more than 100,000 individual insects. The observations show migrating moths to be masters at choosing the most favourable wind, setting off only on nights when it blows close to the direction they need to travel (). On such nights, silver Ys, for example, can hit speeds over the ground of 90 kilometres per hour by finding the fastest-flowing high-altitude airstream and angling their flight to correct for any crosswind drift. Chapman has shown that by using these strategies the insects are able to travel about 40 per cent further than if they were simply blown along in the wind.
Sensing south
For butterflies, there is less of a rush. Painted lady butterflies travel just as far as the silver Ys, but the adults live for weeks rather than days. Painted ladies and related species such as the red admiral tend to fly closer to the ground than moths, and they rely mainly on their wing power rather than the wind. The migration distance record for lepidoptera goes to the monarch butterfly of North America, which over the course of a couple of months at the end of summer travels from as far north as the Canadian border to overwinter in central Mexico (see “Marathon migration”).
Feats like this raise the question of how high-flying insects know where they are heading. They seem to have an inbuilt compass, but how it is set remains controversial. Is it based on the position of the sun – a “sun compass” – or does a magnetic compass like those possessed by migratory birds play a part? Moths are well known to fly towards the light, and butterflies do too. The sun can’t be the whole story, though, because butterflies don’t only fly on clear, sunny days, and moths fly at night.
It is a question that interests Robert Srygley’s team at the Smithsonian Tropical Research Institute in Panama. They have found that altering the direction of the local magnetic field changes butterfly behaviour. For example, insects released into a large cage in which the magnetic field had been reversed tend to fly in the opposite direction to their usual migratory route. However, some of the control insects, released when magnetic north was unaltered, also fly in the opposite direction – so whether they use magnetic fields for migration remains uncertain (). “Although we could show that they were sensitive to the magnetic field, we were not able to show that they use a magnetic compass to orient when migrating,” Srygley says. “Butterflies are problematic to study because they have an escape behaviour towards the sun.”
Experiments with insects less distracted by light have produced clearer results. Srygley has teamed up with physicists in Rio de Janeiro, Brazil, to test whether leafcutter ants respond to magnetic fields by making use of magnetite (iron oxide) crystals in their bodies. The ants do appear to use a magnetic compass to set a direction home (), a finding that adds to a steady build-up of data suggesting that insects possess a magnetic compass.
Steve Reppert at the University of Massachusetts Medical School in Worcester is working on a theory that says butterflies and moths sense magnetic fields using photoreceptors known as cryptochromes, which are also found in birds, other insects and even in plants. Reppert has shown that fruit flies can use their cryptochromes to detect magnetic fields, and had a hunch that monarch butterflies might do the same. To test his hypothesis, he inserted monarch cryptochrome genes into fruit flies whose own cryptochromes don’t work – and sure enough, the flies’ response to magnetic fields was restored (). One oddity about cryptochromes’ magnetic sensitivity is that it only occurs when the photoreceptors are illuminated by blue light; why blue light is required remains unclear.
Chapman reckons that lepidoptera might use a combination of cues to navigate. Butterflies are known to use a sun compass to determine their direction. Even when the sun itself is obscured, its position can be inferred from the polarisation of light from patches of clear sky. What’s more, polarised light remains visible for up to 2 hours after sunset, so Chapman suspects that as moths take off at twilight they use polarised light to set their direction compass. During the night, a magnetic compass could take over, he says.
The team is also using the radar data to build predictive models that will tell us when, where, and in what numbers different migrating insects are likely to arrive. To do this, Chapman used a modified version of the UK Met Office’s model of how airborne particles are dispersed by the wind; this is the model that was recently applied to predict the position of the cloud of ash from an Icelandic volcano as it drifted across Europe. “This new version treats the insects as active flyers with specific behaviours, rather than passively transported particles that just travel downwind,” Chapman says.
As many migratory insects are damaging crop pests, working out where their extraordinary journeys will take them has practical implications. The silver Y’s caterpillars are cabbage and pea-munching pests, and the caterpillars of another migratory moth, the large yellow underwing, are among the group of infamous crop pests better known as the cutworms that cause fatal damage at the base of virtually any herbaceous plant they choose to chew.
Insects are not the only creatures that migrate over long distances; birds do too. But insects have one big advantage over migrating birds: they can afford to make mistakes (see “Moths with maps?”). The two or three offspring of a mating pair of migratory birds have got to land in the right place, otherwise the future of the species is in trouble. The more robust reproductive strategy of butterflies and moths stands them in good stead in the face of climate change, as this year’s “right place” might not be the same as last year’s. Chapman thinks this is giving insects the upper hand. “That’s probably one reason why migratory insects are becoming more common, but many migratory birds are declining.”
Read more: Zoologger: Globetrotters of the animal kingdom
Moths with maps?
When migratory moths and butterflies emerge from their chrysalises in autumn in northern Europe, they immediately start flying south. When the next generation emerges on the Mediterranean coast the following spring, they start flying north. How do these creatures know where their breeding grounds are, when none of them lives to make the return trip?
It seems they don’t. After the adults emerge, they simply travel in a hard-wired direction until they become sexually mature. How far an insect migrates depends on the length of this genetically determined phase. Migrants don’t decide where to land based on the weather or the vegetation: they land when they reach the insect equivalent of puberty.
Many will land in unsuitable locations and fail to breed, but that’s not a huge problem as long as a few of them end up somewhere sensible. A mating pair of insects produces thousands of eggs. The offspring that migrate to inhospitable climes will die, but the chances are that hundreds of their siblings will land somewhere more appropriate. Insects only need about 1 per cent of their offspring to survive to sustain the species.
There is evidence that climate change is already altering insect migration patterns. Steadily increasing numbers of migratory moth and butterfly species are being recorded arriving in a cliff-top garden at the Portland Bird Observatory in Dorset, on England’s south coast. From data collected between 1982 and 2005, it appears that for every 1 °C increase in temperature, an extra 15 species will arrive ().
