LOVE WILDLIFE
Scientists solved the mystery using Rothamstead Research’s two upward-facing radars in Hampshire and Hertfordshire. These special entomological radars identifi ed painted ladies fl ying southwards at the end of the summer: the butterfl y previously evaded detection because it rises to an average of 500 metres to take advantage of prevailing winds, fl ying south at 30mph. These radars have also revealed the miraculous powers of Britain’s only confi rmed obligate migrant moth, the Silver Y. “We suspected they might have some pretty cool capabilities but when we saw the data for the fi rst time it was an exciting surprise,” says Jason Chapman, associate professor at the University of Exeter.
Chapman discovered that the Silver Ys were choosing the altitude that bequeathed most advantageous tail-winds. Radar also revealed that Silver Ys could even orientate their bodies in the correct direction to compensate for cross-winds. Most incredibly, Silver Ys were revealed to be travelling as fast – and sometimes faster – than migrating birds such as
In summer the Silver Y moth moves from southern Europe into Britain, the Netherlands and Scandinavia
Many gardeners will not welcome another “cabbage white”. Some insect migrations are portrayed as problematic. Tabloid newspapers reported “swarms” of harlequin ladybirds “invading” last autumn. Ladybird expert Professor Helen Roy worked with Rothamstead’s radar to identify harlequin and seven-spot ladybirds fl ying as high as 1,100m. Ladybirds, says Roy, are dispersers rather than true migrants. “Generally ladybirds are a little bit bumbly. The harlequin ladybird is a real opportunist. Because it is generalist in what it eats, it doesn’t matter so much which way it’s going – its main motivation is to move out of overcrowded habitat. They can get across the Channel but mostly they are making more local movements.” The harlequin ladybird’s rapid spread across Europe has been assisted by us, however. New research from
38 Spring 2019
thrushes. This was because the moths identifi ed the best spot in the air column for favourable tail-winds, whereas the larger birds didn’t bother.
But how do these insects know which way to go? Lab tests have revealed that the lengthening or shortening of days is the painted ladies’ cue: caterpillars growing while days are lengthening become adults who fl y northwards. When days shorten, the butterfl ies are born with an awareness of the need to travel south, to warmer climes. The painted lady orientates itself using the sun, but night-fl ying moths can’t use a sun compass. A study of Australia’s bogong moth reveals they use the Earth’s magnetic fi eld to guide them. As the climate changes, some dispersing insects will take up permanent residence in Britain. Charismatic arrivals could include the continental swallowtail and the long-tailed blue, but here’s a tip: the southern small white is making remarkable progress across northern Europe and was fi rst spotted in the Netherlands in 2015. It could soon cross the North Sea into East Anglia.
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MARGARET HOLLAND
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