Thirty years ago, if you were walking along the coast of the Netherlands and picked a two-spot ladybug off the leaf of a European lime tree, chances were that the bug would be red with black spots. If you were farther inland, you'd have had a good chance of finding a bug that was black with red spots. In the past 3 decades, however, researchers have been finding more red bugs inland. The reason, they believe, is that a warming climate and fewer sunny days might be driving a shift in ladybug color.
Paul Brakefield, an ecological geneticist at the University of Cambridge in the United Kingdom and colleagues have studied the two-spotted ladybugs for decades, intrigued by why different populations of ladybugs have different colors. In 1980, 10% of the ladybugs living near the coast were black with red spots, or melanic, whereas 90% were red with black spots, or nonmelanic. In contrast, 40 kilometers inland, 40% of the ladybugs were melanic and 60% were nonmelanic. The color difference was likely an adaptation that allows the darker melanic ladybugs to stay warm in the cooler inland climes, whereas a lighter color prevents the nonmelanic bugs from getting too hot on the coast. One protein is responsible for the difference between the two colors, and the genetic switch from one to the other is very simple. But the researchers still don't know how exactly sunlight or temperature influence ladybugs' color.
Over 25 years of sampling insects, Brakefield and his colleagues noticed a change. As they continued to catch hundreds of ladybugs every 5 years or so, over 50 generations of ladybugs, they found more of the nonmelanic bugs, even when sampling inland. In 2004, the last year in which the researchers could gather enough ladybugs to see a significant trend, they found that only 20% of the ladybugs in any area were melanic. The trend seemed to fit with temperature data over the period, which showed that the entire area had been consistently warming, the researchers report in the December issue of Heredity.
In some ways, Brakefield says, the case is not unlike that of the famous peppered moths in Liverpool, which became darker as the trees in the area blackened with soot, protecting them from birds. "The story is one of very strong natural selection," he says. "But moths survive by being camouflaged, and [ladybugs] survive by being conspicuous and telling birds not to eat them." The melanic and nonmelanic ladybugs each mimic a different species of beetle which is very poisonous to birds, he says.
The results are "intriguing," says Ary Hoffmann, a geneticist at the University of Melbourne in Australia. By linking climate change to the shift in ladybug color, the authors present a "workable hypothesis, but it's not quite there yet" he says. Cementing the link would require further data showing that the bugs change color over generations when they are experimentally manipulated in the lab, he says.
For his part, Brakefield says the field studies are over. The numbers of two-spotted ladybugs of either stripe have plummeted, outcompeted by an invasive species called the Japanese harlequin ladybug, an escapee from a greenhouse in Belgium where it is used to control aphids. Brakefield says the two-spot ladybugs are now so extraordinarily difficult to find that he can't continue the study. "In the good old days, we could find individual leaves at the right time of year covered in pupae," he says. "It's very frustrating, very sad."