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Hoverflies stripe off in summer heat

December 10, 2001

Ecologists are unraveling the reasons behind seasonal differences in insects’ colour patterns, a question that has puzzled them since the 1970s. Speaking at the British Ecological Society’s Winter Meeting, to be held at the University of Warwick on 18–20 December 2001, Anne Halpin and Dr Graham Holloway of the University of Reading will explain that hoverflies become darker in colder weather so that they can warm up more efficiently.

Such “colour plasticity” evolved because it confers an advantage over animals of the same species that cannot change colour. Halpin says: “Flies that are able to predict the weather are at an advantage over those that just wear the same coat whatever the weather. By choosing the colours that best suit the prevailing conditions they are able either to warm up more quickly under cool conditions, or stay cool in the heat of the summer.”

According to Halpin: “In cold-blooded creatures like hoverflies, darker flies are able to warm up more rapidly and, therefore, have an advantage over paler flies when the weather is cold as they can move around to find food or mates more quickly.”

More than 250 species of hoverfly live in the UK. They are familiar and welcome visitors to our gardens, where many help to control aphid pests and pollinate flowers. Many adult hoverflies mimic bees and wasps, but their yellow and black stripes vary in thickness during the course of the summer.

Thirty years ago, ecologists found that hoverfly pupae reared at cold temperatures produce adults with thicker black stripes than those reared in warmer conditions. This variation is an example of “phenotypic plasticity” – the ability of genetically identical flies to react to environmental conditions (such as temperature) and adjust their appearance to best suit those conditions. However, the reason remained unclear until now.

Ecologists theorised that it was an example of “thermal melanism” – where darker flies are able to warm up more quickly in the sun than paler individuals. Working with the Marmalade hoverfly (Episyrphus balteatus), Halpin has proved that these darker flies do indeed get hotter – the first time that thermal melanism has been shown in hoverflies.

Halpin will next turn her attention to what happens when hoverflies are faced with weather conditions that they did not expect. “I aim to rear hoverflies at different temperatures, then release them into the field to find out whether or not this affects their survival and dispersal,” she says. She will also try to discover whether or not colour plasticity is common throughout the hoverflies and other groups of insects. “If colour plasticity is common in other species and groups, this would suggest that it appeared early on in insect evolution,” Halpin says.

Anne Halpin will present her full findings at 15.20 on Tuesday 18 December 2001.

British Ecological Society (BES)

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