I’d just knocked a raspberry cane out of its pot when a metallic flash of emerald green caught my eye. A 2cm-long, lozenge-shaped rose chafer beetle (Cetonia aurata) crawled out of the compost. As the light struck its forewings, they shimmered iridescent violet, copper and rose gold, like oil on water.
This iridescence is a form of structural coloration – the production of colour by complex nanostructures found in the beetle’s exoskeleton that reflect light at different angles, causing the colour-changing effect. For many years, scientists have believed that biological iridescence has two primary purposes: attracting a mate, or warning off potential predators. These jewel-like scarab beetles may appear impossible to overlook when spotted feeding on flowers, but recent research suggests that their iridescence actually works as a highly effective form of camouflage in grassland, scrub and woodland edges.
The sun-loving adults usually emerge between late May and mid-June, so it was a surprise to find one in the garden more than a month earlier than in previous years. Posting on the Natural History Museum’s UK biodiversity Facebook page, I discovered that mine wasn’t the only unseasonable sighting. One had been seen squashed on a path near Reading, and there were another three in a conservatory in Colchester, believed to have emerged from a pot recently brought in from the garden, where they had likely overwintered as pupae.
Like many insect species, rose chafers have been affected by climate change. The average daytime temperature in Langstone in March is 11C, but high pressure had brought a lengthy spell of sunshine, with the temperature peaking at 18C, triggering the beetle’s premature emergence.
Unfortunately, this meant that the beetle was out of sync with the flowering stage of its typical food plants. As their name suggests, they are particularly fond of roses, but will also feed on elder, viburnum, hogweed and soft fruit, consuming pollen, nectar and petals. With none of these species in flower, I encouraged the chafer to cling to my finger with its grappling-hook-like feet and relocated it to a winter-flowering honeysuckle.
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