My neighbour recently emailed asking if I could help identify a beetle infestation on a tree in his back garden. When I popped over to take a look, I recognised them as leaf beetles, though I was unsure of the species. I couldn’t find them in my field guides, but if my tree-identification skills were a bit sharper, I might have guessed, as a quick Google revealed that they were alder leaf beetles (Agelastica alni).
They had colonised the lower third of the alder, devouring the foliage from the ground up. The leaves were riddled with perforations, many munched down to the midrib, leaving just enough structure for the beetles to get back to the main stem. Though winged and capable of flight, they seemed to prefer grazing on foot.
With their voracious appetites, it seemed it would only take a matter of days for them to defoliate the whole tree. But I was able to reassure my neighbour that there is just one generation a year, and while the damage is unsightly, afflicted trees usually make a full recovery.
The beetles themselves are striking. At 7mm-8mm long, they are smaller than ladybirds and look like tiny polished pebbles. Their smooth, domed wing cases appear jet black until the light hits them. Then, they flash iridescent hues of petrol blue and violet. Their heads are tucked under a curved thorax, and their delicate segmented antennae twitch constantly.
Alder leaf beetles were once thought to be extinct or a rare migrant in Britain, with no records between 1946 and 2003. That long absence explains why they were missing from my insect books. In 2004, larvae were rediscovered in Manchester, and the diminutive creatures began quietly expanding their range across northern and central England and Wales. The first confirmed sighting here on the south coast didn’t appear until 2014.
The reasons behind their resurgence remain uncertain. It’s been suggested that a reduction in pesticide use or climate change may be factors. Either way, at a time when so much of our biodiversity is in decline, a comeback story, however small, is something to celebrate.
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