Eastern leaf beetles mass in southern Finland

Large numbers of eastern leaf beetles have been observed in several locations across southern Finland, including Lahti, Kouvola, Lappeenranta and Helsinki, Yle reports. The small, metallic‑shining beetles have been seen swarming around trees and landing on people’s clothing in recent days.

Jaakko Kullberg, a senior nature expert at Sitowise and an insect specialist for Yle’s Luontoilta programme, said he had recently observed many of the beetles in Kouvola’s Voikkaa district and in Helsinki. He attributed the abundance to a winter that started and ended with clear temperature transitions and without strong fluctuations.

Adult eastern leaf beetles overwinter and become active in spring. “When they want to reproduce quickly, they come out of the forest to the best spots where the sun shines, so that their young can grow fast on their host plants,” Kullberg said. That is why the insects can appear in very large numbers locally.

The beetle, measuring about six to seven millimetres, feeds on alder leaves but will also eat silver birch leaves when abundant, according to Juho Kukkonen, a forest management expert at the Finnish Forest Centre. Kukkonen said he had found beetles in Lappeenranta, including on a daycare sandbox and even inside his office. Alder trees generally survive defoliation, though young saplings are most attractive to the beetles.

The species undergoes strong population cycles. Kullberg noted that eastern leaf beetles were common in the 1950s, nearly disappeared in the 1970s and 1980s, and have increased again since the early 2000s. The article also cites the Natural Resources Institute Finland’s (Luke) Luonnonvaratieto service as a source.