Meet the brassy willow beetle (Phratora vitellinae), a small metallic leaf beetle measuring 4 to 5 millimeters that is widely distributed across Europe. Both adults and larvae feed heavily on the leaves of poplars and willows. In a brilliant feat of evolutionary chemistry, these beetles hijack the willow’s natural defense mechanism; they sequester the plant’s toxic salicin and convert it into salicylaldehyde—a volatile, foul-smelling liquid. When threatened by predators like spiders, the larvae exude this chemical from specialized dorsal glands, creating a protective, defensive shield on their backs. During population booms, they can become significant pests, completely skeletonizing leaves until the foliage looks scorched. They overwinter as adults inside bark crevices and dead wood, ready to emerge as soon as spring arrives. I crossed paths with these beetles multiple times during my bicycle tour along the coasts of Sweden and Finland. Depending on the shifting natural light, their metallic elytra danced between bronze, copper, green, and deep blue tones. During one encounter, I even managed to capture an intimate look at a mating couple, when the male seemed to be differently coloured than the female.
