On a hot autumn day, a gleaming beetle with purple and green striped wing cases has been spotted for the first time in a garden in Allendale, Northumberland. The rosemary beetle, Chrysolina americana, is a Mediterranean species with no natural predators in the UK. While shimmering and beautiful, it can reduce plants of the Lamiaceae family to bare sticks and cause them to die back, including lavender, sage, thyme and rosemary.
First sighting in a herb bed
The beetle was found among the resinous leaves of rosemary in a herb bed. The gardener, who had not seen the species before, immediately recognised its significance. The insects were mating at the top of branches, particularly favouring lavender seedheads over flowers. Touching a stem causes them to drop, a defence mechanism common to other beetles, making them hard to spot against the soil.
The grubs are dull grey with longitudinal darker stripes, and both larvae and adults feed on the leaves. Rosemary beetles were first discovered living outdoors in the UK in 1994 and have increased rapidly since then in the warming climate.
Monitoring and advice
The Royal Horticultural Society is monitoring their spread and has asked for sightings, which the gardener submits. The advice is to hand-pick this non-native invasive species and tolerate it “where possible”, seeing them as “a colourful addition to the garden”. The gardener wonders how the beetles arrived, as they have quite short wings and can't fly far, preferring to walk, and all herbs were propagated in the garden.
Other insects spotted while studying the bushes include cinnamon bugs, seven-spot ladybirds, a garden snail sheltering in a sage leaf, a dramatically patterned cabbage bug, and a tiny froghopper. Red admirals flicker over calendulas in the unusual autumn warmth, but colder days are to come. Rosemary beetles are only likely to survive in the frost hollow if the winter is mild.
