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Rosemary beetle

Watch out for Rosemary beetle on herbs in May and September, says Plant Doctor Helen Derrin

An eye-catching pest is munching its way through my purple sage at the moment…and judging by the number of emails I’ve received it’s a widespread problem in herb gardens up and down the country this year. The rosemary beetle (called Chrysolina americana by the scientists) is a relatively new pest to these shores, migrating north from southern Europe.

What to look for

The rounded adult beetles are quite distinctive, about 8mm long, purple-striped, with a greyish-green metallic sheen. There have been sightings of this pest in isolated places for ten years or more, but more recently thriving colonies have become established in urban areas in London and the South East of England. They were demolishing my purple sage throughout May, but would have soon disappeared from view through the summer months. Then in autumn they reappear to gorge themselves on lavender, rosemary, sage, thyme…and even Russian sage (Perovskia), before mating and laying eggs.

Control

I think I’ve managed to get rid of my infestation by hand-picking the adults routinely throughout the month. The adult beetles and their larvae are also easy to control using an insecticide containing the active ingredients bifenthrin, imidacloprid or thiocloprid. Bear in mind to spray at dusk if the plants are in flower to avoid harming bees and other beneficial insects.

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