The rosemary beetle (Chrysolina americana) devours the leaves of rosemary, lavender, thyme, sage and some other related plants.
It is an attractive 8mm long metallic green beetle with purple stripes on its wing cases and thorax.
The beetle is a native of southern Europe that has become an established pest in Britain since the 1990s.
The soft-bodied grubs are greyish white with five darker longitudinal lines; fully grown larvae are 5-8mm long. Sausage-shaped eggs, 2mm long, may be found on the underside of the leaves from early autumn to spring.
Distribution and establishment in Britain
The rosemary beetle was first found living out of doors in the UK at RHS Garden Wisley, Surrey in 1994 but this population soon died out and it was not seen again at Wisley until 2003. By 1998 established colonies of the beetle had been discovered near London’s Waterloo Station and Winnersh, near Reading, Berkshire.
In 1999 the first enquiry concerning rosemary beetle was received by the RHS Members’ Advisory Service from a garden in Weybridge, Surrey. By the end of 2005 the beetle had become widespread throughout London and surrounding areas, and had become one of the top 10 most frequent pest enquiries made to the RHS Members’ Advisory Service.
Rosemary beetle is now widespread in England and has been found in parts of Scotland, Wales, Northern Ireland and the Irish Republic.
Research in the entomology laboratory at Wisley Garden indicated that rosemary beetle adults remain relatively inactive on their host plants during July and August. In late August and September the beetles resume feeding, mate and lay eggs, which they continue to do on warm winter days until spring. The eggs hatch within two weeks and the larvae feed for as little as three weeks before entering the soil to pupate. The pupal stage lasts for a further two to three weeks before adults emerge.
Most enquiries on rosemary beetle received by the RHS Members' Advisory Service concern the beetle as a pest on rosemary (Rosmarinus officinalis) and lavender (Lavandula species). However, the beetle is able to breed on thyme (Thymus species), sage (Salvia species), Russian sage (Perovskia atriplicifolia), and it is possible that some other plants in the Lamiaceae family may also act as hosts.
- Rosemary beetle can be controlled either with insecticides or hand picking of adults and larvae.
- The adults and larvae can be collected by placing newspaper or an open upturned umbrella under the branches and tapping the plants to dislodge the insects.
- If pesticides are used then these are best applied in late summer to early autumn or in the spring.
- Suitable insecticides that can be used include pyrethrum, acetamiprid, deltamethrin, lambda-cyhalothrin, or thiacloprid.
- Insecticides should not be used while plants are in flower because of the danger to bees. Products approved for use on edible herbs include Pyrethrum ( e.g. Py Spray Garden Pest Killer), lambda-cyhalothrin (Westland Plant Rescue Fruit and Vegetable Bug Killer), deltamethrin (Killer Bayer Sprayday Greenfly Killer) and thiacloprid (Bayer Provado Ultimate Bug Killer).