Common name: rosemary beetle
Latin name: Chrysolina americana
Size & characteristics: an attractive, 8mm long, metallic green beetle with purple stripes. The larvae are grey with darker stripes and up to 8mm (516 in) in length, hatching from sausage-shaped eggs approximately 2mm (18in) long. Can be found feeding on the underside of leaves of rosemary, lavender and related plants, from early autumn to spring. The beetle is a native of southern Europe that has become established in Britain since the 1990s.
Distribution: The rosemary beetle was first found out of doors in the UK at RHS Garden Wisley, Surrey in 1994 but this population died out and it was not seen at Wisley again until 2003. By 1998 colonies of the beetle had been discovered near London’s Waterloo Station and near Reading, Berkshire and in1999 the first enquiry concerning rosemary beetle was received by the RHS Advisory Service from a garden in Weybridge, Surrey. The beetle spread quickly, and by the end of 2005 it had become widespread throughout London and the surrounding areas.
Rosemary beetle is now widespread in England and has been found in parts of Scotland, Wales, Northern Ireland and the Irish Republic
Thank you to those who took part in the survey from 2008 to 2013. More than 3,700 records of the beetle have been received, greatly adding to our knowledge of this pest.
Research by the RHS indicated that most rosemary beetle adults remain fairly inactive on 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 Advisory Service concern it 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 some other plants in the Lamiaceae family may also act as hosts.
Rosemary beetle can be controlled either with insecticides or hand picking.
Adults and larvae can be collected by placing newspaper or an open upturned umbrella under the branches and tapping the plants dislodging the insects.
If pesticides are used then these are best applied in late summer to early autumn or in the spring, these should not be used while plants are in flower because of the danger to pollinating insects. Provided the instructions regarding harvest interval and other restrictions are followed insecticides approved for use on edible herbs, include Pyrethrum (e.g. Py Spray Garden Pest Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Bayer Sprayday Greenfly Killer) and thiacloprid (e.g. Bayer Provado Ultimate Fruit & Vegetable Bug Killer).
Help us with research
The RHS is keen for the data to be used in further research projects and collaborations. Email us at Advisory_Entomology@RHS.org.uk if you would like to use RHS data for research. Data is already shared with the National Biodiversity Network.
RHS Advice on Rosemary beetle