The rosemary beetle (Chrysolina americana) is an attractive 8mm long metallic green beetle with purple stripes. Despite its scientific name, it is a native of southern Europe that has recently become established in Britain, probably imported with plants.

The grubs are greyish white with five darker longitudinal lines; fully grown larvae are 5-8mm long. The 2mm long sausage-shaped eggs are laid on the underside of the leaves. Both the adults and the larvae feed on the leaves and flowers of rosemary (Rosmarinus officinalis), lavender (Lavandula spp.) and thyme (Thymus spp.).

Distribution in Britain

The rosemary beetle was first seen living out of doors in the UK at RHS Garden Wisley, Surrey, in 1994 but this population died out and it was not seen again at Wisley until 2003. By 1998 established colonies of the beetle had been reported by entomologists from other sites in south east England, including near London’s Waterloo Station and Winnersh near Reading, Berkshire. In 1999 the first enquiry concerning this beetle was received by the RHS Members’ Advisory Service, from near Weybridge, Surrey. By the end of 2003 the beetle had become widespread in London and East Surrey. Outside of the south east, the beetle is established in Norwich and a few adult specimens have been found in other areas of the country including Leicestershire and south east Yorkshire. For a map click here

Life cycle

A laboratory culture of the rosemary beetle has been established in the entomology laboratory at Wisley where its life cycle is under investigation. Preliminary results of this research indicate that rosemary beetle adults remain inactive on their host plants during the summer months (June to August). In late August and September the beetles resume feeding, mate and begin to lay eggs, which they continue to do on warm winter days until spring. The eggs hatch within two weeks and the larvae feed for approximately three weeks before entering the soil to pupate. The pupal stage lasts for a further two to three weeks before adults emerge.

Host range

Most enquiries on rosemary beetle received by the RHS Members' Advisory Service concern the beetle as a pest on rosemary (Rosmarinus officinalis), lavender (Lavandula species) and thyme (Thymus species). Attempts have been made to rear the beetle on other labiate plants and one additional host has been found: sage (Salvia officinalis). It is possible that there may be additional host plants in the  Lamiaceae plant family.

Control

Rosemary beetle can be controlled either with insecticides or hand picking of the adults and larvae. There are no recommended insecticides that can be used on rosemary if they are to be used for culinary purposes. If pesticides are used then these are best applied in late summer to early autumn or in the spring, when the beetles and larvae are active on the plants. Suitable insecticides that can be used include bifenthrin or imidacloprid. Insecticides should not be used while plants are in flower because of the danger to bees.