Research

Rosemary leaf beetle

Introduction and description

The rosemary beetle ( Chrysolina americana ) is an attractive 8mm (1/4in) long metallic green beetle with purple stripes. Despite its scientific name, it is a native of southern Europe. During the last decade it has become an established pest in Britain on rosemary and lavender and related plants.

The soft-bodied grubs are greyish white with five darker longitudinal lines; fully grown larvae are 5-8mm (about 1/4in) long. Sausage-shaped eggs, 2mm (1/8in) long, may be found on the underside of the leaves during early autumn to spring.

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 a garden in Weybridge, Surrey. By the end of 2005 the beetle had become widespread throughout London and surrounding areas, and had become a top 10 most frequent pest enquiry to the RHS Members’ Advisory Service. Outside of the south east, the beetle is established in Norwich and specimens have been found in other areas of the including Edinburgh, Pembrokeshire and southeast Yorkshire.

Click here to submit a record of Rosemary beetle

Distribution of Rosemary beetle in Britain. From Records held by the RHS (at January 2008). Produced using DMAP©

Life cycle

Preliminary research at the Entomology laboratory at Wisley Garden indicated 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 ) or lavender ( Lavandula species). However, the beetle is able to breed on thyme ( Thymus species), sage ( Salvia species), Russian sage ( Perovskia atriplicifolia ), and it is likely that other plants in the Lamiaceae plant family may also act as hosts.

Control

Rosemary beetle can be controlled either with insecticides or hand picking of adults and larvae. The adults and larvae can be collected by placing a newspaper or open upturned umbrella under the branches and tapping them to dislodge the insects. 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, thiacloprid or imidacloprid. Insecticides should not be used while plants are in flower because of the danger to bees. Only the ready-to-use formulation of thiacloprid (Provado Ultimate Bug Killer Ready to Use) is approved for use on edible herbs. For further advice see the rosemary beetle help and advice profile.

Publications

Halstead, A J. 1996. Possible breeding by the rosemary beetle, Chrysolina americana (L.), in Britain. British Journal of Entomology and Natural History 9: 107-108

Mabbott, P., and A. Salisbury. 2006. The establishment of the Rosemary beetle Chrysolina americana (L.) in London, 1998-2005. The London Naturalist 85 : 163-166

Salisbury, A. 2002. The Rosemary beetle, Chrysolina americana (L.) (Col., Chrysomelidae) in Britain. Entomologists Monthly Magazine 138 : 77-80

Salisbury, A. 2003. Further records of Chrysolina americana (Linnaeus) (Chrysomelidae) in Britain. The Coleopterist 12 (2): 61-62