Advice
RHS Help & Advice
Rosemary beetle (Chrysolina americana)
Despite its Latin name, rosemary beetle originates from southern Europe and has been found in Britain since the mid-1990s.
Plants affected
Mainly rosemary and lavender but also thyme, sage and Perovskia.
Symptoms
The adult beetles are 6-7mm long and shiny insects with metallic purple and green stripes on their wing cases and thorax. The larvae are greyish-white with darker stripes running along their bodies; when fully grown the larvae are 8mm long. Both the adult beetles and the larvae feed on the leaves, which can be reduced to short stumps with greyish-brown discoloration where the damaged tissues have dried up. The flowers can also be damaged.
Biology
Initially rosemary beetle was found mainly in London gardens, but it is rapidly spreading and will become widespread throughout England and Wales, and possibly further north. During mid-summer the pest is present on host plants as adult beetles that do little or no feeding. In late summer they commence feeding, mating and laying eggs. These hatch after about 10 days and both adults and larvae will feed on the foliage throughout autumn to spring during periods of mild weather. When fully fed, the larvae go into the soil to pupate.
Control
Hand picking can help to keep infestations below the level at which serious damage occurs. With the taller forms of rosemary and lavender, the beetles and larvae can be collected by tapping or shaking the branches over newspaper spread underneath the plant.
If the plants are used for culinary purposes, the only pesticides that can be used are thiacloprid (Provado Ultimate Bug Killer Ready To Use) or the organic types, such as pyrethrum (e.g. Py Spray Garden Insect Killer) or rotenone (e.g. Bio Liquid Derris Plus). Organic insecticides may control the young larvae but are unlikely to deal with the adult beetles. On plants grown as non-edible ornamental plants, other synthetic insecticides could be used. These include bifenthrin (e.g. Scotts Bug Clear, Bayer Sprayday Greenfly Killer Plus, Doff All In One Garden Pest Killer) or imidacloprid (Provado Ultimate Bug Killer concentrate). Do not apply insecticides while the plants are in flower.
Andrew Halstead