Despite its Latin name of Chrysolina americana, rosemary beetle originates from southern Europe and has been found in Britain since the mid-1990s.
What is rosemary beetle?
Rosemary beetle is a pest that eats the foliage and flowers of various aromatic plants, such as rosemary, lavender, sage and thyme.
Initially rosemary beetle was found mainly in London gardens, but it is rapidly spreading and is becoming widespread throughout England and Wales, and possibly further north.
Both the adult beetles and the larvae feed on the foliage and flowers, with most of the damage occurring between late summer and the spring. Heavily infested plants can look very shabby by March, by which time most of the old foliage will be discoloured as a result of the feeding damage. The plants' appearance will improve as new growth develops in late spring-summer.
- How to spot rosemary beetles on your plants: The adult beetles are shiny insects, 6-7mm long, 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
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 being used for culinary purposes, the only pesticides that can be used are thiacloprid (Provado Ultimate Bug Killer Ready To Use), deltamethrin (Bayer Sprayday Greenfly Killer), lambda-cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer) or the organic types, such as pyrethrum (e.g. Py Spray Garden Insect Killer, Scotts Bug Clear Gun for Fruit & Veg, Doff All in One Bug Spray). Pyrethrum may control the young larvae but is unlikely to deal with the adult beetles. The interval that needs to be left between treatment and using the leaves for cooking purposes is thiacloprid (14 days), deltamethrin and lambda-cyhalothrin (7 days) and pyrethrum (1 day).
On plants grown as non-edible ornamental plants, the spray concentrate form of thiacloprid (Provado Ultimate Bug Killer concentrate) can be used.
Do not apply insecticides while the plants are in flower.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
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. Adult rosemary beetles emerge from pupae in the soil in early summer. There is one generation a year but because the adults are long-lived, there can be some overlap between the new and old generations of adult beetles. Because of this, adult beetles can be found at almost any time of year.