Rosemary Leaf Beetle
On Crops: Rosemary, lavender, sage and thyme
England, central and southern Europe
Strikingly colourful small beetles 6-8 mm in size. The adults have metallic green backs with purplish-bronze stripes, and are usually seen feeding on leaf tips in groups. Larvae are soft-bodied and grey in colour with darker stripes. The beetles are most active in late spring and again in late summer.
Rosemary leaf beetles and their larvae feed on tender stem tips of herbs that usually produce high levels of camphor, causing the new leaves to develop brown shrivelled edges. Feeding is usually not serious enough to kill plants, but new growth is distorted and affected plants appear unhealthy. However, if left untreated plants will often start to die after 2-3 years.
If this pest has been seen in your area, check plants often for signs of trouble. Refresh mulches so they provide habitat for ground beetles, which consume the beetles when they pupate in the ground.
Place a box or towel beneath affected plants and quickly shake them to knock the beetles to the ground. Drown them in a bucket of soapy water. Prune off damaged tips to keep the plants looking neat. Rosemary leaf beetles are active over a long season, so plants should be checked often for re-infestation. Where the infestations are severe, as a last resort organic Pyrethrum-based products are available from garden suppliers. These will need to be applied following the label instructions.