A native of southern Europe, this beetle was first discovered in the UK in 1994 at RHS Wisley. Since then, it has established throughout southern England and is now reported from most English counties.
Rosemary Beetle is less prevalent in northern and south west England, Northern Ireland, Scotland and Wales. The adults are not very good fliers, so the movement of infected plant material is the main way this pest spreads.
The 6-8mm long adult beetle is unmistakable! The larvae are like small greyish-white slugs with darker stripes and are 8mm in length. Both the the larvae and adults eat the leaves and flowers of members of the mint family such as rosemary, lavender, sage and thyme. A severe infestation can strip a plant of all its leaves.
Plant First Aid
Give the plant a vigorous shake to dislodge the beetles into a suitable container (some recommend an upturned umbrella). Please dispose of the beetles humanely. Squash any grubs. Prune out badly damaged areas of the rosemary and feed the plant to promote regrowth.
The only effective control is Homo hortus - the gardener!
In our opinion, there are no totally effective (or safe) sprays. Manual control is best. If a plant is badly damaged the cost of a replacement plant may be less expensive than buying a chemical spray.
If you must spray, then do so only when there is no risk of the spray affecting beneficial or pollinating insects. For example, late evening or when the plant is not in flower (ideally both).