Rosemary beetle
Chrysolina americana
A small metallic green-and-purple striped beetle that grazes rosemary, lavender, sage, and thyme. Shake branches over a tray to collect adults and larvae.
How to know it
A jewel of a beetle, 7–8 mm, in metallic green with purple stripes running down the wing cases; unmistakable once you have knelt down and looked. The gray-white grubs, marked with dark stripes, feed alongside the adults on the shoot tips.
What you'll see
- Notched, ragged shoot tips on rosemary, lavender and sage from late summer.
- Metallic striped beetles sitting openly on the foliage, especially in autumn sun.
- Pale grubs curled among the leaves at the ends of shoots.
- Thinning, scruffy growth on old woody herbs that were fine the year before.
- Fine droppings caught in the foliage below the feeding.
Where it comes from
Adults are active from late summer through autumn and again in spring, with a summer lull. Eggs are laid on the shoot tips from late summer, larvae feed through autumn and mild winter spells, then pupate in the soil. Effectively one long generation, active across the cool half of the year.
A recent arrival in northern Europe that has spread rapidly. Worst on old, congested, unpruned herbs in warm sheltered gardens; young vigorous plants take it lightly.
Dealing with it
- Spread a sheet or a tray under the plant and shake the branches hard — adults and grubs both fall straight off.
- Pick by hand on autumn afternoons, when the beetles bask openly on the foliage.
- Clip and bin the worst-grazed shoot tips, which removes eggs and grubs with them.
- Do not spray a herb you intend to cook with, and never one in flower with bees on it.
- Repeat the shaking every couple of weeks in autumn rather than doing it once.
Keeping it out
- Prune rosemary and lavender annually after flowering to keep the plants young, open and easy to inspect.
- Replace woody, leggy lavenders every few years — old plants carry the heaviest populations.
- Check the foliage of bought herbs, which is how the beetle usually arrives in a garden.
- Grow herbs in an airy sunny spot rather than a sheltered congested corner.
- Look in autumn, not summer: the mid-season lull hides the population.
What it goes for
Rosemary, lavender, sage, thyme and other woody Mediterranean herbs.
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com