Why Are My Lavender Leaves Being Eaten by Beetles?

If you are finding distinctive ragged feeding damage on lavender foliage and can spot small, metallic beetles on the plant, you are almost certainly dealing with the rosemary beetle (Chrysolina americana). Despite its common name, this pest attacks multiple plants in the Lamiaceae family, including lavender, rosemary, thyme, sage and marjoram. It arrived in the UK in the late 1990s and has spread widely across England and into Wales, becoming a significant pest of herb gardens. Both adult beetles and their larvae feed on the plant.

Identifying the rosemary beetle

Adult beetles are about 6 to 8 mm long, with striking metallic green and purple longitudinal stripes on the back. The larvae are pale grey with darker stripes along the body and reach about 8 mm when fully grown. Both adults and larvae feed on the foliage, stripping leaves and leaving ragged, partially eaten tissue. Adults are most active from late summer through autumn and into early spring; larvae are most damaging from autumn to late winter as they feed and grow before pupating in the soil.

Hand-picking — the most effective control

Both adults and larvae are easily spotted due to their distinctive appearance. Lay a piece of white paper or cloth under the plant and shake the branches — adults and larvae drop readily when disturbed and can be collected and destroyed. This approach, repeated weekly from August through to March, can dramatically reduce populations with no chemical input. Checking plants after dusk is particularly effective as the beetles are more active in the evening.

Protecting plants with fleece

Where infestations are severe, covering lavender plants with horticultural fleece from August to April physically prevents adults from landing and laying eggs. This is most practical for container-grown plants or smaller bushes; large established lavender hedges are less easily managed this way.

Insecticide options

Where damage is severe and hand-picking is not keeping pace with the population, a pyrethrin-based insecticide applied to affected plants can reduce beetle numbers. Apply in the evening to minimise impact on pollinating insects. Avoid systemic insecticides on lavender plants during the flowering season. Repeat applications are needed as the treatment has no residual effect on newly arriving adults.

The long-term picture

Rosemary beetle populations tend to fluctuate from year to year. Cold winters significantly reduce survival, while mild winters allow populations to build. In gardens where the pest is established, regular vigilance in late summer and autumn — spending a few minutes checking and removing beetles each week — keeps damage to a manageable level without significant chemical use.