Rosemary Beetle is a minor pest that affects rosemary, lavender and sage.
What does it look like?
Adults are shiny, 6 to 7mm long (8mm in Gardeners' World), with metallic purple and green stripes on the wing cases and thorax. Larvae are greyish-white with darker stripes along the body, 8mm long when fully grown. The eggs are tiny and elongated, laid beneath the leaves from September.
The hosts are rosemary, lavender, sage, thyme and some related plants, including Russian sage.
What causes it
This beetle has no true off-season. The active window runs from August to April, adults are present throughout the year, and egg-laying continues on warm days right through winter.
Is this pest active now?
August to April is the window to check plants, although adult beetles are present throughout the year. Spotting an adult is the cue to look under the leaves for the greyish, dark-striped larvae too, since the larvae feed from autumn to spring.
Symptoms
Foliage can be reduced to short, greyish-brown stumps where feeding has dried out the damaged tissue, and holes appear in leaves. Flowers can be eaten too. In most cases the plant's overall health isn't affected.
Life cycle
One generation a year, but adults are long-lived enough that old and new generations overlap, so adult beetles can turn up at almost any time of year rather than in one clear window.
Why this matters in Scotland
Rosemary beetle was first found breeding in central London in the late 1990s and is now established in Scotland.
It is established in Scotland and present in Northern Ireland, and widespread in England and Wales (RHS). No Scottish figures for how common it is were found.
Controls: what works
Not every method here works equally well: some are backed by real evidence, and one has been tested and shown not to help despite how often it gets repeated.
Hand-picking: pick off adults and larvae, and shake or tap branches over paper to collect them
Pick off the adults and larvae and destroy them, and tap or shake taller branches over newspaper to collect them. Check plants regularly through the active window from August, including on mild days in winter.
Check container-grown rosemary, lavender, sage and thyme as closely as plants in a border.
Encourage natural predators: birds, frogs and predatory ground beetles
Birds, frogs and predatory ground beetles eat the larvae and sometimes the adults.
Tolerate the beetle as part of garden biodiversity
Treat the beetle as a colourful addition to the garden: healthy plants usually survive it, so accept it rather than working to remove it.
Chemical: thiacloprid insecticide spray (no longer approved)
Gardeners' World names thiacloprid, a neonicotinoid, for use from late summer to spring, with a warning not to spray flowering plants because bees may be killed. Its approval was withdrawn from February 2020 (EU decision, followed in Great Britain), so it is not an option, and hand-picking is the practical control.
Check the label of any stored product, and the HSE register, before using any insecticide on flowering herbs.
Myths about rosemary beetle
“Rosemary beetle only attacks rosemary, so other herbs nearby are safe”
The name is misleading. The beetle feeds on lavender, sage and thyme as well as rosemary, not on rosemary alone. A gardener who checks only the rosemary bush and assumes the lavender next to it is out of danger is working from the wrong picture.
Our read Rosemary, lavender, sage and thyme are all hosts, and rosemary isn't singled out as the primary or preferred plant.
What to do: Check lavender, sage and thyme for the same metallic purple-and-green beetles and greyish larvae whenever checking rosemary, especially where the plants are grown close together.
1 source
- RHSThe beetle feeds on rosemary, lavender, sage, thyme and some other related plants, not rosemary exclusively.
“Rosemary beetle is a summer pest, so it's safe to stop checking plants once autumn arrives”
This one runs backwards from most garden pests. Adults are most active between late summer and spring, and larvae feed through autumn and winter during mild spells, while high summer is a quieter period for feeding. Stopping checks in autumn removes the plant just as its most active season is starting.
Our read The beetle is active from August to April, with larvae feeding through autumn to spring, which puts the quiet period in summer rather than winter.
What to do: Keep checking rosemary, lavender, sage and thyme from August through to spring, on any mild day, rather than treating autumn as the point to stop looking.
In Scotland: The beetle is established in Scotland (RHS). No Scottish figures were found for how long the active season runs.
1 source
- RHSAdults are most active between late summer and spring, and larvae feed throughout autumn to spring during mild weather.
“Hand-picking rosemary beetle keeps numbers down, it's not just a token gesture”
The RHS, Gardeners' World and Garden Organic all recommend hand-picking as the practical control. Picking beetles and larvae off by hand, or tapping branches over paper to collect them, is described as keeping numbers below the level where serious damage occurs.
Our read Hand removal can keep beetle numbers below the level at which serious damage occurs, and it's the main recommended action through the active season.
What to do: Check rosemary, lavender, sage and thyme regularly from August into any mild day through winter and spring, picking off adults and larvae or tapping branches over paper to collect them, and destroy what's collected.
2 sources
- RHSRemoving beetles by hand where practical can help keep numbers below the level at which serious damage occurs.
- Garden OrganicRecommends picking off and destroying rosemary leaf beetle by hand as the main action through August and into autumn.
“Rosemary beetle is a new arrival in the UK, not a pest gardeners had to deal with a generation ago”
This one holds up. The beetle originates from southern Europe and was first found breeding in central London in the late 1990s, then spread to become widespread across England and Wales and established in Scotland within a few decades. A gardener who planted rosemary in the 1980s wouldn't have met this pest at the time.
Our read The RHS dates the beetle's UK establishment to breeding first recorded in central London in the late 1990s, and it is now widespread in England and Wales and established in Scotland.
What to do: Treat rosemary beetle as an established Scottish pest now rather than an occasional stray, since the timeline shows steady spread rather than a one-off sighting.
1 source
- RHSThe beetle originates from southern Europe, was first found breeding in central London in the late 1990s, and is now established in Scotland.
Frequently asked questions
What should I do first if I find rosemary beetle in my garden?
The first thing to try: Hand-picking. Then: Encourage natural predators.
Will rosemary beetle kill my plants?
Healthy plants usually survive it. Leaves can be reduced to short greyish-brown stumps, holes appear in the leaves and flowers can be eaten, but overall plant health is usually unaffected.
When should I watch for rosemary beetle?
August to April is the window to check plants, although adult beetles are present throughout the year. Spotting an adult is the cue to look under the leaves for the greyish, dark-striped larvae too, since the larvae feed from autumn to spring.
Should I just leave rosemary beetle alone?
Mostly, yes: the beetle can be treated as a colourful addition to the garden. Hand-pick adults and larvae if you wish, and encourage birds, frogs and ground beetles.
Sources
Show the 3 sources
- RHS1: adult and larva description, damage, host plants, treating the beetle as a colourful addition, one generation a year with long-lived adults, eggs from late summer, larvae autumn to spring, pupating in soil, adults emerging in early summer, active August to April, established in Scotland, tapping branches over newspaper, predators
- Gardener's World12: 8mm adults, holes in leaves and elongated eggs, eggs from September hatching after a couple of weeks, egg-laying through winter on warm days, hand-picking, the bee warning on thiacloprid (the product is no longer approved)
- Garden Organic8: pick off and destroy rosemary leaf beetle; active from August until the autumn