Understanding and Managing the Rosemary Leaf Beetle in Britain

The largest order of insects on Earth is Coleoptera, the beetles, of which there are over 300,000 species recorded so far. Here in Britain, around 4,000 species have been recorded, with ladybirds being one of the most easily recognised groups. 

Britain’s beetles encompass a very diverse range of species with not only many unique shapes and biological characteristics, but a vast array of colours.  This includes a few that display iridescent metallic colours on their wing cases, giving them the appearance of living jewels.

One of these jewel-like beetles is called the Rosemary Leaf Beetle (Chrysolina americana), a beetle that’s often seen in many gardens across Britain. Easily recognised by the metallic green and purple stripes on their wing cases, the beetles will shimmer in the sunlight as they clasp onto the stems of their camphor-rich host plants, namely the aromatic herbs Rosemary, Lavender, Sage and Thyme, where they consume the foliage and flowers.

Often close by will be the beetle’s much less vibrant, light and dark, grey-striped larvae which will also be feeding on the leaves of the host plants.  Despite the current abundance of Rosemary Leaf beetles across Britain, they are an invasive species that originate from Southern Europe and were first recorded here in the mid-90s on a Rosemary plant in central London.

As they progressively dispersed over the following years, they became widespread throughout Britain and can often be seen all year on their host plants, only leaving them to shelter underground during the coldest days of winter.

Being such a visually attractive insect, many people will probably enjoy seeing these beetles in their garden and most likely will tolerate them feeding on their plants. However, it’s important to know that despite their beauty, Rosemary Leaf beetles can unfortunately cause harm to the plants that they infest by destroying the plant’s growing tips and inducing die-back. And so, Rosemary Leaf beetles are usually regarded as plant pests when they infest commercial herb crops such as lavender and rosemary and are often eradicated with an appropriate insecticide.

How to control the Rosemary Leaf beetle?

Although Rosemary Leaf beetles could also be controlled with chemical-based insecticides in a home garden, it is not advisable to do so nowadays in light of Britain’s rapidly declining biodiversity. Since most chemical insecticides, (which include those certified as organic), will be toxic to a wide range of beneficial insects, such as the pollinator and predatory species. 

Alternatively, a simple environmentally friendly ‘physical’ method could be used to reduce Rosemary Leaf beetle numbers. Either picking the beetles and their larvae off the plants by hand, or by placing an old bed sheet around the base of the infested plants, and vigorously shaking the plant.  The adult beetles and their grubs are easily dislodged by the shaking, falling onto the sheet where they can be collected and removed.