Rosemary beetle is a reasonably recent pest in the UK. It attacks the leaves and flowers of rosemary, and lavender, sage and thyme, causing extensive damage.
- Metallic purple and green adult beetles on plants
- Striped greyish-white grubs (larvae) on plants
- Plants stripped of leaves
- Greyish-brown discolouration of the leaves
What are rosemary beetles?
The adult beetles look very attractive with metallic purple and green stripes and measure 6-8mm (around 1/3in) long. They feed on the leaves and flowers for much of the year, with most damage occurring from late summer to spring.
The greyish-white larvae have dark stripes running along their body and measure up to 8mm (around 1/3in) long. They feed on the leaves and flowers from autumn to spring.
In late summer, the adult beetles mate and the females lay their eggs on the plants. When the larvae are fully grown, in spring, they move into the soil to pupate. The adult beetles emerge in early summer. The adult beetles are long lived and can be found on plants nearly all year round.
The first rosemary beetles were recorded in the UK in the 1990s in London, but they have spread rapidly to other parts of the UK since.
What do they affect?
- Rosemary (Rosmarinus)
- Lavender (Lavandula)
- Sage (Salvia)
- Thyme (Thymus)
What do they do?
The larvae do most of the damage, feeding on the leaves and flowers from autumn to spring. The adult beetles also feed on leaves and flowers, although their damage is less extensive, and their most extensive feeding is carried out during late summer.
Affected leaves can be reduced to short stumps with a greyish-brown discoloration, caused by the leaves drying up.
How to control rosemary beetles
Check susceptible plants regularly throughout the year, looking for both the adults and the larvae, and deal with them as soon as you see them.
Pick off the beetles and larvae by hand, or prune out shoots and branches where they are feeding.
You can collect both the larvae and the adult beetles by spreading polythene sheets or newspaper underneath the plant and shaking or tapping the branches.
Careful hoeing around the plants in spring, so as not to cause any damage to the roots, may kill the pupae, or expose them to foraging birds and other animals.
There are numerous chemical insecticides available that will kill soft-bodied insect pests, such as rosemary beetle larvae. Just bear in mind that the majority of these are what is known as “contact” insecticides. That is, you have to spray the actual insect (make contact with it) to kill it. If you miss some, these will not be affected. Also, most are based on “organic” or “naturally-occurring” materials and have very little persistence once sprayed on to the plant.
Insecticides used on plants used for culinary purposes, must have approval for use on edible plants. Check the label, and follow the manufacturer’s instruction, before spraying.
A systemic insecticide (BugClear Ultra) – which also kills on contact – is absorbed by the leaves and moves all around inside the plant, so can kill the larvae and adult beetles as soon as they start to eat the foliage. This does not have approval for use on susceptible plants that are used for culinary purposes.
Use plant protection products safely. Always read the label and product information before use. Plants in flower should not be sprayed due to the danger to pollinating insects. Either spray early in the morning or late in the evening when pollinating insects are less likely to be active.
There is nothing you can do to prevent rosemary beetle attacking its host plants.