Blue mint beetle

The blue mint beetle, Chrysolina coerulans, was detected breeding in the UK for the first time in July 2011. The foliage on mint plants in a garden at Newenden, Kent was being eaten by the adults and larvae of this insect. In September 2012 the beetle was confirmed as breeding in several locations in the vicinity of Ely, Cambridgeshire. The beetle is of widespread occurrence on the mainland of Europe and it may now spread and become a problem in the UK. The Royal Horticultural Society wants to hear from anyone who finds this beetle on their mint plants so the extent of the infestation can be assessed and its spread monitored.

Blue mint beetle Image: RHS/Science

Quick facts

Common name Blue mint beetle
Scientific name Chrysolina coerulans
Plants affected Mint (Mentha spp)
Main symptoms Holes in foliage, beetles on leaves
Most active Spring to autumn

What does it look like?

  • The adult beetle is 7mm long and is a shiny dark blue colour. 
  • The larvae have soft rotund bodies that are blackish in colour. 
  • Both adults and larvae feed on the foliage of cultivated and wild mints, Mentha spp.

    A blue mint beetle larvae.A blue mint beetle.

    Are there other insects that eat mint leaves?

    Yes. There is another beetle native to Britain, known as the green mint beetle, Chrysolina herbacea. This is 8-9mm in length and is a shiny emerald green. The black larvae are similar to those of the blue mint beetle. Pale green caterpillars of several moths, Pyrausta spp. loosely spin together foliage at the shoot tips with silk threads and eat the leaves.

    Are there other dark blue beetles?

    Yes. They are likely to be seen on plants such as willows, poplars, alder, fuchsia, Zauschneria and wildflowers known as willowherbs, Epilobium spp. These other blue beetles are mostly less than 7mm in length. The leaf beetle that occurs on alder is about 7mm long. A rare blue form of the green mint beetle (see above) sometimes occurs.

    How can I be involved in the blue mint beetle survey?

    Check your mint plants for holes in the leaves and search the plants for beetles. Correct identification of the beetle is important, so please send either a digital image of the beetle to gardeningadvice@, or bring or post samples of the insect to Gardening Advice, RHS Garden, Wisley, Woking, Surrey, GU23 6QB. The samples should be sent alive in a stout container from which they will not escape while in the post. Always provide information on the type of plant on which the beetle was seen and the address and postcode of the garden where it was found.

    Data protection: The RHS will primarily use the information you provide for recording the distribution of the blue mint beetle. However, the RHS and its subsidiaries may be in touch with special offers relating to products, services and fundraising. We'll never sell your data to any third party and all emails will include an unsubscribe option. If you would prefer we don't keep in touch, you can email membership@ at any time.

    If blue mint beetle is a problem, how can I control it?

    On small clumps of mint, it is feasible to remove the beetles and their larvae by hand. 

    For more extensive infestations, it may be necessary to apply a pesticide. An organic insecticide, pyrethrum, (Py Spray Garden Insect Killer, Scotts Bug Clear Gun for Fruit & Veg, Doff All in One Bug Spray, or Growing Success Fruit & Veg Bug Killer) can be used on mint to control pests. This short persistence pesticide should deal with young larvae but may be less effective against the adult beetles. Other pesticides approved for use on outdoor edible herbs are lambda-cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer) and deltamethrin (Bayer Ultimate Fruit & Vegetable Bug Killer). The minimum period that needs to be left between treatment and using the leaves is one day for pyrethrum and seven days for lambda-cyhalothrin and deltamethrin.

    Advertise here

    We love free entry to our local RHS garden

    Lucy, mum, part-time lectureer & RHS member

    Become a member

    Discuss this

    for the site or to share your experiences on this topic and seek advice from our community of gardeners.