Blue mint beetle

The blue mint beetle, Chrysolina coerulans, was detected breeding in the UK for the first time in July 2011, from Kent.  Since 2012 the beetle has been found in Cambridgeshire, Lincolnshire, Hampshire and parts of London. The beetle is of widespread occurrence on the mainland of Europe and it may spread further in the UK. The Royal Horticultural Society wants to hear from anyone who finds this beetle on their mint plants so that 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
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What does it look like?

  • The adult beetle is 7mm long and is a metallic dark blue
  • The larvae have soft rotund bodies that are blackish 
  • Both adults and larvae feed on the foliage of cultivated and wild mints, Mentha spp.
  • It can easily be confused with alder leaf beetle and fuchsia flea beetle, blue mint beetle is only found on mint and is larger than the other two species.  

A blue mint beetle.
A blue mint beetle larvae.
    A blue mint beetle. A blue mint beetle larvae.

    Are there other insects that eat mint leaves?

    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. Some other species of moth caterpillar and slugs and snails  also sometimes eat mint leaves.

    Are there other dark blue beetles?

    There are several other metallic blue beetles found in the UK. They are unlikely to be found on mint. These include willow leaf beetles found on plants willows and poplars, the alder leaf beetle found on alder, and fuchsia flea beetle found on 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@rhs.org.uk, 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@rhs.org.uk at any time.

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

    Light infestations of this insect can be tolerated, it is unusual for this beetle to be numerous enough to cause significant damage to mint plants

    Non-chemical

    • If necessary on small clumps of mint, it is feasible to remove the beetles and their larvae by hand. 

    Chemical

    • If the plants are being used for culinary purposes, herbs must be listed on the product label and the manufacturer’s instructions must be followed, particularly the harvest interval that needs to be left between treatment and using the leaves.
    • Pesticides are likely to be more effective on larvae than adults
    • Heavy infestations which are impractical to remove by hand can be treated with pesticides
    • Organic insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit or Defenders Bug Killer, ecofective Bug Killer (also contains fatty acids)). Several application of these short persistence products may be necessary to give good control
    • Synthetic pyrethroid pesticides such as lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), or deltamethrin (e.g. Sprayday Greenfly Killer) can be used
    • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) can also be used 
    • Plants in flower should not be sprayed due to the danger to pollinating insects
    • Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

    Download

    Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

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