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, parts of London and Edinburgh. The beetle is of widespread occurrence on the mainland of Europe and it may spread further in Britain.

Blue mint beetle Image: RHS/Science

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 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

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.

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

Blue mint beetle can usually be tolerated, it is unlikely to be numerous enough to cause significant damage to healthy mint plants.

Non-pesticide control

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

Pesticide control

  • 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
  • Do not use pesticides on mint growing in or near water such as ponds and streams
  • Pesticides are likely to be more effective on larvae than adults
  • Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer). Several applications of these short persistence products may be necessary to give good control
  • More persistent contact insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer)
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
  • Follow label instructions when using pesticides
  • 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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