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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
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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.
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.
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.
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
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