RHS warns over blue mint beetle

Call for information on sightings of pest which is widespread on mainland Europe goes out to amateur gardeners

Blue mint beetle (Carol Sheppard)
Blue mint beetle (Carol Sheppard)

The Royal Horticultural Society (RHS) is calling on amateur gardeners to watch out for the latest pest to appear in the UK. The beetle, Chrysolina coerulans or blue mint beetle, was confirmed to be breeding in the country by the charity's entomology department when specimens were sent to them by an RHS member from Kent in July 2011. But as there has only been one reported finding, the charity is keen to find out whether this is an isolated incident and therefore controllable, or whether the insect is more widespread but as yet unreported.

"The beetle is widespread on mainland Europe and the detection of breeding adults in the UK could mean problems for gardeners who grow this herb," says principal scientist in plant health Andrew Halstead. "It is therefore important that we find out if there are any other breeding adults elsewhere in the UK. The adults and the black, soft-bodied larvae both eat the foliage."

The 7mm long blue mint beetle is quite different in colour to our native beetle, the green mint beetle (Chrysolina herbacea) which is shiny, emerald-green. This beetle also eats mint leaves but is generally not a problem because it occurs more frequently on wild mint.

"At the moment our control suggestions are the same both for our native green mint beetle and the new incomer," says Halstead. "If there are only a few then removal of the beetle and larvae by hand works best. If the infestation is more extensive then it may be necessary to apply a pesticide. An organic insecticide, pyrethrum, can be used on mint to control pests. This short-persistence insecticide should deal with young larvae. However, it may not be effective to control the adults and so other insecticides, such as deltamethrin or lambda-cyhalothrin, may have to be considered."

If blue beetles are discovered the charity would like either digital photographs taken and sent to advisory_entomology@rhs.org.uk or live samples posted in stout containers to Advisory Service, RHS Garden Wisley, Woking, Surrey GU23 6QB.

The RHS recommended Pyrethrum products: Py Spray Garden Insect Killer; Doff All in One Bug Spray; Scotts Bug Clear for Fruit & Veg; Westland Earth Matters Insect Control; and Growing Success Fruit & Veg Bug Killer. Deltamethrin is available as Bayer Ultimate Fruit & Vegetable Bug Killer and lambda-cyhalothrin as Westland Plant Rescue Fruit & Vegetable Bug Killer.

For pyrethrum, a minimum of one day should be left between treatment and using the foliage for culinary purposes; for deltamethrin and lambda-cyhalothrin, it is a minimum of seven days.

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