rossett's picture

Differences between Mint and Tansy Beetle?

First of all, My apologies if this is an idiotic posting.

Is there an easy way to distringuish a Tansy from a Mint Beetle as online they look the same to my untrained eye?

Last year when walking along the banks of my Local River I came across a lot of bright iridescent beetles. Never having seem them before I went home and Googled to discover that they were most likely to be mint beetles.

However, the River banks does have a lot of tansy and although some were on the grass and sand banks a lot of the beetles were on the Tansy itself.

(Unfertilized and unsprayed fields exist either side of the river and only occasional organic cow grazing in small areas, most parts of the river bank are untouched and ungrazed) -if such a things matters?

I know that Tansy Beetles are only on the river Ouse so most likely to be Mint Beetles. but for the life of me I couldn't see the difference between the two from the online photos I saw or the descriptions I read.

Is there an easy way to distringuish between the two, or is it simply a matter of grabbing the best picture I can take and posting it for a more expert eye?

(I didn't have a camera last year so couldn't take a picture, but have a fairly fast macro lens now)

Again, apologies if it's a silly question.
Iain

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John Bratton's picture

Mike Cox's PhD key says the

Mike Cox's PhD key says the edge of the pronotum of C. graminis has a slight rim and the elytra are wrinkled between punctures; and C. menthastri (now herbacea) has no rim on the pronotum and the elytra have simple punctuation without wrinkles. David Hubble's draft AIDGAP key says herbacea has the elytral suture the same colour as the rest of the elytra; and in graminis the suture is darker than the rest of the elytra.

I don't have specimens of either so I don't know how well these work.

Don't assume graminis is only on the Ouse. Things get around.

John Bratton