The adult Tansy beetle is approximately 10mm long with a bright iridescent green colouration. The female Tansy beetle is generally larger than the male. The elytra show marked pitting which is also present, though less pronounced, on the thorax.
Care must be taken in identifying the Tansy beetle (Chrysolina graminis) as it can be confused with the similar sized Mint beetle (Chrysolina herbacea). Two key characteristics can be used to separate the species: –
- The lip, or lateral margin, of the elytra runs the full length of the elytra in the Tansy beetle but only occurs along the front half in the Mint beetle (see diagram above).
- The pitting on the thorax (pronotum) is less pronounced than on the elytra in the Tansy beetle. In contrast both the pronotum and elytra of the Mint beetle have a similar depth of pitting.
It must be stressed that the Mint beetle does not occur in northern England.
Confusion can also arise with the Dock beetle (Gastrophysa viridula) as this also has an iridescent green colouration. However, the Dock beetle is much smaller than the Tansy beetle, only 4mm in males and 7mm in females. In addition this beetle is normally confined to dock plants which it feeds upon.