Elachista tetragonella is a moth of the family Elachistidae. It is found from Fennoscandia to Spain and Italy and from France to Bulgaria.
The wingspan is . Adults are on wing in January and again in June.
The larvae feed on Carex montana. They mine the leaves of their host plant. The mine has the form of a white elongate gallery in the top region of the leaf. The frass is deposited irregularly. Pupation takes place outside of the mine. They are dirty yellow with a pair of grey dorsal length lines and a yellowish head. Larvae can be found from April to the end of May.
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