DESCRIPTION: The large oval to elliptical swellings in the stem of the host plant are caused by a picture winged fly. The gall starts off soft and eventually hardens from the centre outwards becoming quite woody. The colour turns from yellowish-green through to brown. Each gall usually has 4 chambers each one with a single larva. The larva pupate in the gall during spring time with the adults emerging in mid-summer when the gall begins to rot down. DISTRIBUTION: Common in southern Britain. HABITAT: Common on Creeping Thistle.
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