Several of these are used in biological control of invasive knapweeds and starthistles. They are strongly attracted to purple flowers such as thistles and knapweeds. Adults feed on the nectar of bramble, thistles, and knapweeds, and also on aphid honeydew. The males, on the contrary, visits clovers, green alkanets, hound's-tongues, knapweeds, and many others. Several species are used in biological control of invasive weeds, namely knapweeds ("Centaurea").