Terry Whittaker / Common knapweed (Centaurea nigra). Chalk cliffs, Kent. July

Terry Whittaker / Common knapweed (Centaurea nigra) on chalk cliffs, Kent. July

Alan Williams / Sweat bee (Halictus rubicundus) feeding on Knapweed (Centaurea nigra) flower in meadow Cheshire, UK, August.

Alan Williams / Rose-spotted plant bug (Calocoris roseomaculatus) on Knapweed (Centaurea nigra) on grassland, North Wales, UK. September.

Nick Upton / Meadow brown butterfly (Maniola jurtina) sunning on Common knapweed (Centaurea nigra) in a perennial wildflower meadow planted on an urban common by Bristol University's Urban Pollinators project, Hor...

Nick Upton / Blue mason bee (Osmia leaiana) nectaring on Common knapweed (Centaurea nigra) in a perennial wildflower meadow planted on an urban common by Bristol University's Urban Pollinators project, Horfield, B...

Nick Upton / Early bumblebee (Bombus pratorum) nectaring on Common knapweed (Centaurea nigra) in a perennial wildflower meadow planted on an urban common by Bristol University's Urban Pollinators project, Horfield...

Nick Upton / Six spot burnet moths (Zygaena filipendulae) mating on Common knapweed (Centaurea nigra) in a perennial wildflower meadow planted on an urban common by Bristol University's Urban Pollinators project,...

Nick Upton / Six spot burnet moths (Zygaena filipendulae) mating on Common knapweed (Centaurea nigra) in a perennial wildflower meadow planted on an urban common by Bristol University's Urban Pollinators project,...

Rod Williams / Thick headed fly (Physocephala rufipes) feeding on Black knapweed (Centaurea nigra) flower, Sutcliffe Park, Eltham, London, England, June.

Rod Williams / Six-spot burnet moth (Zygaena filipendulae) on Black knapweed (Centaurea nigra) flower, Sutcliffe Park Nature Reserve, Eltham, London, England, July.

Alan Williams / Hoverfly (Epistrophe grossulariae) feeding on Knapweed (Centaurea nigra) in meadow, Cheshire, UK, August.