A Common Resource - Gower - page 216

The main larval host plant is devil's-bit scabious Succisa pratensis . The females lay their eggs on the
larger scabious plants, typically those growing where the height of the vegetation is between 8 and 20
centimetres. The fritillary is therefore very susceptible to grazing pressure and most colonies occur where
there is light grazing by cattle or horses. Very few occur in areas of the commons grazed by sheep, since
sheeparehighlyselectivefeedersandgrazethefoodplantpreferentially,renderingitsmallandunsuitable
for egg laying. In contrast Welsh ponies are less selective feeders and avoid devil's-bit scabious flower
heads.Populationsfluctuategreatlyinsizefromyeartoyear,withlarvaeoccasionallyreachingenormous
densities. The fluctuations appear to be dependent upon weather, food supply and the proportion of cater-
pillars killed by two parasitic braconid wasps, Cotesia melitaearum and C. bignellii , the latter apparently
being more common in Wales than the former. In some years parasitic wasps can kill 75 per cent of the
larvalpopulation.Itisthoughtthatthewaspscontrolthesizeofmarshfritillarycolonies,preventingthem
from outstripping the supply of food plant. They are thus an integral element in the butterfly's population
dynamics and are themselves of significant conservation value.
Thenationallyscarcedouble-linemoth Mythimna turca hasbeenrecordedfromCefnBryn,Pengwern
and Clyne commons. In Britain the species is now confined to South Wales and southwest England
(Somerset, Devon and Cornwall), where it is chiefly associated with wet grassland. The adult, which flies
in June and July, is reddish-brown in colour with black cross-lines on the forewings. It occurs in wet
grasslands, and in woodland clearings and rides. Little is known about its requirements, although the pale
brown larvae feed at night on grasses, such as cock's-foot, wood meadow-grass Poa nemoralis and field
wood-rush.
Along with the marsh fritillary and double-line can be found some scarce beetles such as the flea
beetle Longitarsus holsaticus , which feeds on louseworts Pedicularis spp., and the metallic-blue jewel
beetle Trachys troglodytes , which also feeds on devil's-bit scabious. The jewel beetle has been recor-
ded from Fairwood Common and Oxwich Point and seems to be more common in coastal areas such as
Gower.Thetussocksandleaflitterofpurplemoor-grassprovideshelterforawidevarietyofgrasslandin-
vertebrates during bad weather. The money spider Glyphesis servulus , which appears to need rank moor-
grass, has been recorded from this habitat on Fairwood Common.