The sole caterpillar foodplant of the Northern Brown Argus
Common Rock-rose (Helianthemum nummularium) is not as its name suggests a rose, but a low-growing evergreen shrub that in summer, carpets in golden yellow the steep open bare slopes where it can get the sun. The five crinkly rose-like petals open in the sun revealing a source of nectar for day-flying lepidoptera, but close at night.
There are many garden varieties but it's always best to go for the native species, as others may lack the special qualities that attract native invertebrates. Also it should not be confused with other completely unrelated plants also called Rock Rose.
Rock-rose is the only known foodplant of caterpillars of the NORTHERN BROWN ARGUS, a small butterfly with attractive velvety brown upper wings, and a white spot on each forewing, which lives in Scotland and the North of England. Its dependency on Rock Rose means that without it there will be no Northern Brown Argus. It is rare enough to be designated a UK Biodiversity Action Plan Species.
On limestone soils it is also the main food plant of the BROWN ARGUS, found in England and Wales, a close relative of its northerly relative.
Rock-rose is also a significant foodplant (along with Birds-foot Trefoil) of the SILVER-STUDDED BLUE found in southern England, and is one of the options for the GREEN HAIRSTREAK. It is also eaten by the caterpillars of several species of moth including those of the attractive WOOD TIGER MOTH, a partly day-flying species.
Attractive, easy to grow and providing a useful supply of nectar, it is a welcome addition to any garden.
INCREASE BIODIVERSITY BY GROWING ROCK-ROSE TO SUPPORT THE BUTTERFLIES (AND MOTH) BELOW
BROWN ARGUS Aricia agestis
Rock-rose is the main foodplant on limestone soils of the Brown Argus.
(The image appears to be of the Brown Argus but is not confirmed).
NORTHERN BROWN ARGUS Aricia artaxerxes Image to come
Fastidious in its food requirements, its caterpillars must have Rock-rose. In appearance it is very similar to its southerly relative the Brown Argus, but with a prominent tiny white spot on each forewing.
GREEN HAIRSTREAK Callophrys rubi
Rock-rose is one of the fairly large selection of plants eaten by Green Hairstreak caterpillars, especially those living on chalk and limestone downs.
A tiny green butterfly it is found in small scattered colonies throughout Britain and Ireland.
WOOD TIGER MOTH Parasemia plantaginis
The male Wood Tiger Moth is often seen flying during the day, although the females fly mainly at night.
Its caterpillars feed on an exceptionally wide range of plants but Rock-rose when available, is one of the best known.
Interestingly, the Wood Tiger Moth doesn't drink nectar from the Rock Rose. In fact it doesn't feed at all as an adult, but depends on food reserves laid down as a caterpillar. A well-fed caterpillar is clearly essential for a healthy adult with the energy to find a mate and lay eggs if it is to give rise to a new generation.
This starvation diet is a feature of the adults of several of our native species including our huge Emperor Moth, whose caterpillars eat a variety of tree leaves.
CISTUS FORESTER MOTH (Adscita geryon) Another day-flying moth for which Rock-rose is a significant caterpillar food plant is the iridescent green Cistus Forester Moth found in England and North Wales. The adult feeds on nectar.