On Tuesday Vanna & I visited West Blean & Thornden Woods in Kent, a lovely ancient woodland nature reserve managed by Kent Wildlife Trust. We were on the hunt for the Heath Fritillary butterfly, Melitaea athalia, a rare butterfly in Britain. We saw our first Heath Fritillaries in France, on a holiday in the Vendée in May 1990, but wanted to see them again, and in Britain, hence our trip to Kent.

The main food-plant of the Heath Fritillary’s caterpillars is Common Cow-wheat, Melampyrum pratense (note 1).

A patch of Common Cow-wheat, Melampyrum pratense in West Blean & Thornden Woods, Kent, June 2021.

Common Cow-wheat, Melampyrum pratense, is a close relative of the showier and much rarer Field Cow-wheat, Melampyrum arvense, that I wrote about in May 2019 after a visit to the Isle of Wight.  It is a member of the family Orobanchaceae and is an annual hemi-parasite: it possesses chlorophyll and makes its own sugars but it also takes some of its nutrients from its host plants (note 2).

Common Cow-wheat is widely distributed in the British Isles (see map), though here in Norfolk it is not particularly common. “A Flora of Norfolk” (1999) says it is found in five sites in the county, a decline from a pre-1978 total of nine sites. Fortunately my nearest colonies (not mapped in the 1999 Flora) are in Norwich, just a short walk from home, in Bluebell Wood, at the west end of Eaton Park.

Common Cow-wheat flowers from May to September, peaking in June or July. It can often carpet the ground where it grows. It was abundant in the woods near Watendlath in the Lake District which we visited in June 2019 and there were large patches of it in West Blean & Thornden Woods, though only patches in open sunny parts of the woods are suitable for Heath Fritillaries.

Melampyrum pratense has two sub-species in the British Isles: pratense and commutatum. The commoner subspecies, pratense, grows in woods, scrub, heaths and moors on well-drained, nutrient-poor acidic soils. Less commonly, subspecies commutatum grows on chalk and limestone, in scrub, hedgerows and deciduous woodland. The Wildflower Finder website has lots of excellent photographs and more details of both subspecies (note 3).

Once it has flowered, Common Cow-wheat sets seed and these are attached to a fleshy structure known as an elaiosome. This is rich in lipids  and proteins and highly attractive to ants, which aid seed dispersal. Seed set and dispersal is vital as Common Cow-wheat is an annual and must reproduce by seed each year. Fortunately self-pollination will allow the plants to set seed when suitable pollinating insects are scarce. Seeds germinate in late autumn or early winter and put on a spurt of growth in spring.

Like Field Cow-wheat, Common Cow-wheat contains aucubin, an iridoid glycoside and high concentrations are found in the seeds. This is slightly toxic and probably makes the plants less attractive to herbivores.

Common Cow-wheat, Melampyrum pratense. Near Watendlath, Cumbria, June 2019.

Not all stories have a happy ending but ours did.

We found our Heath Fritillaries, which were just as delightful as the ones we saw in France over thirty years ago. On both occasions we travelled with our friends Steve & Janice, who I’d like to thank for letting us see this butterfly (and its foodplant) once again.

Common Cow-wheat, Melampyrum pratense, with a Heath Fritillary. West Blean & Thornden Woods, Kent, June 2021

Heath Fritillaries, West Blean & Thornden Woods, Kent. June 2021.

Notes

Note 1 – Heath Fritillary caterpillars will also feed on Ribwort Plantain (Plantago lanceolata), Germander Speedwell (Veronica chamaedrys), and occasionally other speedwells (Veronica spp.). Foxglove (Digitalis purpurea) can be a secondary foodplant, especially on Exmoor. The butterfly also occurs in woods in Devon and Cornwall, on Exmoor and (as a result of re-introductions) in Essex.

Note 2 – Common Cow-wheat seems to parasitise woody plants but there seems to be some doubt as to which ones.

The NatureGate website mentions Pine and Cowberry as hosts in Finland.

A 1963 paper, “Variation in Melanpyrum pratense” by A. J. E. Smith (Watsonia Vol 5(6), pp 336 – 367) suggests Ling (Calluna vulgaris), heaths (Erica) and Bog Myrtle (Myrica gale) as major hosts in Britain, as well as other plants such as Beech (Fagus sylvatica), Rowan (Sorbus aucuparia) and Sessile Oak (Quercus petraea).

But in his paper “Transplanting and sowing seed of common cow-wheat Melampyrum pratense to increase its distribution at Blean Woods RSPB Reserve, Kent, England” (Conservation Evidence (2005) vol 2, pp 41-42) Michael Walter concludes that “the principal host and the host-plant(s) remains at Blean a mystery”.

Note 3 – Another difference is that the uppermost leaves (below the bracts) are 7 to 15 times longer than wide in ssp. pratense and only 3-8 times longer than wide in ssp. commutatum.

Subspecies pratense has rich yellow flowers and grows on acidic soils whereas commutatum has pale yellow flowers and grows on alkaline calcareous soils, mainly in the south.

See the Wildflower Finder website and Clive Stace’s Flora for more information.