I’ve returned to the limestone grassland slopes above Portland Bill to settle a score with bastard-toadflax (Thesium humifusum). Earlier in the week, my eyes more accustomed to scanning vast seascapes for guillemots, razorbills and gannets, I hunted for this semi-parasitic plant among the vetches, bird’s-foot trefoil, yellow wort and eyebright. But though I was in an area where it’s known to grow, bastard-toadflax refused to be found. My error – it turns out – was one of magnitude.
So today my approach is lower and slower. Nose down, bottom up. From this undignified position, it takes only a few minutes to locate the inconspicuous straggler. It’s smaller than I’d imagined. A low-growing perennial with trailing stems and narrow leaves, it scatters white starbursts left and right as it weaves through the vegetation. Through my hand lens, I can see that the 3mm-wide star-flowers enclose five stamens with creamy anthers and a white stigma floating in a green pool – actually a short tube from which the petal-like sepals arise.
Bastard-toadflax’s uncharitable common name is a nod to its supposed similarity to toadflax, though it’s far more closely related to mistletoe, another hemiparasite. But its disparaging title doesn’t bother the insects that feed on this rare plant. Glistening green swollen-thighed beetles sup from the flowers, while underneath the sprawling foliage, tiny black and red blobs trundle back and forth.
Though no bigger than the flowers, under the hand lens the blobs transform into domed bugs with black bands on their blood-red bodies. These are young down shieldbugs (Canthophorus impressus). The adults are dark metallic blue and only half the size of the more common green shieldbug. I glimpse one before it slips beneath the leaves. They are outnumbered by nymphs, though, which go through five instars or juvenile stages, and by late summer will have matured into the next generation of adults.
These monophagous shieldbugs (feeding only on one type of plant) rely on bastard-toadflax, itself mostly restricted to downland in southern England. A specialist insect dependent on a specialist plant. So it’s a privilege to watch this ground-hugging bug and to think that, from a down shieldbug’s point of view, this prostrate plant is an entire world.
• Nic’s new book, Land Beneath the Waves, is out on 12 June
• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com and get a 15% discount
