The Nursery Inside the Thistle

Imogen looks at a unique habitat for the Urophora stylata

Not all of nature’s stories shout for attention. Some hum along quietly, half-hidden among the thorns and stems of a summer garden.

Female Urophora stylata with its long black ovipositer

The spear thistle Cirsium vulgare (aka common thistle or on the US often called the bull thistle) is a plant most people recognise and to be fair they usually avoid. It’s tall, coarse, and seems to have spines on every surface. The leaves are long and narrow. Each leaf has jagged, forward-pointing segments that give it a spear-like silhouette and presumably the provience of it’s name. Along the edges of the leaves and on the prominent midrib beneath, sharp yellowish spines jut out, making even a casual brush a memorable experience!

A Hefty specimin - Spear thistle

The surface is rough and bristly, with a slightly hairy or webbed texture, especially on younger growth. Leaves grow alternately up the stem, their base often forming winged spines that continue down along the stem itself, giving the whole plant this rather heavily fortified appearance if its been allowed to grow its full height and girth.

However this native plant that we might avoid, is loved by many of our native Highland insects. It’s flowerhead is made up of hundreds of tiny tubular florets packed tightly together, creating a soft-looking dome that’s simply irresistible to bees, butterflies, and hoverflies. In fact their blooms are among the top nectar producing wildflowers in Scotland!

One insect that loves the thistle more than most is the spear thistle gall fly (Urophora stylata). Its a pretty small insect and you’re unlikely to notice it unless you know exactly what to look for. Their wings are banded black on a surface like smoke-stained glass and their bodies barely the size of a grain of rice.

It has evolved to live (almost) entirely on this one species of thistle. The adults emerge in early summer May into June flying short distances, feeding on nectar, and searching for mates. I saw them today, in early July, mating on the spiny crown of a spear thistle in the garden. And on researching online, I saw that mating usually happens directly on the flowerheads of this plant. Once mated, the female gets to work. She probes the developing flowerheads with her slender ovipositor, laying her eggs into the soft tissue beneath.

Photo credit: Bj.schoenmakers - Mating Urophora stylata

The Gall Nursery Begins

When the larvae hatches, they begin feeding on the tissues at the base of the seed head. This activity prompts the thistle to form a gall (a small, hardened swelling) that actually shelters the developing insect from predators. From the outside, the flower may look only slightly unusual, perhaps firmer or oddly shaped, but inside, the larvae continues to feed through the summer. They then pupate within the gall and overwinter inside it, safe from frost and predators. The following year, they will emerge as adults to begin the cycle anew. One healthy plant might reasonably support 50-200 larvae.

In areas where the spear thistle has becomes overly dominent and in parts of the world where it’s not native, this tiny fly has actually been introduced as a form of biological control. It doesn’t destroy thistle plants outright, but by reducing their seed production capability, it helps to limit their spread and because Urophora stylata is a uniquely adapted species to this specific plant host, it poses no threat to other native wildflowers.

The fly itself isn’t immune from predation. Parasitic wasps may invade the gall, and birds or small mammals can sometimes prise them open in search of a meal. Like most of nature’s specialists, the spear thistle gall fly exists in a web of connected relationships.

So next time you pass a spear thistle, out on your travels, in a field corner, or sprouting from the edge of a garden path - take a moment to look closer. Is one flowerhead unusually solid or bulbous? You may be looking at the home of a gall fly larva. You might even spot the adult fly itself with its wings patterned like stained glass with the distinctive two black stripes resting on a prickly perch.

In gardens and even wild margins it’s tempting to treat thistles as unwanted weeds. The flies chances of survival are greatly reduced if the flowerheads are exposed to wet ground, fungal decay, or disturbance. Leaving a few to stand, especially into late summer, autumn and overwinter offers shelter and continuity for creatures like this fly next spring (and is a natural weed suppressant).

If you are interested in other bugs that make the spear thistle their home (including a different gall fly the Terellia serratulae) I recommend futher reading ‘Life on a spear thistle’ blog by Alan Watson Featherstone, who also has some splendid photographs.

All plants have roles to play beyond our tidy expectations and provide habitat for a whole host of creatures. Letting a few flower and fade on their own terms allows the hidden lives inside them to run their course.

Imogen Furlong, the High Life Highland Countryside Ranger Manager, is known for her enthusiasm, organisation, and drive. Her deep passion for the Scottish Highlands' wildlife and extensive experience in outdoor education and recreation management have been central to this role. Imogen is a practical and collaborative project manager, empowering her teams to work effectively in communities and contribute to local conservation efforts.