Natur am Byth Webinar 2: Rare Insects in North Wales

This time we’ll investigate two charismatic species from North Wales: the cliff-dwelling Large Mason Bee, and the wetland-loving Clubbed General Soldierfly. We’ll reveal why these attractive species are rare, and what makes them unique.

Learn about how and where they can be found and recognised, how they are connected to people, and what is being done to help save them from extinction.

The species featured include:

Clubbed General Soldierfly (Stratiomys chamaeleon)
Large / Cliff Mason Bee (Osmia xanthomelana)

Presenter: Thom Dallimore

What makes an insect rare? There are several ways in which we can consider an insect rare, they are generally more abundant than mammals, and are more mobile than plants, so being able to understand how at risk a species is can be a complex challenge. With some exceptions, we need to know the number of sites. The Great Britain Rarity System looks at how many 10 km2 squares are occupied by a species. Nationally Rare species are believed to be found in only 1 – 15 of these hectads. Two species are the Clubbed General Soldierfly, and the Cliff Mason Bee.

The Clubbed General Soldierfly (Stratiomys chamaeleon) is a large yellow and black fly (Diptera) that is an impressive wasp mimic. Superficially, the appearance of soldierflies, the Stratiomyidae, is similar to hoverflies and the two can be easily confused. Soldierflies can be identified by their particularly flat looking bodies. When resting they hold their wings overlapping tightly over the abdomen, and often have long antennae. Hoverflies tend to sit with their wings open, have more rounded elongate bodies, and short antennae.

The Clubbed General is distinctive within the genus Stratiomys in having two long spines on the scutellum (area of body between the wings) and a long first antennal segment. This species can be identified by its yellow tibia and yellow ‘club’ shaped pattern on the abdomen. This species is particular in its habitat choices, often preferring tufa rich calcareous flushes near umbelliferous plants. Suitable habitat has now become rare, and this species is reduced to a handful of sites on Anglesey and Oxfordshire.

The larvae are surprisingly large and shaped like a baseball bat and can sometimes be seen moving through open vegetation on the margins of slow-moving flushes. It can take up to 3 years for them to mature. Adults are active in late June to early September, and can be found feeding on nearby umbellifers. Long-term data tells us that this species has been through a serious decline, and the Natur am Byth project aims to improve the condition of the available habitat. The Cliff Mason Bee occupies only a single hectad and is one of the UK’s rarest insects. The behaviour of this species is typical of many masonry bees; it uses mud and plant mastic to construct nests and more specifically utilises exposed sandy cliffs with overhanging vegetation near to seeping water. It creates a cluster of nest cups using masticated sand, plant material and the available spring water to create a rudimentary cement. These nests are mostly built within burrowed out tunnels, or in spaces between cracked sand near the bases of sparse grass. The adult females feed almost exclusively on species of Lotus (e.g. Bird’s Foot Trefoil), of which there needs to be a ready supply near to suitable nest sites. These conditions have become increasingly rare, and populations have become isolated and eventually lost.

The cliff mason bee is of the genus Osmia, a group of bees with boxy heads, powerful jaws for chewing tough lignin and mineralised material, and a visible pollen brush beneath the abdomen. The male and female look quite different; the female is large (12 – 13 mm) with a thorax and abdomen of brown-orange hairs and a dark clypeus (area between the eyes) as well as a shiny propedal triangle between the wings. The males are serval millimetres smaller with a similar covering of orangey hairs but with a pale fluffy appearance to the face. Males can be distinguished from other species by a small bump on the ventral surface of the rear basitarsus (most central leg segment).

This species can now only be found at one secret location on the Llŷn Peninsula. Surveys carried out by experts have shown that the number fluctuates but is now worryingly down to only 15 nest sites (2024), with 5 suspected of being lost over winter to landslip. Natur am Byth is working to expand the amount of available nest sites by careful management of the landslips and working closely with local landowners.