Purbeck Mason Wasps - a star of Dorset's heathlands

 

Purbeck Mason Wasp Pseudepipona herrichii - Grange Heath, Dorset

One of the joys of surveying for reptiles on the Dorset heathlands is the likelihood of being distracted by other exciting wildlife. And this morning on Grange Heath, in the growing heat of June, there was much of interest to distract us - mostly of the insect variety. 

Star billing went to the Purbeck Mason Wasps Pseudepipona herrichii (see image above). Within the British Isles, this handsome hymenopteran has an incredibly restricted range. Essentially, it is only found on the lowland heaths around Poole harbour in the east of Dorset, especially in what is generally known as the 'Isle of Purbeck'. (Spoiler alert - it's not even slightly an island). I've added a link to NBN Atlas distribution map for the species here - the pattern is pretty stark.

Purbeck Mason Wasps are one of the UK's rarest invertebrates, which has been recognised by law: they are a 'priority species' for conservation in the Natural Environment and Rural Communities (NERC) Act 2006 (section 41). A 'species action plan' was published in 1999 (link here) describing actions and activities aimed at improving the wasp's conservation status. This has been taken forward by the 'Back from the Brink' project, with a useful summary sheets here and here.

Grange Heath lies in a quiet, and rather isolated, corner of the wider Purbeck Heaths National Nature Reserve (more details about the overall reserve are here). The land is managed by the RSPB, who make no secret of the presence of Purbeck Mason Wasps on the site:

Grange Heath entrance sign

The heath is managed for several species of conservation interest, including birds, reptiles and invertebrates. Populations are monitored, and it was reptile monitoring that took us to the site this morning. Various actions have been carried out on the heath to benefit Purbeck Mason Wasps, but to fully understand these we need to take a quick look at the ecology of this - frankly rather fussy - species. 

Like many insects, these solitary wasps have a bizarrely specialised life cycle, which probably accounts for their rarity. After winter, males are the first to emerge; they have a short life span, existing only to mate with the females before dying after about 10 days. Not much of a life, you might say. After that, the females do all of the work - and their first job is to dig a burrow, as seen in a short iPhone video from this morning:

Although classed as 'solitary wasps', individual females seem to prefer nesting in groups - although this may be due to the relative scarcity of suitable sites for burrowing. The wasps need open ground that has a high clay content: if it is too sandy then this risks the burrows caving in. The holes are neat and round, with the burrows heading vertically down. The spoil from digging is usually moved slightly away from the hole, so it's worth looking for a small pile of soil granules in the vicinity. The photo below, from this morning at Grange Heath, shows Purbeck Mason Wasps at the entrances of two burrows plus a third burrow (arrowed): 

Purbeck Mason Wasp burrows - two occupied and one empty (arrowed)

At Grange Heath, a number of artificial scrapes have been excavated in suitable clayey substrates - and this has obviously worked. Almost all of the Purbeck Mason Wasps that we saw were buzzing around these cleared areas - and we saw well over 20 individuals.

However, finding suitable ground conditions is only part of the challenge for a female Purbeck Mason Wasp. What makes the wasps unique (you might say 'picky') is their feeding strategy. This has two parts. The adult females seek out a high energy food source in the form of nectar. However, only one species of plant will do: Bell Heather Erica cinerea. The wasps' tongues aren't long enough to reach into the tubular openings of these richly purple-coloured flowers, so they have to bite through the base of the flower (the calyx) to obtain nectar. The presence of Bell Heather is therefore another habitat requirement for the species. Fortunately, there's plenty of this on Grange Heath.

The second part of the feeding equation involves provisioning the wasps' offspring, and (once again) only one type of food will do. This time it's the caterpillars (larvae) of a small heathland moth variously known at Heath Tortrix, Heath Button Moth or (more scientifically) Acleris hyemana. The unfortunate larvae are caught by the female wasps, brought back to base and stuffed into the newly dug burrow. An egg is then laid on top of a ready-made food source for the growing wasp larva. Grisly, but not unusual in the insect world.

But it doesn't always go to plan for the wasp. The female will usually leave the burrow unsealed in her search for several moth larvae to feed her future brood. This provides an opportunity for Black Ants Formica fusca to nip in and steal the hapless moth larvae, which will then be taken back to the ants' nest for a similarly unpleasant end. We were fortunate this morning to see a caterpillar in the process of being moved by some ants from one of the many nearby wasp burrows (below). 

Acleris hyemana larva being moved by Black Ants

Any strategy aimed at conserving the wasps must therefore also seek to conserve the moths. While these have a wider distribution than that of the wasps (NBN distribution map link here), they are not always common in heathland environments, showing a clear preference for shorter and younger heather. Managed burns of small areas of heathland can achieve this, as happens at Grange Heath. 

The story of the Purbeck Mason Wasps provides an insight into both the complexity of heathland ecosystems and the challenges of conserving what are often rare species in the British context. But it's surely worth it in order to maintain the interest and diversity of Dorset's fabulous lowland heaths.

Acknowledgements

 

Particular thanks to my sharp-eyed fellow reptile surveyors (Howard, Chris, Sue and Rob) for their help in locating wasps, burrows, larvae and ants at Grange Heath this morning, and to all the RSPB Dorset Reserves team involved in protecting and managing this fabulous heathland site.

Thanks also to Phil Sterling for confirming identification of the Acleris hyemana larva.