Other than the beetles associated with ant nests there are a couple of others that should interest Hymenopterists – and both are rarely recorded.
Firstly is the Wasp Beetle (Metoecus paradoxus), the UK’s only representative of the family Ripiphoridae. The beetle lays its eggs in batches in tiny cracks in the surface of dry wood where wasps will choose to chew off fibres for nest-building. They overwinter as eggs, then tiny triungulin larvae hatch and climb onto a visiting wasp and are transported back to the nest. Once inside the nest they become parasites of wasp larva, first internally, then externally as they grow (life cycle illustration right). It is just possible that a wasp may be captured with a triungulin present, but it may also be worth looking on areas of wood, such as fence-posts, in the spring when wasps are collecting lignin.
The Wasp Beetle is not often recorded, but can be numerous when it is. I found a window-ledge covered in them (dead) in my father’s attic in Sheffield a few years ago. He had had a wasp’s nest in the roof that year. In July 2020 I found a dead specimen on the ground outside our house in Bolton le Sands, Lancashire (VC60) (image below). There is one NBN record for VC60 from 1995 and only a handful in Cumbria in the 2012 checklist, the latest again being 1995. However, there may be more – I doubt the NBN data is up-to-date!
The other group of Hymenoptera-linked beetles came to my attention when working through my collection of bees from 2019. Attached to the side of one female Lasioglossum bee was a tiny creature, looking rather like a tiny beetle larva (image below). I had seen pictures before and recognised it as the juvenile stage of an oil beetle Meloe proscarabeus.
Oil beetles (image of adult at bottom) belong to the genus Meloe and are becoming quite rare. The reason for their rarity and decline is their unusual lifestyle. They are totally dependent on healthy colonies of solitary bees for their survival which, as has been widely reported, are themselves suffering huge declines in many areas due to a combination of habitat loss and exposure to pesticides.
The female oil beetle digs a small burrow and in it lays hundreds of eggs. These hatch into larvae known as triungulins which immediately climb up onto nearby flowers where they wait for a female solitary bee to visit. While the bee feeds, they climb on and are transported into its nest burrow where they climb off and feed on the eggs and food provisions collected by the bee.
In our area we have two species recorded, the Violet Oil Beetle (Meloe violaceus) and Black Oil Beetle (Meloe proscarabaeus). They are quite similar, the key difference being the shape of the rear of the thorax – so a good photograph from above can confirm an identification. Their triungulins are very different, that of violaceus being over 2 mm long and black whereas proscarabaeus is smaller, about 1.5 mm long and orange/yellow.
M. violaceus has been recorded from a number of places across Cumbria and a couple of sites in northern Bowland. However, I have been unable to find any records of M. proscarabeus for northern Lancashire, nor from southern Cumbria so this looks like quite a good record. The adults are most numerous in the spring and early summer and unmistakable, so unlikely to be overlooked by any entomologist. However, the triungulins may turn up more often on bee specimens in the spring so Hymenopterists are the most likely people to find them.
As if all this isn’t fascinating enough, you may also be interested in a short paper by Darren Mann and Clive Turner (British Journal of Entomology & Natural History 16: 2003 pp 7-8) who observed the biting-midge (Ceratopogonidae) Atrichopogon (Meloehelea) winnertzi feeding on M. violaceus, sucking fluids from the leg-joints. This is especially remarkable as the oil beetle is so-called because when threatened it will exude oily fluid containing cantharidin, a toxic, blister-inducing compound, from its joints; although this is also the active ingredient in Spanish Fly beetles – purportedly a potent aphrodisiac!