The solitary wasp Pemphredon austriaca (Kohl, 1888) is a species not currently recorded in the UK. It appears to have been increasing in the Netherlands in recent decades and there is a good chance that it could be overlooked in the UK.
This small, black wasp shows a preference for using Marble Galls, which are caused by the gall wasp Andricus kollari and found on oak. A. kollari emerges from these spherical galls, leaving small circular emergence holes. Other insects then take advantage of these chambers for shelter or nesting, including P. austriaca, which constructs nest cells within the gall.
February and March are the perfect months to collect some of these Marble Galls (where you can see the circular emergence holes) and provide something else to keep an eye out for on those familiar lockdown walk routes. Keep them in a plastic pot with some fine mesh or muslin secured over the top, or a specific mesh rearing container if you happen to have one, and make sure they are labelled with a location and the date collected. They should be kept as close to ambient temperature as possible, so a shed or covered area outside is ideal. The wasps are on the wing from May to August on the continent, so it’s worth starting to check the container every few days (or ideally daily) from mid-April to see if anything emerges.
If you are lucky enough to have some potential wasps emerge and are not sure how to identify them, get in touch with BWARS: https://www.bwars.com/content/bwars-contacts Happy hunting and fingers crossed for another wasp species to add to the UK list!
For more information, see this paper from the Netherlands: https://edepot.wur.nl/347358