Microterys

There are three species of Microterys, parasitic Encyrtid wasps, in the UK. They are tiny which accounts for them usually being overlooked. They parasitise scale insects and have been widely used as biocontrol agents. The males are difficult to identify but the females, although similar, can be identified with a microscope. I recently found large numbers of Microterys seyon overwintering in Ivy. Although tiny (around 2mm), they are distinctive in appearance and stand out in the beating try. Even in the cold they have a tendency to high speed vertical takeoff, so you'll need to be nimble to catch them. 

• Microterys nietneri and Microterys seyon are similar in appearance with strongly patterned wings and black and white antennae. The middle tibia of Microterys seyon is covered in silky hairs which are not present in M. nietneri. 
• Microterys tricoloricornis is larger (nearly twice the size, around 4mm) and slightly darker than the other two species. 

Microterys seyon middle tibia: 

See: Painter, D. (2020). Two parasitoid wasp species, Microterys tricoloricornis and Microterys nietneri (Hymenoptera: Chalcidoidea, Encyrtidae), new to Britain. British Journal of Entomology and Natural History, 33, 1-16. Available here.