There are two UK species of Kleidocerys, K. resedae, the Birch Catkin Bug, and K. ericae. They are very similar and difficult to separate. The Birch Catkin Bug is very common, 4.5-5.5 mm long and with a mostly brown scutellum, although this is variable. (Kleidocerys privignus is similar but somewhat darker than K. resedae and occurs on Alders, but is now generally considered to be an ecoform of K. resedae (Carayon, 1989).) K. ericae is generally slightly smaller than K. resedae at 3.5-4.8 mm and is associated with heathers, both Erica and Calluna. K. ericae is mainly distinguished from the Birch Catkin Bug by its smaller average size, by its lighter ground color on average and by its host plants.
All good so far? Well not really. The two species overlap in terms of size and pigmentation, and K. ericae is also capable of completing its normal development on Birch catkins (Woodroffe, 1960; Carayon, 1989). So are they in fact the same species? No, apparently not. Like many Hemiptera, Kleidocerys communicates by stridulation. In this genus, sound production seems not to be associated with mating but occurs when they are disturbed, perhaps as a territorial statement. K. ericae stridulates at a frequency of 16 Hz while K. resedae uses a lower frequency of 8 Hz (Haskell, 1957).
This is where it gets difficult (you thought it was difficult already?). We are used to thinking of "sound" at much higher frequencies than this. The range of "normal human" hearing is generally quoted as about 20 Hz to 20 KHz. Above 20 KHz is ultrasound - we whip out our bat detectors to took for Soprano Pipistrelles (Pipistrellus pygmaeus) at 55 KHz, or strain to hear the top end of expensive hifi setups (alas, no longer possible at my age). Below 20 Hz is infrasound - put away your bat detectors and your mobile phones, they don't work in this range. What you need down here is laser Doppler vibrometry, and there isn't a mobile phone app or a pocket detector for that. Fun though adventures in the vibrosphere are (Rexhepi et al, 2021), the only reliable way to tell Kleidocerys species apart is to whip out your laser Doppler vibrometer and listen - or to be more strictly accurate, watch, as we are now using light rather than sound to detect vibrations. The good news is that you can pick them up used on eBay for less than £1,000, but they are the size and weight of a suitcase (carry on size, hold luggage weight).
Kleidocerys then, remains frustratingly ubiquitous but annoyingly out of reach. Until, of course, DNA studies are performed to clarify the situation. We already know the mitochondrial DNA sequence of K. resedae - remarkably, the shortest mitochondrial genome of any Hempiteran (Li et al, 2016) - but I am not aware of any comparative studies with K. ericae. I think I need to add a pocket DNA sequencer to my shopping list.
UPDATE: I've been contacted to say that the Darwin Tree of Life Project is currently whole genome sequencing K. resedae and that there are plans to collect K. ericae specimens for DNA barcoding. If there is a difference in the CO1 barcode region they will then produce full genomic data for both species. And if there isn't ... well then we have the answer.
References
Carayon, J. (1989) Systématique et biologie des Kleidocerys d'Europe [Hem. Lygaeidae]. Bulletin de la Société entomologique de France, 94(5), 149-164.
Haskell, P.T. (1957) Stridulation and its analysis in certain Geocorisae (Hemiptera Heteroptera). Proceedings of the Zoological Society of London, 129 (3): 351-358.
Li, T., Yi, W., Zhang, H., Xie, Q., & Bu, W. (2016) Complete mitochondrial genome of the birch catkin bug Kleidocerys resedae resedae, as the first representative from the family Lygaeidae (Hemiptera: Heteroptera: Lygaeoidea). Mitochondrial DNA Part A, 27(1), 618-619.
Rexhepi, B, et al. (2021) Hay meadow vibroscape and interactions within insect vibrational community. Iscience, 24(9), 103070.
Woodroffe, G.E. (1960) Entomologist's Monthly Magazine, 96: 156.