P. rotundipennis is rarely seen or trapped as an adult. Ninety percent of the 700 UK records are as larvae and most from Environment Agency surveys. Permanently flowing medium-sized sandy bottomed streams with a moderate flow rate seem a favoured habitat.
It is usually encountered in ones or twos but in 1975 and 1976 my wife and I found it was common in the stream that exits Hatch Mere. Timber removal from the stream in 1991 led to the species apparently disappearing but wood is now allowed to build up again and in November last year we found a few larvae and many old pupal cases on timber at (SJ55-71-) suggesting that the population has recovered.
However, that population Is eclipsed in size by the one in the Prescot Brook. Large numbers were noted in September 2014 by Chris Felton and Tony Hunter of World Museum Liverpool in the Prescot Brook where it forms the exit of the Mizzy Dam on the Knowsley Estate (SJ457935); also recorded in 2015 by an Environment Agency biologist downstream where the brook flows through Carr Lane Woods Park, Prescot (SJ460920) and that is where my wife and I found large numbers on the 5th of May 2023. It was by far the commonest caddis amongst a restricted range of species that we encountered.
Why is a stream that has well-recorded inorganic and some organic pollution with a highly modified water course such a good site for this very local species? It is noted in the Netherlands as being able to withstand a little organic pollution and also tolerate more silt than its relatives. The bottom at Prescot is covered in silt bound to filamentous algae but under ‘stones’ [mainly bricks and concrete bits] it has less silt and we were surprised to find nests of the Bullhead, Cottus.
Perhaps the pollution has been deleterious to the usual species and allowed P rotundipennis to thrive. The Gresty Brook at Wistaston, Crewe (SJ69-53-) is another interesting suburban stream with a history of past pollution which, until serious diesel pollution last year, had far larger populations of the wood-eating Lype phaeopa (Stephens)and the small stream species Micropterna sequax McLachlan than we have previously encountered anywhere – but lacked many species that would have been expected from an unaffected large stream.
The general pollution problems of the Prescot and Gresty Brooks are so multi-factored that they will probably never become pure clean water bodies but they are nevertheless of interest and give insights into the biology of species.
Potamophylax nigricornis (Pictet) has recently been added to the British list from South Wales and to accommodate it in the keys I need to look at freshly collected early instar larvae of P. rotundipennis, hence my renewed interest in the species, and I am pleased to have places to look for small larvae this autumn.