Dog owners are being warned against letting their pets swim in Sheffield’s rivers, after research found that commonly-used flea treatment can be deadly to aquatic insects.

Sheffield River Rangers and Rivelin Valley Conservation Group say there has not so far been a decline in the numbers of species they are finding, but are concerned about the impact of swimming dogs on the insect population.

A meeting of the conservation group was told flea powder for a medium-sized dog contained enough toxic insecticides (fipronil, permethrin and neonicotinoid imidacloprid) to kill 60 million bees.

River ranger John Blewitt, 76, said: “I’ve been all over the world but the river has got a special affinity. I love the place. 

“And in the 70-odd years that I’ve been walking up and down it, it’s changed dramatically.”

Sheffield River Rangers also advise against open water swimming in these areas, especially as warmer weather approaches. This is known to erode river banks, smothering insects which live in the river bed under dry dirt and rocks.

Sheffield River Rangers analyse numbers of eight invertebrate families including mayflies, caddisflies, shrimp and crane flies, in the Don, Rivelin and Loxley rivers. 

The groups have noticed a worrying trend in the behaviour of cased caddisflies, insect larvae which build protective cases out of sediment found in the river. A rise in litter has caused them to use glass and plastic, leading to further microplastic formation in rivers and harm to other wildlife.

Speaking during his talk entitled The Good, the Bad and the Ugly, Mr Blewitt added: “The ugly is what really does put a blight on our countryside and our rivers. And that’s what we’re trying to monitor.”

Sheffield River Rangers collaborate with the Environment Agency to record incidents of pollution and evaluate water quality.