This is a water flea (Daphnia magna), not even a distant cousin to the fleas on your dog, but very close to the bottom of the freshwater food chain that is being disrupted by your dog’s flea treatment.
Let’s look at the food chains and inner workings of the Lambrok tributary with its series of little pools of still water and the one big pond. Right at the bottom of this food chain are phytoplankton, tiny single-celled organisms that photosynthesise, constructing sugars from carbon dioxide, water and sunshine. These are the organisms that turn the little pools green in the summer or sometimes join forces to make thread-like structures that float in bubbling green mats on the water’s surface.
 Phytoplankton;  zooplankton, the larva of a cyclops.
Next come zooplankton, tiny animals that eat the phytoplankton, and each other, and any passing bacteria they can catch; water fleas are zooplankton. Everything in the stream eats zooplankton: fish, fry, tadpoles of all kinds, water-fly larvae, various filter-feeding mussels and worms.
It only takes tiny amounts of fipronil, an insecticide commonly used in flea treatments for dogs, to affect the reproduction and development of freshwater zooplankton. Another insecticide commonly used in flea treatments, a neonicotinoid called imidacloprid, is highly toxic to water fleas in what the scientists call trace concentrations: 3.5 micrograms per litre of water.
A microgram is a millionth of a gram and there are a thousand grams of water in a litre: this translates into 3.5 parts per billion. How many parts per billion of these highly toxic insecticides is left behind in the big pond when an energetic dog, recently treated with spot on flea treatment or wearing a nice new flea collar, swims across after its ball?
 Mayfly;  mayfly larva or nymph;  caddis fly;  caddis fly larvae.
Fipronil and imidacloprid are insecticides, designed to kill insects in all the stages of their life cycles. As well as reducing the population of zooplankton in our pond, these chemicals will be actively poisoning the larvae of water-flies. Insects such as mosquitoes, mayfly, damselfly and dragonfly are all aquatic at their larval stages and all susceptible to waterborne insecticides. There will be consequences further up the chain.
When the water-fly hatch, the energy originally derived from the sun by the photosynthesising phytoplankton at the food chain’s base is flung into the air for hawking birds and dragonflies, for wasps and hornets to carry away into the wider ecosystem. The Lambrok freshwater food chain ends not only with its apex predator, the otter, but also with the swallows, swifts and house martins that hunted all summer over the stream and will redistribute its energy far and wide.
 Swallow;  swift.
It is so short sighted of us to respond to small domestic problems with such enormously dangerous solutions. We do it all the time: an ant in the sugar, poison the nest; a wolf in the sheep fold, slaughter the pack; fleas on the dog, destroy a whole ecosystem. Let’s put the sugar in a jar, shut the sheep in at night and buy a flea comb. If we destroy one part of our freshwater food chain, we destroy it all.
We need to act: we need to question the manufacturers and the vets and look for alternative solutions to what is, after all, a flea-sized problem.