Argulus another crustacean parasite, round and up to 1cm wide. They have a sucker to hold on to the Koi with needle-like mouth parts which they stick into the Koi and inject a toxin. This causes intense irritation to the Koi and they scratch and jump and can cause bacterial infection. The punctured areas of the koi will also be open to secondary bacterial infection.
This parasite can be introduced to the koi pond by visiting frogs and toads.
Argulus reproduce by laying eggs. After mating the female detaches from the host koi to deposit her fertilized eggs on the pond walls or any other surface. Upto 500 eggs may be laid. Dependent on water temperature the eggs take between two and four weeks to hatch. The newly hatched parasites will swim to find a host koi. They will reach sexual maturity after 20 to 50 days depending again on the water temperature of the pond, then the cycle starts again.
To treat an Argulus outbreak you will need to sedate and inspect every koi in the pond, removing all adult parasites with tweezers. The pond must then be treated to kill off the juveniles. This treatment should be repeated two or three times at intervals of seven to ten days. The most effective treatment is Masoten.