Nematodes Sf are live, microscopic worms that come within a small sponge that can be soaked in water within a watering can for targeted application to just a few plants. They are parasitic predators for fungus gnat larvae primarily, but will also eat thrips pupae in the growing medium. They can be used for both active infestations or preventatively. One sponge of 1 million nematodes treats about 10-15 plants.