Abstract: Larvae swimming in a mass of fresh water and having interiors substantially constituted of highly electrically conductive substance are immobilized or killed. Two capacitor plates, which include exterior insulation, are located on opposite sides of the mass of fresh water. A low power AC voltage is applied across the plate. The AC voltage is sufficient to establish an alternating electric field in the mass of fresh water between the plates and to alternately polarize the mass of fresh water and the larvae. Thereby is generated an AC current in the substance in the interiors of the larvae sufficient to cause therein a concentrated power dissipation of immobilizing or killing magnitude.