Or cistern tank shouldn't be treated with insecticides so I thought I'd share some safe solutions. Sheet plastic or wax paper kitchen wrap gently spread on surface will stay on surface as long as not disturbed. If kept in place continuously, 10% surface coverage is adequate but if wigglers are already present, 25% or more coverage is advisable. Larvae must surface often to breath via special tube and this blocks access to air. An alternate requiring more preparation but works better where surface is disturbed by wind,birds or such. Just to give you idea how it works,some of those little Styrofoam hats used for faucet freeze protection do well. Pull stopper,submurge 50%,replace stopper and let go. There's a strong vacuum in air space inside cup that pulls larvae inside-out the moment his guzzle breaks surface. To fabricate a larger version, cut upper portion,including cap from plastic, 5 gallon distilled water jug, split and glue a fat pool noddle around rim. There's plenty options using Fed-X packing and discarded plastic containers. As long as container is highly buoyant there's no need for cap/plug. Partly submerge then flip container into position.
A tablespoon of most any oil will cover a few sq feet surface to accomplish same thing but makes a terrible mess when emptying barrel. Vegetable oil doesn't harm irrigation water but again it sure makes barrel slick.
Floating containers do fairly well in livestock troughs but for large tanks I place a few small fish/minnows in them each Spring.
A tablespoon of most any oil will cover a few sq feet surface to accomplish same thing but makes a terrible mess when emptying barrel. Vegetable oil doesn't harm irrigation water but again it sure makes barrel slick.
Floating containers do fairly well in livestock troughs but for large tanks I place a few small fish/minnows in them each Spring.
