Water “lice”

We are using a drip system in a soilless mix. The other day we moved a plant and saw hundreds of tiny white bugs in the water standing at the bottom of the pail. They are very quick and constantly jump around in the water and tend to collect into colonies.
What are they? Should we kill them?


The little white bugs in the water are immature fungus gnats. They suck on the roots and are vectors for disease. Once they mature they eat plant debris and continue to spread pathogens. They should be eliminated.

There are several ways to kill them. Fungus gnat solutions were described in detail in the answer to the expert question in issue #37. I’ll briefly list a few below. All of these pest killers listed below are harmless to you and your warm-bodied pets. They are all organic and are suitable for use on food crops. Many of these products are available at garden shops, indoor grow stores or over the internet.

Beneficial Nematodes: are tiny worm-like creatures that feed on many kinds of soil insects and larvae. They are very effective against gnat larvae. The nematodes come as a dry powder that is added to the water. They can be re-introduced as a prophylactic once every 4-6 weeks. Many nurseries and indoor grow shops carry them.

Botanigard: mentioned above in relation to grubs and caterpillars, also works on gnats. It contains a living fungus that infects and kills soft-bodied insects. The fungus is not interested in other life forms. It’s sort of like athlete’s foot gone super bad. It comes as a liquid that is added to the water.

Gnatrol: The active ingredient, a bacterium named bacillus thurengensis israelensis, causes plague in gnats. It is harmless to other creatures. It comes as a liquid concentrate. It is very effective because the bacteria remain alive for a couple of weeks, plaguing the larvae newly emerging from the eggs.

Pyrethrum: also mentioned above, is a pesticide made from a chrysanthemum-like flower. Pyrethrum is toxic to fish so it should never be used around water bodies. Once pyrethrum is used it remains active for just a short time before it deteriorates. A spray will eliminate the flying gnats. It comes in a spray that can be used as a bomb, a liquid concentrate or a wet-able powder. If you use the bomb, leave the room for a few hours. It’s not that the active ingredient is harmful, it’s just not good to breathe propellant fumes. The concentrate or powders can be prepared and watered into the soil to kill the larvae.

Readers with grow questions (or answers) should send them to Ed at: Ask Ed, PMB 147, 530 Divisadero St., San Francisco, California 94117, USA
You can also email Ed at [email protected], and send queries via his website at www.ask-ed.net.
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