Issue: August 7, 2004

Mosquito control and garden pond


In your recent article about mosquitoes and West Nile Virus, you recommended draining all standing water on a property. Does that mean I must drain the water from my lily pond? I don't want West Nile Virus, but I also don't want to lose my beautiful water lilies.


Thank you for asking. I didn't have space to thoroughly discuss this topic in a previous article, so this is a good opportunity to continue.

Having fish in your pond will solve the problem because fish eat mosquito larvae. You can purchase mosquito fish through several suppliers (local or mail order). Gambusia affinis and other species of Gambusia are known to eat mosquito larvae but have a reputation for also eating the young of other fish and insects. Some sources recommend them for small ponds but not for larger bodies of water, especially those containing other reproducing fish. Don't release them into the wild because they can harm native fish populations.

Try adding Koi (large relatives of goldfish) to the pond. They are more expensive, but they are prettier than mosquito fish and they also eat mosquito larvae. However, like mosquito fish, Koi should not be released into natural bodies of water.

If you don't want to care for fish, or if you want an extra measure of security to control mosquitoes, try using an insecticide called BTi. It is a natural toxin produced by Bacillus thuringiensis var. israelensis. It is available in many nurseries and garden centers as a granular material or as "dunks," which are donut-like disks with a hole in the center that float on the surface of the pond and slowly release the bacteria and toxic agent into the water. This material is not toxic to humans, pets, fish or most other insects. Still, they may kill some non-harmful insect relatives (midges) that are food for fish, so you might want to provide additional food for fish in the pond. Although BTi is a natural product and non-toxic to people, please read and follow directions carefully. Apply enough BTi for the surface area of your pond. Given that BTi decomposes in sunlight, it must be reapplied occasionally.

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Send gardening questions to Southwest Yard and Garden, Attn: Dr. Marisa Thompson at or at Please include your county Extension Agent ( and your county of residence with your question!

For more gardening information, visit the NMSU Extension Horticulture page:

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505-865-7340, ext. 113.