April 2005
Notes On Mosquito Abatement and Turtle Ponds
by Michael J. Connor
Outdoor ponds can provide stimulating, semi-natural environments that adult aquatic turtles can thrive in captivity as well as providing interesting enhancements to backyard settings. Ponds have several advantages over aquaria including exposure of the turtles to beneficial natural sunlight, a reduced need for equipment (heaters, filters, heat lamps and broad-spectrum fluorescent lights) as well as water-conditioning chemicals, and the opportunity to introduce aquatic and marginal plants into the turtles' environment, creating a more naturalistic setting and providing natural, whole foods to supplement their diet (Cohen, 1993). However ponds do bring with them there own maintenance issues. Standing water acts like a magnet for mosquitoes, midges and other flying pests. Mosquitoes are particular issue these days, because they are a vector for West Nile Virus which can cause serious disease and even fatality in humans.
The length of time for the egg, larvae and pupae stages depend on temperature, so mosquito control is particularly important in the warmer months. Culex tarsalis, the mosquito that carries West Nile Virus, might go through its life cycle in 14 days at 70 F and but take only 10 days at 80 F. There are a number of steps that the pond turtle keeper can take to control mosquitoes.
(a) If feasible, drain ponds weekly to break the mosquito life cycle.
(b) Stock ponds with fish such as Gambesia that eat mosquito larvae. Smaller turtles may eat mosquito larvae, and most turtles will eat Gambesia and other fish if they get in easy reach. This dynamic set of interactions while interesting means that fish may need to be added very regularly!
(c) Agitate the water surface using a pump with a fountain or waterfall. You may be able to combine this with an external filter set-up. Mosquitoes prefer still water to lay their rafts of eggs, and mosquito larvae must come to the surface at frequent intervals to obtain oxygen through their breathing siphon.
(d) Use the donut shaped "Mosquito Dunks" or similar commercial preparations of Bacillus thuringiensis subsp. israelensis. "Mosquito Dunks" are available from home supply stores such Home Depot or Lowes. Mosquito larvae eat algae and small organisms which live in the water. Bacillus thuringiensis israelensis is a bacterium that when eaten by the mosquito larvae induces a fatal dysentery-like disease that kills them. It is highly selective, killing only the larvae of mosquitoes and a few other related flies. Fish and turtles are not affected by Bacillus thuringiensis and I have used "Dunks" in my turtle ponds without any evident ill effects.
(e) Examine the pond surface closely on a regular basis. If you see any of the little wriggly mosquito larvae you must take prompt action. Mosquitoes do not respect fences and even if you are willing to tolerate a few bites you do not want to create a problem for your neighbors.