Many gardeners have fountains, ponds, and other water features in their landscapes. Water gardens are beautiful and calming, but, if not managed properly, can add an unpleasant element to the landscape—mosquitoes. How can you prevent mosquito infestations?
How do mosquitoes develop?
First, it is important to understand mosquito biology. Mosquitoes are small flies that lay their eggs in, on, or near stagnant water. The larvae, or wigglers that hatch from the eggs live in water and feed on organic debris until they transform into a motile pupa, or tumbler, and finally into the familiar adults. This process, from egg to adult, requires as little as one week when conditions are favorable. Emerging females must mate and ingest blood to produce new eggs.
Mosquitoes can be managed using an integrated approach that relies mostly on prevention, using biological and chemical controls when necessary. The key strategy is to eliminate all potential breeding sites; even one ounce of standing water can support a population of larvae. What can be done, however, when an outdoor space contains a water element?
Tips for managing mosquitoes in water features
Water features in the landscape will invariably attract adult mosquitoes, but attempting to control them or prevent their egg laying is difficult. Larvae are easier to manage, since they are concentrated in known areas, don't yet bite, and can't fly away. Larvae prefer shallow water that is less than 24 inches deep, so install water features that are deeper than 2 feet. Ponds or features that provide a steep slope or have vertical walls that quickly drop off into deep water will also be less favorable to mosquitoes. Add a fountain, waterfall, or other device that increases water circulation and reduces the stagnation that allows mosquitoes to breed.
Remove excess vegetation and organic debris that provide mosquito larvae with food, shelter from the sun, and hiding places from predators. For larger ponds, a pond skimmer will help keep mosquitoes and the algae that favors them under control. If you have a smaller pond, you can use pond skimmer nets. If fertilization is required, use pond spikes designed to prevent algae blooms.
In natural environments, bacteria, nematodes, other insects, crustaceans, and fish often keep numbers of mosquito larvae low. Protect predators such as dragonflies and backswimmers, which may have colonized ponds, by avoiding broad-spectrum insecticides and consider introducing fish. County vector control services may provide free mosquito fish, voracious consumers of mosquito larvae and pupae. Never release mosquito fish into natural water bodies, since these fish aren't native to California and can disrupt ecosystems.
Although these measures will prevent problems in most cases, mosquito larvae may still develop in some ponds. In gardens with lots of plants growing in still water, it may be impossible to keep mosquitoes from breeding. Regularly check their water features for larvae, which periodically come to the surface to breathe through abdominal siphons. Watch for the larvae's characteristic wriggling movement, or use fine dip nets to monitor for larvae. It is important to act quickly to kill mosquitoes when they are small, easiest to manage, and before they become adults and start biting.
Pesticides for mosquito larvae
Use larvicides specially designed for use against mosquitoes in water. Larvicides containing spores or metabolites of the bacterium Bacillus thuringiensis ssp. israelensis (Bti) (e.g., Mosquito Dunks, Mosquito Bits, Microbe-Lift, and other products) act as stomach poisons when ingested, killing larvae within a few days. Bti affects only fly larvae, so it won't harm predatory insects living in the pond or water feature. Another effective larvicide is the insect growth regulator (IGR) methoprene (e.g., Pre-strike Torpedos). IGRs interfere with larval molting and also take a few days to kill, but they have a broader spectrum of activity, affecting most juvenile insects and other arthropods that might be in the pond. Both Bti and methoprene are available as granules or pellets and remain effective for about a month. As with all pesticides, use these only according to label directions.
For more information about mosquitoes, visit https://ipm.ucanr.edu/PMG/PESTNOTES/mosquitoes.html.
[Featured in the Summer 2024 edition of the Home and Garden Pest Newsletter]