Biological control is the use of specially chosen living organisms to control a particular pest. This chosen organism might be a predator, parasite, or pathogen which will attack the harmful insect resulting in a desired reduction of pest population levels. The most successful biological tool against immature mosquitoes in California is the mosquitofish, Gambusia affinis. When introduced to a mosquito breeding source, the mosquitofish quickly adapts, multiplies and becomes numerically capable of sustaining an effective control level.
The most successful biological tool against immature mosquitoes
in California is the mosquitofish,
Gambusia affinis. Two other beneficial fish species include
Threespine Stickleback
and Guppies. All three fish
species are bred right here at the District Headquarters. Other predators include Water
Striders, Giant Water Bugs, Dragonflies, Predaceous Diving Beetles, Damselflies,
Water Scavenger Beetles, and Back Swimmers.
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Mosquitofish
When introduced to a mosquito breeding source, the mosquitofish quickly adapts, multiplies and becomes numerically capable of sustaining an effective control level. This relative of the common guppy feeds primarily on aquatic insects, and are effective predators of mosquito larvae in many diverse aquatic habitats throughout the world. A comparatively small species, the full-grown females are typically under 1 1/2 inches. The muted silver and light olive green color is common in both sexes.
Gambusia exhibits a tremendous tolerance for a wide range of water temperatures, although the preferred temperatures appear to lie between 77º and 86ºF. When surface water temperatures approach higher lethal limits, mosquitofish usually swim down to cooler water strata. Conversely, in the cooler seasons mosquitofish will move into shoal areas to reach the sun-warmed shallow waters.
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Threespine Stickleback
The District has raised and stocked the threespine stickleback, Gasterosteus aculeatus,
since 1998, when our original stocks were seined from Grizzly Island Wildlife Area. During initial
studies, the District discovered that the threespine stickleback preferred to feed on benthic
organisms (invertebrates living in or on the pond bottom).
Mosquito larvae are found on the surface of most sources and therefore not in the preferred
feeding zone of this species. In most artificial containers—unused swimming pools and backyard
ponds—there is not a benthic community, so the sticklebacks have to expand their feeding range
to the surface.
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Guppies
The guppy, Poecilia reticulata, has been used for biological mosquito control
since World War I. It has been introduced almost all over the world from the areas of
tropical South America to which it is indigenous. In many areas the guppy has provided
good control of mosquitoes in highly polluted sources, such as sewage pools, dairy lagoons,
chicken ranch ditches and slightly acidic sources. Unlike the mosquitofish, the guppy's
ability to reproduce or control mosquitoes is not reduced by low levels of dissolved oxygen.
Many of the guppies at the District are cultured indoors in large aluminum raceways. It's possible
to manipulate the photo period as well as the temperature in these indoor tanks to ensure year-round
production. The breeder tanks have several fry retrieval baskets that collect the newly released
free-swimming fry. These fish are moved to grow-out tanks, where they mature in about 40 days.
The guppies that are cultured indoors are primarily used for stocking koi and goldfish ponds.
Since they have been reared indoors, they are exposed to very few parasites, which are commonly
found among outdoor or pond-cultured fish.
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