July 14, 2003

OVERTON – As the summer heats up, so do fears from mosquito-borne West Nile Virus risks and so does the hype about magic-bullet type mosquito controls.

In reality, there are no magic bullets, but although draining stagnant pools or treating them with a pesticide remains the most effective means of controlling mosquito populations, there are biological controls that will help lower insect populations, said Dr. Billy Higginbotham, wildlife and fisheries specialist with Texas Cooperative Extension.

These biological controls range from fish to fowl, and from bacterial to what some people might consider creepy – bats, Higginbotham said.

First, to control mosquitoes biologically, it helps to review a little about the insect's life cycle. All species of mosquitoes must deposit their eggs in standing water sources. Momma mosquitoes prefer small stagnant pools because this offers the most protection from things that eat mosquito developing larva. Marshes are a favorite hatchery, but mosquitoes will also deposit eggs in buckets, water-filled tires, stump holes, birdbaths, water-filled buckets, ornamental pools and even farm ponds. Usually farm ponds are too deep and have too many larva eating fish, but such is not always the case.

The larval stage is aquatic and feeds on subsurface micro-organisms. Mosquito larva when first hatched are about one-sixteenth inch long. When mature, they measure as much as a quarter inch long. They don't have gills and must move to the surface to breathe. To do so, they don't so much swim but wiggle. Hence, among those on familiar terms with insects, such as Dr. James Robinson, Extension entomologist, the larva are known as "wigglers."

The simplest way to take care of the "wigglers" in birdbaths and the like is simply to wash them out. Like a fish out of water, the larva will soon die.

For pools and ponds too large to be dumped, one simple option is the mosquito dunk that utilizes a bacteria that is only lethal to insect larva. The dunks, shaped like small donuts, use a bacterially derived pesticide called Bacillus thuringiensis israelensis (Bti). Bti disrupts the life cycle of insects that lay their eggs in standing or running water.

It is non-toxic to humans, amphibians, fish, crustaceans, adult insects, flatworms and mollusks. Neither is it toxic to insect predators of the mosquitoes, such as dragonflies. The dunks are inexpensive, can be simply dropped into the breeding pools and generally last for several weeks to months. The dunks will usually survive if the pool dries out and is then refilled with rain or drainage water, according to Robinson.

In farm ponds, fathead minnows, bluegill and many other species of sunfish will readily eat wigglers. Gambusia, or mosquitofish as they are called, particularly like wigglers. In ornamental backyard ponds, goldfish or koi carp will control the wigglers as well. Amphibians such as frogs and salamanders present in ponds may also help.

Fish farms will have sunfish species and fathead minnows for sale, but mosquitofish may not be as easily available. Ornamental pond dealers can help with sources for goldfish and koi, according to Higginbotham.

"Texas has close to a million farm ponds, but we do not know to what extent they contribute to mosquito production versus pools or containers holding stagnant water," he said. "The stagnant water sources such as birdbaths left unattended, old tires and cans are probably more of a problem since they don't already contain fish – and it's doubtful that stocking fish into stagnant water will result in very good survival. In other words, stagnant water should be eliminated instead of stocked with fish."

Another way to control for flying insects is to establish air superiority. For mosquitoes, ruling the skies means encouraging allies such as bats.

Primarily because of their association with rabies, bats have gotten a bad rap, Higginbotham said. In truth, bats are mammals and many mammals can contract rabies. However, even the less than half of one percent of bats carry the disease.

On the plus side, a single little brown bat can catch 1,200 mosquitoes in just one hour. Although mosquito populations may number as much as a million in wetland areas, a large population of bats can take a big chunk out of mosquito counts. For example, the 20 million Mexican free-tail bats from Bracken Cave, Texas, eat approximately 200 tons of insects nightly.

No one wants 20 million bats in their backyards, but a small population can help keep the number of mosquito bites down. Bats normally bite only in self-defense and pose little threat to people who do not handle them. Of course, noted Higginbotham, children should be warned not to pick up a sick or dead bat, but this warning should apply to all wildlife including birds, not just bats.

Purple martins have also been attributed to eating thousands of mosquitoes, but this is a myth started years ago by a birdhouse manufacturer. Purple martins do eat flying insects, but mosquitoes constitute only a small percentage of their diet.

As for biological controls in general, Higginbotham and Robinson agree they aren't the sole solution to mosquito control. The best bet remains to use biological control measures in conjunction with draining stagnant water sources, and using environmentally safe insecticides such as Bti when necessary.

Additional information on bats can be found at the Bat Conservation International webpage at http://www.batcon.org/, Higginbotham said.

More information on the effectiveness of mechanical control devices can be found in a previous news article at http://overton.tamu.edu/news/news2003/mosquito_control_2003.htm