Operation Swat!

By Dr. Sherwood B. Idso

How do you get rid of a pesky mosquito when you find one buzzing around the room? You swat it, right? Well, what do you do if you find the entire countryside swarming with them? That was the problem facing the first pioneers to arrive in California. And they couldn’t run down to the local hardware store for a handy repellent; nor could they swat them all. Moreover, the mosquitoes (Anopheles freeborni and Culex tarsalis) were not just a nuisance, they were carriers of the often fatal diseases malaria and encephalitis. But the early settlers of the state surmounted the problem; and the story of how they did it and how their descendants continue to keep the problem under control is one of the great success stories of applied science.

The first serious efforts at mosquito control in California were begun in 1905, with the first antimalaria campaign getting underway in 1910. Through the use of chemical insecticides and other measures to get rid of the mosquito breeding areas, the dreaded disease was nearly eliminated by 1920.

But then something unexpected happened—the mosquitoes became resistant to chemical insecticides. For a while, mosquito-eating fish were employed in the fight. Then intensive research efforts during World War II led to the development of DDT, a chemical that proved very effective against mosquitoes. By 1954, however, the mosquitoes had become resistant to DDT. The mosquito fighters then went back to their laboratories and developed a whole new arsenal of chemical weapons. But ten years later the story was repeated. The mosquitoes once again became resistant to the new chemicals. Consequently, the last decade has seen the development of an intensive research program designed to find new remedies.

The watchword this time around was “biological control.” Mosquito-eating fish such as the pupfish were once again employed, as well as various types of aquatic insects. Still other natural enemies of mosquitoes to be engaged in the battle were flatworms, nematodes, hydra, fungi, and bacteria (see photos). Of course some new chemical agents were also developed, and the elimination of breeding areas continues to be important.

As a result of these new efforts, the mosquito is now being effectively controlled. Due to its adaptability to changing conditions, however, it will probably never be completely eliminated. But the many different approaches to the problem provided by ongoing research should ensure that it never again gets out of control.

1 A World Health Organization technician in Kenya collects mosquitoes from volunteers in malaria-control research project. (Photo by Russel Fontaine.)

2 Large tents keep mosquitoes in while scientists study their behavior in natural situations. (Photo by Jack Kelly Clark.)

3 A scientist siphons water from cavity in dead tree limb to get count of number of larvae living there. Such “treeholes” are a favorite breeding place of the western treehole mosquito. (Photo by Jack Kelly Clark.)

Mosquitoes beware! Here is a group of some of the mosquito’s deadliest foes that scientists have enlisted in the battle to curtail the number of pesky insects.

4 A mosquito fish (Gambusia affinis) about to gobble up a tasty mosquito larva morsel. (Photo by Jack Kelly Clark.)

5 Another mosquito-larva-eating fish, the desert pupfish (Cyprinodon macularious), is tolerant of salty water and can withstand temperatures from freezing to 115 degrees F. (Photo by Laurel Walters.)

6 One of several species of flatworms that have proven to be effective predators of mosquito larvae. many are tolerant of foul waters filled with decaying organic matter. (Photo by Max Badgley.)

7 Fresh-water hydra feed on mosquito larvae and are useful in sloughs, rice fields, roadside ditches, canals, and pools. (Photo by Jack Kelly Clark.)

8 Larvae clumped together have been killed by a substance produced by a special strain of bacteria that is deadly only to them. It does not harm the fish you see or the neonectid (backswimmer) that also feed on the larvae. (Photo by Jack Kelly Clark.)

9 This mosquito larva is being infected by a fungus that will shortly bring about its death. (Photo by Jack Kelly Clark.)

10 Here is a mosquito larva infected by a nematode parasite. Four of the nematode larvae can be seen coiled within the mosquito larva’s body cavity. (Photo by Edward Platzer.)