Insecticides and Flies

W. N. Bruce.

Today spray applications that almost eradicated house flies on treated premises a few years ago are not noticeably reducing the fly populations found in the field. Heavy and frequent treatments with DDT, methoxyclor, chlordane, dieldrin, and lindane have failed to give satisfactory control of certain field strains of flies.

Resistance to insecticides was probably first noticed by the farmer or the field observer as a failure in the control of house flies, but it was not proved until several laboratory methods of determining the degrees of resistance were perfected.

The methods are of two kinds those that treat individual flies and those that treat large numbers of flies in one operation. The first is represented by the long-used microsyringe method of testing, which is used by the California Citrus Experiment Station and the Illinois Natural History Survey, and by the micro-loop method, which is used by the Department of Agriculture and the Public Health Service. The second is represented by the spray-chamber method, used by Department of Agriculture workers, and by the residual panel test method, which is used quite extensively by the Public Health Service.

The topical, or local, application with a microsyringe is a good laboratory method of obtaining quantitative data on the amounts of insecticides needed to kill adult house flies. Individual females are selected from flies anesthetized with carbon dioxide and treated with acetone solutions of the insecticide. The actual treatment is accomplished within a carbon dioxide anesthetizing chamber, in which a 0.25-milliliter syringe is actuated by a micrometer caliper. A minute, measured amount of insecticidal solution is applied to the prothorax of the fly. Treated flies are placed in clean paper containers and fed. The numbers of dead and live flies are recorded 24 hours later and the percentage of mortality is calculated.

The micro-loop method also is valuable in computing the amounts of insecticides needed to kill adult house flies. A micro-loop is a very small loop made on the end of a piece of fine, noncorrosive wire. The loop is dipped into the insecticidal solution. The liquid retained in the loop is transferred to an anesthetized fly. Treated flies are fed and retained for a 24-hour mortality count.

We also can get the relative degrees of resistance of groups of house flies by using a spray chamber. The usual procedure is to place caged flies in the chamber, which we then fill with mist of the insecticidal solution by means of a small atomizer. We then transfer the flies to clean cages to be fed and retained for the 24-hour mortality count. A comparison between mortality produced in cages containing flies of standard laboratory strains and mortality in cages containing flies of questionable resistance reveals the relative degrees of resistance. By adjusting spray concentrations, we can measure very high or low degrees of fly resistance.

From a practical standpoint, the relative resistance can best be determined by exposing the groups to panels treated with the insecticide. Panel tests cannot be used to determine quantitatively the amount of insecticide needed to kill flies but rather gives the practical answer to the question of effective kill by surface treatments. Degrees of relative resistance are determined by varying the length of fly exposure to the treated panels and sometimes by varying the amount of insecticide on the panel. After flies are exposed to a treated panel, they are fed and held for the 24-hour mortality counts.

Often in comparing the effectiveness of insecticides we use the term "median lethal dosage (LD-50)" to express relative toxicity values. In studies on resistant flies, a median lethal dosage is the amount of insecticide in micrograms per fly required to kill one-half of the sample of flies treated.

LD-50 values of insecticides on flies are influenced significantly by the room temperature during and after treatment. In cool holding temperatures, flies are more easily killed by DDT-like compounds and less easily by chlordane or dieldrin. Flies used in computing the LD-50 values in tables 1 and 2 were retained at 80 . Flies used for test results shown in tables 3 and 4 were held at 60 . The temperatures explain differences in the LD-50 values of the various insecticides in relation to standard laboratory strains of flies.

THE FIRST RECOGNIZED OCCURRENCE of DDT-resistant flies was reported in 1947 by Giuseppe Sacch and A. Missiroli of Italy. The first widespread use of DDT was made by American occupation forces in Italy. Dr. R. Weismann, also in 1947, reported a strain of flies in Sweden that exhibited a significant amount of resistance to DDT.

In 1948 an alarming amount of DDT resistance was discovered among flies infesting southern California and scattered places in the Southern, Eastern, and Central States. By the end of the 1949 growing season, resistance to DDT had become prevalent among flies in most parts of the United States. A survey conducted in 1949 by the Illinois Natural History Survey showed that 87 percent of the farms in Illinois were infested with DDT-resistant flies. A survey in 1950 revealed the presence of DDT resistance in all populations of wild flies that were tested. The surveys gave evidence that the wild susceptible strains were becoming resistant to DDT over a period of 2 or 3 years.

The actual trends in development of DDT-resistant strains on two Illinois farms from 1945 to 1950 are shown in table 1. The 1950 levels of DDT resistance for the farms are significantly higher than the 1948 or 1949 levels, even in the absence of applications of DDT. Investigators in California observed the same phenomenon after the use of DDT for fly control had been discontinued for 2 years.

Several investigators have attempted since 1947 to produce DDT-resistant flies by exposing successive generations of susceptible flies to DDT in the laboratory. Richard Fay and his associates of the Public Health Service exposed adult flies in partially treated stock cages to produce a strain of a rather low order of resistance in 45 generations of adults. Starting in 1946, W. V. King and the staff of the laboratory in Orlando, Fla., produced a strain of flies highly resistant to DDT by exposing 55 generations of adults to sprays of DDT solutions. At the Illinois Natural History Survey laboratory, George C. Decker and I got spectacular results by exposing both larvae and adults to DDT. We contaminated the larval media and treated the adult stock cages with near-lethal dosages of DDT solutions. In that way we could select strains highly resistant to DDT in 9 to 18 generations from the standard laboratory strain. We attempted to simulate field conditions in which barn surfaces and manure piles are treated with insecticides.

As to the nature of the trend in the acquisition of DDT resistance by the standard laboratory strain when both larvae and adults were selected by DDT treatment, it appears that the process of segregation or the initial establishment of resistance to DDT is slow, but, when resistance is once established, its intensification is rapid and proceeds to a maximum level, which is reached when the DDT found in the environment no longer acts as a selective agent.

RESEARCH RESULTS that I reported in December 1949 revealed the development of strains of flies resistant to dieldrin, chlordane, lindane, toxaphene, methoxychlor, pyrethrins, paraoxon, and a mixture of all the effective chlorinated hydrocarbons. Since that report, all of the resistant laboratory strains except the paraoxon and pyrethrins strains have reached a maximum resistance point or have risen as high as is selectively possible by the method I used. A strain that showed a threefold increase in tolerance for dieldrin in November 1949 had risen to a 2,000-fold tolerance by July 1950. The dosages needed to kill individuals of these strains far exceed those which could be applied in the control of field populations of house flies.

Three kinds of trends characterized increased tolerance or resistance in experiments in 1948 and 1949.

The DDT type of acquisition trend, in which a susceptible fly strain slowly developed characters that permitted a more rapid selection in succeeding generations, was characteristic of dieldrin, methoxychlor, and chlordane, as well as DDT.