Attempts to learn the genetics of resistance show that both males and females carry DDT-resistant characters and that crosses produce what would be called physiological blends. In other words, the first hybrid generation possesses an intermediate degree of, resistance.

From the standpoint of genetics, we have little hope of eliminating resistance by crossing resistant flies with susceptible flies, because resistance probably results from several gene changes, which cannot easily be defined as dominant or recessive. The resistant flies, although well adapted to a DDT environment, however, may not be so well adapted as susceptible flies to a DDT-free environment. Along with the advantageous mutation occurring during the development of resistant stock, there are probably many disadvantageous mutations that may be reflected in a decreased rate of reproduction.

SEVERAL POSSIBLE CHANGES might make field strains of house flies more and more difficult to control : The permeability of the cuticula might change, the rate of detoxification might increase, the toxicant might repel the insect, selection might produce flies with protective habits of resting in places not ordinarily treated, and morphological changes in the dimensions of the fly might occur.

We easily can demonstrate some of these changes. Workers in Illinois found that their Multi I strain of DDT-resistant flies decomposed DDT as quickly as it entered the body to produce a nontoxic DDE [2,2-bis (p-chlorophenyl) - 1, 1 dichloroethylene], whereas susceptible strains did not. Other DDT-resistant strains of flies were found to break down only part of the DDT that enters the body of the fly. Because not all the DDT can be recovered as DDT or DDE, an unidentified fraction remains. Flies of the DDT-resistant laboratory strain from the Orlando laboratories are apparently larger than susceptible flies. There also seems be a close correlation between the rate of larval development and resistance to insecticides. Insecticide-resistant house flies grow more slowly than the susceptible flies.

Field observers have noticed changes in resting-site habits of some resistant flies. Those insects rested on floors and lower parts of buildings but not on treated ceilings and walls. Some field strains of house flies are paralyzed by DDT more rapidly than are susceptible laboratory strains, but they completely recover from the initial paralysis of DDT poisoning. The quick paralysis and complete recovery serves as a protective mechanism in the field by preventing flies from resting on walls or ceilings long enough to absorb a lethal dose of poison.

Resistance to insecticides seems to be closely correlated with the ability of the fly to degrade chemically the toxicant to a nontoxic chemical. The site of the degradation appears to be in the hypodermal layer of the body wall of the insect. We do not know the actual mechanics of the process.

WHAT IS THE SOLUTION to our problem? If we had an easy solution we would have no serious problem. A solution is needed, and to that end I offer four items for investigation and thought.

1. The usefulness of DDT and other chlorinated compounds is coming to an end in the control of house flies unless some promising chemicals can be found that will prevent the flies from chemically degrading or evading the toxic effects of them. The Public Health Service has tested a large number of DDT activators and found that 1,1-bis(p-chlorophenyl) ethanol greatly increased the effectiveness of DDT on DDT-resistant house flies. It remains to be seen whether flies will become resistant to the combination.

2. New insecticides will have to be developed that are not chemically related to the chlorinated hydrocarbons. Some of the new phosphate compounds may become useful in controlling resistant house flies now found in the field. Dilan, a nitro-paraffin derivative of 1,1-bis(p-chlorophenyl), has shown some promise in field and laboratory tests, according to Ralph March. But the usefulness of Dilan may be short-lived because Dilan-resistant flies have been developed from a multi-resistant strain in five generations.

3. Some investigators believe that field strains will revert to susceptibility in the absence of treatment. Field strains tested by workers in California and Illinois and investigators of the Public Health Service showed no loss of DDT resistance 2 years after DDT was discontinued as a residual treatment for fly control. DDT-resistant strains retained in the laboratory in the absence of DDT retained their high levels of tolerance for 30 to 5o generations. On the other hand, some investigators have reported instances of reversion of DDT-resistant flies to more susceptible strains of flies. We of the Illinois Natural History Survey have laboratory data that indicate that resistant flies may produce fewer progeny and in some cases have a longer life cycle than susceptible flies. Gordon Bender, of the University of Illinois, working with fly-muscle preparations, found that respiration rates were higher in resistant than in the susceptible flies. These two facts suggest that resistant flies may not be so well adapted physiologically to their environment and are not so efficient as susceptible flies. If so, then one can expect a gradual loss of resistance to insecticides among field strains of houseflies, as the susceptible strains literally outproduce the resistant strains. If the flies in the field lose their resistance to the insecticides, when treatments have been discontinued and residues on walls and in the soil disappear, then the chlorinated hydrocarbons will again become useful as chemicals to reduce fly populations.

4. Still greater emphasis should be placed on sanitation. Insecticides should be used only to augment the fly control obtained by strict sanitation. There is no substitute for sanitation.

W. N. BRUCE, a native of Nebraska and a graduate of the University of Nebraska, is associate entomologist for the Illinois Natural History Survey. He worked at Iowa State College for 2 years as instructor and research associate and has conducted research on insects affecting man and animals for the Illinois Natural History Survey since 1945.