Results with some chlorinated insecticides have been extremely inconsistent. Some workers reported fair results with benzene hexachloride and protection for several days with a combination of benzene hexachloride and methoxychlor or DDT. I have observed little or no protection against one species of horse fly (Tabanus abactor), however, with rather high concentrations of DDT, methoxychlor, TDE, toxaphene, chlordane, benzene hexachloride, aldrin, or combinations of benzene hexachloride and DDT, or benzene hexachloride and methoxychlor. Practically all the materials caused some mortality to the feeding flies, the greatest resulting from DDT followed by methoxychlor. These materials caused fairly high mortality of the flies for the first 5 days after they were applied. The insecticides alone proved as toxic to the flies as the combinations tested, and some workers have reported a definite reduction in horse fly populations following the use of DDT alone.

I suggest that methoxychlor be used on dairy animals and DDT on beef or range stock at a concentration of 0.5 to 1.0 percent. In areas where the fly season is relatively short, weekly applications of the insecticide should be feasible and worth while. Properly timed sprays should also be helpful even in areas where the horse fly season lasts 3 to 5 months.

Several compounds are relatively new. Neither their effectiveness against flies attacking livestock nor their toxicity to animals has been fully determined.

p-Aminophenol is used as an antioxidant for pyrethrins. It was reported to prolong greatly the residual toxicity of pyrethrins to stable flies. It has been tested in combination with pyrethrins against horn flies, stable flies, and horse flies. In laboratory, semifield, and field trials p-aminophenol was more effective than piperonyl butoxide. p-Aminophenol is apparently not highly toxic to cattle, but is known to be a photo-sensitizing agent. It stains or discolors light-colored animals if it is applied at concentrations much higher than 0.1 percent.

Tall oil was found to prolong the residual toxicity of pyrethrins in laboratory tests against stable flies. It was more effective at concentrations of 5 to 10 percent than at lower concentrations. In semi-field tests against stable flies and in field tests against horn flies it was no more effective than a pyrethrum-piperonyl butoxide combination. However, against several species of horse flies a combination of tall oil and pyrethrum gave protection lasting 4 to 5 days, with 2 or 3 days' protection afforded by pyrethrum-piperonyl butoxide combination.

Allethrin is commonly referred to as allyl cinerin, or synthetic pyrethrins. Allethrin is as toxic ( perhaps more toxic) to some insects as pyrethrins.

Against horn flies, stable flies, and horse flies, however, allethrin was considerably less effective or less toxic than pyrethrins. It showed little repellency to those insects in concentrations that normally are used for the pyrethrins.

RESEARCH ON THE CONTROL of flies attacking livestock has progressed considerably since 1945. Most progress has been made against horn flies in 1940, for example, the control measures employed against them consisted of destruction of larvae in the manure and the use of fly traps and pyrethrum oil sprays against adults. The sprays usually afforded only a few hours of protection. About 3 weeks of protection can be obtained now with one application of several different insecticides.

Less striking advances have been made on the control of stable flies and horse flies than on horn flies, but the measures employed have been greatly improved. The protection afforded by pyrethrum against these flies has been increased from a few hours to 2 or 3 days by the use of synergists, activators, and antioxidants, which increase the toxicity and duration of effectiveness of the pyrethrins.

A NUMBER OF laboratory methods have been developed for evaluating materials as insecticides and repellents.

Tests on the toxicity of materials to biting flies are made by exposing flies tc, residual deposits of the insecticides. The materials are applied to glass, wood, or wire screen as solutions, emulsions, or as wettable powders. In routine tests, for example, pyrethrins are tested at 5 to 25 milligrams to the square foot; most organic compounds are tested at 25 to 200 milligrams to the square foot.

Small wire-screen cages 3.5 inches in diameter and 8 inches high are dipped in solutions of test insecticides, and flies are exposed in the cages. The insecticide residues on glass or wood are tested by confining flies under petri dishes on treated surfaces. Exposure is usually for a relatively short period (30 minutes to 2 hours) or for a continuous 24-hour period. Knock-down and kill of the flies are recorded for a 24-hour period or longer. The materials are then retested at intervals until they are no longer toxic. The relative toxicity of insecticides to flies can be determined easily by these methods.

White mice are utilized in the laboratory as test animals for determining the toxicity of insecticides to stable flies and deer flies. The mice are sprayed with the insecticide, usually in the form of an acetone solution, and then exposed to hungry flies. Mortality of the flies is recorded 24 hours after feeding. This method enables one to make tests with a small amount of insecticide, and the work can be done in winter when no field tests can be conducted. There is also a large saving in research cost when such preliminary tests are made on mice instead of on livestock.

Materials that show promise by those methods are sprayed on cattle, usually on only one or two animals at first, depending on what is known about the insecticide. For testing, the treated animals are placed inside large wire-screen cages (8 by 10 by 7 feet), and flies are released in the cages. The animals are tested at intervals until the treatments are no longer toxic to the flies. Preliminary tests against horn flies and stable flies on cattle are made in that manner. Laboratory colonies of the flies are maintained for this purpose. In many ways the results obtained are considered more accurate than those obtained in field tests where fly populations may fluctuate considerably or disappear entirely.

The materials that prove effective are then field-tested on larger numbers of animals. For tests against horn flies, the animals are usually sprayed with 1 to 2 quarts of the insecticidal preparation. When the fly population returns to an average of 25 flies per animal, the material is considered as having failed or lost its effectiveness, and the animals are resprayed or the test is ended. The toxicity of the insecticides to stable flies, horse flies, and deer flies under field conditions is determined through observations and collection of flies as they feed on treated animals. The flies are then observed to determine whether they die later as a result of contact with the insecticide during feeding.

GAINES W. EDDY has been an entomologist in the Bureau of Entomology and Plant Quarantine since 1942. He is in charge of the research on flies meeting livestock at the Bureau's laboratory in Kerrville, Tex.