INSECTICIDES AND REPELLENTS 55 



Radio communication from ground to air for instructions to the 

 pilot is highly desirable, but if the proper equipment is not available 

 prearranged code signals with flags should be established. 



CONTROL OF FLIES 



It was thought that DDT residual treatments, which had caused 

 miraculous reductions or practical elimination of house flies around 

 military installations, dumps, stables, and dairy barns, offered the 

 final solution to the fly-control problem. However, resistance to 

 DDT began to be noticeable after several years of intensive use. 

 The degree of resistance has now become so high that DDT can no 

 longer be used successively in a large part of the United States. 

 Similar developments have been reported in other countries. Chlor- 

 dane, BHC, and dieldrin were effective substitutes for a time, but 

 tolerance to them developed rapidly, frequently in one season among 

 flies already resistant to DDT. Resistance to closely related com- 

 pounds also occurred. Fly populations soon showed a high tolerance 

 to all chlorinated hydrocarbon insecticides. 



Research was undertaken to find means of blocking or neutralizing 

 the resistant factor and to find other classes of insecticides against 

 which it is believed resistance is not readily developed. While 

 awaiting information from these studies it has been necessary to 

 return to older methods of control, such as space spraying with 

 synergized pyrethrum, trapping, use of poisoned baits, and especially 

 sanitation. However, residual and space sprays may still be effective 

 in many areas, and the methods of using them for fly control are 

 discussed briefly below. 



RESIDUAL SPRAYS 



For residual treatments on painted or papered walls in residences 

 either emulsions or refined kerosene solutions are usually employed 

 as they are less visible than suspensions. A 5-percent DDT emulsion 

 Or oil solution is applied at the rate of about 1 gallon per 1,000 square 

 feet. This dosage gives approximately 200 mg. of DDT per square 

 foot, or 2 grams per square meter. Wettable-powder suspensions 

 are also used at slightly lower concentrations. The sprays are often 

 applied to the point of runoff, and the amount required will depend 

 on the nature of the surface. Some types of surfaces may need 1 

 gallon to 750 square feet. For this rate the 5-percent spray may be 

 diluted to 3.75 percent to maintain the 200-mg. dosage. The amount 

 of spray that surfaces hold before runoff begins also varies with 

 the formulation used. 



A wettable-powder suspension is preferred for use on porous sur- 

 faces, such as masonry, clay, or rough unpainted wood surfaces, as 

 less of the DDT is absorbed in this form. However, the concentra- 

 tion of DDT that can be used satisfactorily without excessive clog- 

 ging of spray equipment is about 2.5 percent in a suspension made 

 with 50-percent or 3.75 percent in one made with a 75-percent wettable 

 powder. 



A hand-compression sprayer equipped with a nozzle producing 

 a flat, fan-shaped spray is preferred for most indoor residual spraying. 



