INSECTICIDES AND REPELLENTS 41 



tions may be relatively long lasting. Supplemental pyre thrum or 

 allethrin space sprays may be necessary to provide protection at 

 dusk or night. When mosquitoes are migrating, the use of these 

 fast-acting insecticides is necessary for effective control. DDT space 

 sprays are not satisfactory under such conditions. 



Control of Adult Mosquitoes Indoors 



Liguefied-gas aerosols. — The pyrethrum-DDT aerosols now used ex- 

 tensively by civilians and the military services are modifications of 

 the aerosol bomb first developed by members of the Bureau of Ento- 

 mology and Plant Quarantine (see pp. 16-17). 



A low-pressure aerosol adopted by the militarj^ (No. 2, table 3) is 

 put up in 12-ounce dispensers. The directions for using this dispenser 

 state that 7 seconds of spraying is ample for each 1,000 cubic feet of 

 space. A valve delivery rate of 0.65 to 1.25 grams per second at 

 80° F. is specified. For its use outdoors see page 33. Repeated 

 aerosol treatments in a room provide some deposit on the walls and 

 floors, but coarse sprays are much better for residual treatments. 



Liquefied-gas aerosols in 5- or 10-pound containers may be used in 

 automatic dispensing systems in airplanes, messhalls, and ware- 

 houses. Solenoid valves at each of the outlets are activated as 

 required from a central electric switch. Automatic timing devices 

 may also be used for throwing the switch. 



Besides DDT and pyre thrum, other insecticides, especially lindane, 

 are being studied as to their use in aerosols. 



Space sprays. — Household sprays containing 5 percent of DDT in 

 deodorized kerosene are still widely used in hand spray guns for 

 killing mosquitoes and other household pests. The 5-percent DDT 

 spray (No. 23) is intended primarily for residual treatments. The 

 space sprays (No. 26) contain 1 percent of DDT, 0.1 percent of 

 pyrethrins or 0.16 percent of allethrin, and 0.8 percent of piperonyl 

 butoxide. Other space sprays (No. 27) contain 1.5 percent of DDT 

 and 2.5 percent of Lethane (type L) or 1.25 percent of Thanite 

 (typeT). 



Lindane is coming into some use as an indoor space spray. The 

 concentration used is 0.1 percent. Chlordane is not at present 

 recommended for indoor space spraying. 



For use in messhalls, warehouses, and other large spaces, electric 

 and small gasoline-powered sprayers are available. The cart- 

 mounted pneumatic sprayer developed at the Orlando laboratory 

 and used for outdoor spraying also proved highly effective against 

 stored-product insects in large warehouses. Several types of small 

 pneumatic electric sprayers and a portable electric Microsol machine 

 (fig. 10) for indoor use are available, 



Residual sprays. — The long-lasting properties of DDT when applied 

 to walls and other surfaces made possible the development of the 

 residual method of insect control. This method has. become of 

 inestimable value in malaria control throughout the world and is 

 used in areas where malaria control had not been possible because of 

 high costs or adverse physical conditions that precluded the use of 

 such methods as drainage or larvicide applications. 



The standard practice in many malarious areas is to spray the in- 



