Circular 21 - Inseoticides and Subsidiary Materials 
Spreaders and wetters are materials that 
are added to insecticidal sprays or dusts to 
make them spread over and wet the surfaces of 
the insects or materials to which they are 
applied and thus increase their effectiveness. 
Many wetting and spreading agents used in the 
dyeing industry have been found during recent 
years to be valuable when used with such con- 
tact insecticides as nicotine, pyrethrum, and 
derris. These materials are usually harmless 
to plants, generally available, and are sold 
under various trade names. 
Spray emulsions are mechanical mixtures 
of two liquids. One of the liquids is kept 
finely divided, at least until applied, in 
small droplets in the other by the use of a 
third substance. Tliis third substance is 
called an emulsifier. Commonly used emulsifi- 
ers include soaps, casein, sulfated alcohols, 
sulfonated oils, blood albumin, and vegetable 
guius . The emulsions most coimnonly encountered 
in insect control are of the oil-in-water type. 
The oil droplets are prevented from uniting 
through the action of the emulsifier. Emul- 
sions may be prepared by stirring or by both 
heating and agitation, aepending on the kind 
of ingredients and quantities used, method of 
mixing, and other factors. 
Insecticidal dusts are prepared in three 
ways. In one type, the active insecticidal 
agent is used in the undiluted form. The second 
type is prepared by mixing the undiluted insec- 
ticidal powder with some inert dust such as 
talc, clay, or similar material which serves to 
dilute the pov;der and act as a carrier for the 
active ingredient. The third type is prepared 
by mixing a liquid concentrated toxic agent, 
with a pov/der such as clay or talc and is knovm 
as an impregnated dust. Dusts are generally 
