INSECT ENEMIES AND DISEASES 
87 
times infest cabbage after the plants are too large to 
permit the use of barrel or power sprayers, and in addi- 
tion, they colonize on the under sides of the leaves as 
well as on the upper. These conditions make the knap- 
sack sprayer the most desirable pump for this work, 
especially if used with a crooked extension rod. 
Barrel pumps are more satisfactory and less laborious 
to operate than knapsack sprayers, and are popular with 
commercial growers. Chain and sprocket power machines 
are frequently employed. Gasoline engine pumps are 
used extensively in some commercial gardening sections. 
Dusters, of the knapsack type and other sorts, are now 
on the market and have proved popular with many vege- 
table growers. They are time-savers, as compared with 
sprayers of similar types, are easier to handle, but usually 
use a somewhat greater amount of material. They are 
giving satisfactory results in the control of some pests. 
127. Nozzles. — For its value in spraying vegetables a 
nozzle depends upon its ability to break up solutions and 
mixtures into the finest particles. In orchard spraying 
and sometimes in garden treatment, an additional factor 
is important; namely, the ability of a nozzle to project 
the spray with the greatest possible force. The best 
known and most popular nozzles used by commercial 
vegetable growers are the “Vermorel” and the “Friend.” 
128. Insecticides. — Insecticides may be divided into 
three classes, namely: Stomach poisons, contact poisons 
and repellents. Stomach poisons are used in destroying 
insects with biting mouth parts; for example, the potato 
beetle and the asparagus beetle. Contact poisons are 
used in killing sucking insects, as aphides and the stink 
squash bug. Repellents, as lime, may not kill insect 
foes, but they may be effective as deterrents. 
129. Arsenate of lead is the most valuable of the arsen- 
ical poisons. It is a stomach poison and has three dis- 
tinct advantages over other arsenical mixtures, which 
