INSECT ENEMIES AND DISEASES 87 



best pump under certain conditions. For example, aphides 

 sometimes infest cabbage after the plants are too large 

 to permit the use of barrel or power sprayers, and in 

 addition, they colonize on the under sides of the leaves 

 as well as on the upper. These conditions make the 

 knapsack sprayer the most desirable pump for this work, 

 especially if used with a crooked extension rod. 



Barrel pumps are more satisfactory and less labori- 

 ous to operate than knapsack sprayers, and are most pop- 

 ular with commercial growers. Chain and sprocket 

 power machines are frequently employed. By their use 

 large areas may be covered in a day. With a good pump 

 and efficient nozzles properly placed and adjusted, the 

 work is satisfactory. Gas, compressed air and gasoline 

 engine pumps are not used extensively in commercial 

 gardening. Various powder guns on the market are 

 effective in the application of powders. 



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 and tobacco, 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 



