ENTOMOLOGY 369 



the cabbage, dusted as recommended above, 28 heads would have to 

 be eaten at one meal to reach this result. It is preferable, however, 

 to use other insecticides in the case of vegetables soon to be eaten, 

 and thus avoid all appearance of danger. 



INSECTICIDES FOE EXTERNAL SUCKING INSECTS (CONTACT POISONS). 



The simple remedies for this class of insects, such as soap, insect 

 powder, sulphur, tobacco decoction, etc., are frequently of value, but 

 need little special explanation. Some brief notes will be given, how- 

 ever, describing the methods of using some of these substances which 

 are easily available and will often be of service, particularly where 

 few plants are to be treated. The standard remedies for this group 

 of insects, viz., crude petroleum, kerosene, and kerosene emulsions, 

 resin washes, lime-sulphur wash, hydrocyanic acid gas, and vapor of 

 bisulphid of carbon, will be treated farther on. 



Soaps as Insecticides. Any good soap is effective in destroying 

 soft-bodied insects, such as aphides and young or soft-bodied larvae. 

 As winter washes in very strong solution, they furnish one of the 

 safest and most effective means against scale insects. The soaps made 

 of fish oil and sold under the name of whale-oil soaps are often espe- 

 cially valuable, but they are variable in composition and merits. A 

 soap made with caustic potash rather than with caustic soda which is 

 commonly used, and not containing more than 30 per cent of water, 

 should be demanded, the potash soap yielding a liquid in dilution 

 more readily sprayed and more effective against insects. The soda 

 soap washes are apt to be gelatinous when cold, and difficult or impos- 

 sible to spray except when kept at a very high temperature. 



For aphides and delicate Iarva3, such as the pear slug, a strength 

 obtained by dissolving half a pound of soap in a gallon of water is 

 sufficient. For the pea aphis as little as 1 pound of potash fish -oil 

 soap to 6 gallons has been effective. Soft soap will answer as well as 

 hard, but at least double quantity should be taken. 



As a winter wash for the San Jose and allied scale insects, whale., 

 oil or fish-oil soap is dissolved in water by boiling at the rate of 2 

 pounds of soap to the gallon of water. If applied hot and on a com- 

 paratively warm day in winter, it can be easily put on trees with an 

 ordinary spray pump. On a very cold day, or with a cold solution, 

 the mixture will clog the pump and difficulty will be experienced in 

 getting it on the trees. Trees should be thoroughly coated with this 

 soap wash. Pear and apple trees may be sprayed at any time during 

 the winter. Peach and plum trees are best sprayed in the spring, 

 shortly before the buds swell. If sprayed in midwinter or earlier, the 

 soap solution seems to prevent the development of the fruit buds, 

 and a loss of fruit for one year is apt to be experienced, the trees 

 leafing out and growing, however, perhaps more vigorously on this 

 account. The soap treatment is perfectly safe for all kinds of trees, 

 and is very effective against the scale. With large trees, or badly in- 

 fested trees, as a preliminary treatment, it is desirable with this as 

 well as other applications to prune them back very rigorously. This 

 results in an economy of spray and makes much more thorough and 

 effective work possible. The soap can be secured in large quantities 



