;[76 MISC. PUBLICATION 273, U. S. DEPT. OF AGRICULTURE 



Contact insecticides are used against small sucking insects such as 

 the aphids, scales, and bugs, and sometimes against small caterpillars 

 and leaf miners, and are applied in the form of liquid sprays or dusts 

 so as to come in direct contact with the insects. These poisons kill 

 the insects by entering the respiratory system, or by penetrating 

 through the thin body membranes, or they may destroy them by 

 plugging their breathing tubes, paralyzing the nerve centers, or 

 actually burning the insects through the caustic properties of the 

 chemicals. The young insects are much more easily killed by such 

 treatment than the older forms, Avhich often become protected or 

 more or less resistant. 



There are a large number of contact sprays and dusts, each with 

 certain advantages and special fields of usefulness. Only a few will 

 be mentioned here, since the contact sprays, while very useful in the 

 control of shade-tree pests, have not as yet been used in the forests 

 of the West. 



One of the most useful contact sprays is 40-percent nicotine sul- 

 phate diluted with water in the ratio of 1 to 800 or 1 to 1,000, to 

 which is added about 1 ounce of soap to each gallon of spray in order 

 to give good spread and adhesion. This spray is particularly useful 

 in the control of aphids and other soft-bodied insects. A typical 

 formula is — 



Nicotine sulphate, 40-percent 1 pint 



Fish-oil soap 5 pounds 



Water, to make 100 gallons 



Miscible oils, or oils that will mix with water, have come into 

 prominence in recent years as effective contact sprays for shade and 

 orchard trees, and are useful in nurseries and plantations against 

 such forest pests as the spruce bark louse, the green spruce aphid, 

 and various scales. These oils are proprietary products, sold under 

 a variety of names, and vary somewhat in strength. The Avinter oils 

 are used as dormant sprays and will injure foliage if applied when 

 the trees are in leaf. Lighter or summer oils are also available that 

 can be applied to the foliage without injury. Instructions as to 

 dilution of the oils come with the product. Fish-oil soap is usually 

 used as a spreader. 



Soap and kerosene emulsion have been used in the past, but 

 results from their use are uncertain, and they are no longer recom- 

 mended for shade-tree sprays. 



Lime sulphur is a very effective and inexpensive spray which is 

 used in the control of the armored scales. It is applied during the 

 winter or early in the spring as a dormant spray. The concentrated 

 preparation should have a density of about 30° Baume and should 

 be diluted with eight parts of water. 



Many other insecticides are available, and various combinations 

 of ingredients are used for special purposes. Some of these may be 

 very harmful to plants and dangerous to use. If special problems 

 come up requiring the use of chemical insecticides, it is best to con- 

 sult an entomologist before entering on any extensive control i^roject. 



Liquid insecticides, either stomach or contact poisons, when used 

 ■ the control of tree-defoliating insects, are applied in the form of a 



