INSECTICIDE, any agent which destroys insects. This definition includes natural as well as artificial means of control, the latter being those operated by man. The most important of the former are adverse temperatures, excessive or insufficient moisture, fire, bacteria, fungi, mites, spiders, fish, reptiles, insects and birds. The artificial controls may be grouped according to their mode of action. Two principal groups are recognized: those intended to reach the alimentary tract through which they act, and those that act through the respiratory apparatus. The former are effective only with such insects as bite off and swallow pieces of plant tissue; the latter more or less also with these, but most frequently used upon insects which suck the plant juices from beneath the punctured epidermis. Beetles and their larvæ, caterpillars, grasshoppers, the larvæ of various saw flies (e.g., the currant worm), etc., all chew their food and have been most effectively controlled by arsenate of lead, Paris green, hellebore, etc., applied to the foliage before it becomes badly infected either as a spray or as a powder. Plant-lice, plant-bugs and other sucking insects are checked best by kerosene emulsion, whale-oil soap, tobacco extract or other substances that choke the breathing-pores in the insects' bodies. Some of these sucking insects are dreaded because of their great prolificacy, their small size and resistance to treatment. Gases are often used under favorable circumstances to reach insects troublesome in stored grain, among clothes, upon plants, in greenhouses, and even upon plants in the open air—these last being covered with tents or boxes while being fumigated. Since the development and use of the lime-sulphur wash which is an effective fungicide, San José and related scale insects have been more economically and conveniently held in check than by fumigation methods, though in California and other sections where citrous fruits are grown commercially, fumigation is still popular. Various chewing insects which tunnel through the tissues cannot be controlled by sprays, and are usually beyond the reach of gases. The leaf-miners, which burrow just beneath the epidermis of leaves and green stems, have never been effectively controlled, though the adults are sometimes held in partial check by sweetened poisoned sprays. Some borers (currant-borer) can be kept in check by burning the twigs they infest, by cutting them out of their burrows (peach-borer and squash borer), the method being suggested by the nature of attack. Other chewing insects (plum curculio) are jarred into kerosene. Lastly there are various oils and greases which are used upon animals and man to destroy fleas, lice, etc. Fir-tree oil in water and carbolic acid are also similarly employed.
Formulæ and Methods of Application.— Paris green and arsenate of lead if to be used wet should be mixed with a little water to form a creamy fluid, and then added to water or Bordeaux mixture (see Fungicide), the former at the rate of one pound to 200 to 250 gallons. If to be used dry, use one part to 200 or 100 of finely sifted plaster, air slaked lime, road dust or wood ashes and mix thoroughly before applying. Arsenate of lead, the most popularly used poison, may be applied in strength varying from 4 to 10 pounds to 100 gallons of water, depending on the kind of insect to be controlled. These poisons are often sprayed on short clover or grass which is cut toward evening and spread thinly on bare ground to kill cut worms which destroy transplanted plants. A little molasses makes the bait more attractive. Hellebore may be mixed with water (1 ounce to 3 gallons) and a little glue or flour paste to increase adhesiveness. Each of these may be applied as a powder sifted on the plant, through a salt sack or blown upon them through a powder-gun. A little flour aids the sticking quality. Pyrethrum made from flower heads of certain plants of this genus is useful dry or in solution but soon loses its effectiveness. When powders are used, the plant should still be wet with dew or rain. Tobacco extract in several commercial forms is highly effective in controlling plant lice (aphis), if used before these insects have crumpled the leaves. Lime-sulphur wash, the best and most extensively used insecticide for scale insects, has also fungicidal properties. It is made by slaking and men boiling 20 pounds of quick lime with 15 pounds of sulphur in an iron kettle for one hour, then diluting with water to make 50 gallons and applying preferably while hot. There are various other formulæ. Because of the discomforts of making the mixture, commercial brands, of which there are many, are preferred by small growers of fruits. Miscible oils of many brands are very effective against San José and other scale insects. They are used at the rate of one part to 10 or 12 of water and are useful only on dormant deciduous trees when the temperature is above freezing and the trees not wet. Kerosene emulsion is made by intimately mixing a solution of hard soap (one pound to two gallons of hot water) with four gallons of kerosene, and diluting as needed for use with from 30 to 60 gallons of water. Pure kerosene and crude petroleum can be safely applied only to dormant plants, and then only upon bright breezy days, which will hasten evaporation. They have almost wholly given place to lime sulphur wash applied in winter. Whale oil soap is mixed with water (1 pound to 1 or up to 10 gallons), and applied as a wash or spray. Carbon disulphide may be used in confined receptacles where there is no danger of its inflammable fumes coming in contact with fire. An ounce is sufficient for from 50 to 75 cubic feet of air-tight space; and the exposure should be for 24 hours or longer. Hydrocyanic acid gas is prepared by adding cyanide of potassium (98-99 per cent pure) to water and sulphuric acid, one and one-half ounces for every 250 cubic feet of greenhouse; 100 cubic feet of nursery stock room and 125 feet of dwelling-house rooms, flour mills, trees, etc. Exposures may be from 30 to 60 minutes for trees, the former time being for plants in active growth, the latter for dormant ones; from 12 to 24 hours is usual for rooms, granaries, etc. Since these gases are violent poisons the greatest care should be exercised in their application. Oils and greases are merely rubbed on infested animals and man. Boiling water is effective in destroying both lice and eggs in clothing, but the clothing must be boiled for hours to destroy the eggs of the body-louse and the crab-louse. Dust, tobacco-dust, etc., are useful in poultry houses for the birds to wallow in. Carbolic soap is the favorite remedy for insects or pet animals. But with all stock, poultry, pets and man, cleanliness is the great preventive.
For information concerning insecticides and their use consult bulletins of the various State Agricultural Experiment Stations and of the Bureau of Entomology of the United States Department of Agriculture and Farmers' Bulletins of this department; also the following books: Smith, ‘Economic Entomology’ (Philadelphia 1896); Johnson, ‘Fumigation Methods’ (New York 1902); O'Kane, ‘Injurious Insects’ (New York and London 1912); Sanderson, ‘Insect pests of Farm, Garden and Orchard’ (New York 1912); Slingerland and Crosby. ‘Manual of Insects’ (New York and London 1914); Herrick, ‘Insects Injurious to the Household and Annoying to Man’ (New York and London 1914); ‘Insects of Economic Importance’ (New York 1915); Chittenden, ‘Insects Injurious to Vegetables’ (New York 1907).