Poisonous gases are widely used in killing insects under certain conditions. Hydrocyanic acid gas is employed in the fumigation of greenhouses and citrus trees. It is a most deadly and effective material. In Europe, fumigation with this gas is known as cyaniding and cyanization. Nicotine preparations are used extensively in greenhouse fumigation. Carbon bisulfid is employed almost exclusively for the treatment of stored grains and seeds.

Hydrocyanic Acid Gas

This gas is generated by adding potassium or sodium cyanide to dilute sulfuric acid. The gas is a deadly poison, and great care should be taken not to inhale it. One breath is fatal!

Potassium cyanide is a white amorphous salt that readily absorbs moisture when exposed to the air. Pure potassium cyanide contains 40 per cent of cyanogen (CN) by weight. When potassium cyanide (KCN) is placed in dilute sulfuric acid the cyanogen (CN) unites with the hydrogen (H) of the acid (H2SO4) to form hydrocyanic acid gas (HCN). In the preparation of this gas for fumigation purposes use a potassium-cyanide which is at least 98 per cent pure. The chemicals should always be combined in the following proportions: Potassium cyanide, one ounce; sulfuric acid, one fluid ounce; water, three fluid ounces.

Always use an earthen dish, pour in the water first, and add the sulfuric acid. When all is ready, drop in the proper quantity of potassium cyanide and retire immediately, before the gas arises. Fig. 1323 shows a device used abroad (from the "Gardening World") for dumping the cyanide (at 4) into the acid by means of a cord that extends outside the house.

Shed for the fumigation of nursery stock.

Fig. 1324. Shed for the fumigation of nursery stock.

White-Fly

The quantity of chemicals used for a given space will depend on the nature of the insects to be killed and the susceptibility of the plants to injury. This quantity is usually indicated by amount of potassium cyanide required for each 100 cubic feet of space. For treating white-fly on tomatoes in greenhouses, use one ounce to 3,000 cubic feet, letting the fumigation continue all night. The same treatment applies for cucumber. Fumigate on dry, dark nights when there is no wind. The house should be as dry as practicable and the temperature not above 60° F.

Greenhouses

No one formula can be given for fumigating with hydrocyanic acid gas the different kinds of plants grown in greenhouses, as the species and varieties differ greatly in their ability to withstand the effects of the gas. For the general run of greenhouse subjects, the practice is to use one ounce of potassium cyanide, one ounce of sulfuric acid, two ounces water, to each 2,000 cubic feet of space. The cyanide should be 98 per cent pure. Fumigate at night when there is no wind and when the plants are dry and the house cool; leave the house closed till morning, and open it up and let it air out before entering it. This applies to chrysanthemums, cinerarias, azaleas, bulbs, carnations and other common plants.

Ferns and roses are very susceptible to injury, and fumigation, if attempted at all, should be performed with great care. In cases of doubt, or when there is reason to suspect that the plants are particularly susceptible, and when one does not have definite instructions, it is well to fumigate with the weakest strength in use, and increase it in subsequent fumigations if the insects are not killed and if the plants are not injured.

Violets are very susceptible to injury from tobacco fumigation, and commercial growers therefore regularly use hydrocyanic acid gas for the control of green-fly" and "black-fly," two species of plant-lice. The latter is much more difficult to kill. For over-night fumigation from one-fourth to one-half ounce potassium cyanide to each 1,000 cubic feet is generally used. Sometimes one ounce potassium cyanide to each 1,000 cubic feet is used, the fumigation continuing only from twenty-five to thirty-five minutes This treatment is more likely to injure the plants. Violets may be injured severely by the gas without the leaves being burned. This injury consists in a weakening of the plants which defers blooming for several weeks.

Dormant nursery stock may be fumigated with hydrocyanic acid gas in a tight box or fumigating-house made especially for the purpose. Fumigating-houses are built of two thicknesses of matched boards with building-paper between, and are provided with a tight-fitting door and ventilators. The stock should be reasonably dry to avoid injury, and should be piled loosely in the house to permit a free circulation of the gas. Use one ounce of potassium cyanide for each 100 cubic feet of space, and let the fumigation continue forty minutes to one hour.

A fumigating-house is shown in Fig. 1324 (from a bulletin on "The San Jose Scale," by A. E. Stene, of the Rhode Island State Board of Agriculture and College of Agriculture). It is a house or box as nearly airtight as possible. The floor should have a movable slat grating on which the plants may be laid, some distance from the ground, to allow of circulation of the gas. The house shown in the cut is 8 feet high in front and 6 feet in rear, and the larger room contains 980 cubic feet, requiring approximately ten ounces of cyanide. The other rooms allow of smaller quantities to be fumigated. The doors opening from the outside provide quick discharge of the air when fumigation is completed.