An effective means of destroying insects in glass houses is by fumigating with various kinds of smoke or vapors. The best material to use for general purposes is some form of tobacco or tobacco compounds. The old method of fumigating with tobacco is to burn slowly slightly dampened tobacco stems in a kettle or scuttle, allowing the house to be filled with the pungent smoke. Lately, however, fluid extracts and other preparations of tobacco have been brought into use, and these are so effective that the tobacco-stem method is becoming obsolete. The use of hydrocyanic acid gas in greenhouses is now coming to be common, for plant-lice, white-fly, and other insects. It is also used to fumigate nursery stock for San Jos scale, and mills and dwellings for such pests and vermin as become established in them. The following directions are from Cornell Bulletin 252 (from which the formulas in the succeeding pages, and most of the advice, are also taken):--
"No general formula can be given for fumigating 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. Ferns and roses are very susceptible to injury, and fumigation if attempted at all should be performed with great caution. Fumigation will not kill insect eggs and thus must be repeated when the new brood appears. Fumigate only at night when there is no wind. Have the house as dry as possible and the temperature as near 60 as practicable.
"Hydrocyanic acid gas is a deadly poison, and the greatest care is required in its use. Always use 98 to 100 per cent pure potassium cyanide and a good grade of commercial sulfuric acid. The chemicals are always combined in the following proportion: Potassium cyanide, 1 oz.; sulfuric acid, 2 fluid oz.; water, 4 fluid oz. Always use an earthen dish, _pour in the water first,_ and add the sulfuric acid to it. Put the required amount of cyanide in a thin paper bag and when all is ready, drop it into the liquid and leave the room immediately. For mills and dwellings, use 1 oz. of cyanide for every 100 cu. ft. of space. Make the doors and windows as tight as possible by pasting strips of paper over the cracks. Remove the silverware and food, and if brass and nickel work cannot be removed, cover with vaseline. Place the proper amount of the acid and water for every room in 2-gal. jars. Use two or more in large rooms or halls. Weigh out the potassium cyanide in paper bags, and place them near the jars. When all is ready, drop the cyanide into the jars, beginning on the top floors, since the fumes are lighter than air. In large buildings, it is frequently necessary to suspend the bags of cyanide over the jars by cords running through screw eyes and all leading to a place near the door. By cutting all the cords at once the cyanide will be lowered into the jars and the operator may escape without injury. Let the fumigation continue all night, locking all outside doors and placing danger signs on the house."
In greenhouses, the white-fly on cucumbers and tomatoes may be killed by overnight fumigation with 1 oz. of potassium cyanide to every 1000 cu. ft. of space; or with a kerosene emulsion spray or whale-oil soap, on plants not injured by these materials.
The green aphis is dispatched in houses by fumigation with any of the tobacco preparations; on violets, by fumigation with 1/2 to 3/4 oz. potassium cyanide for every 1000 cu. ft. of space, leaving the gas in from 1/2 to 1 hr.
The black aphis is more difficult to kill than the green aphis, but may be controlled by the same methods thoroughly used.