50 THE WHITE-FLY OF GREENHOUSES 



Our experiments showed that the nymphs are readily killed by 

 the kerosene spray. Doubtless kerosene emulsion could be 

 used to equal advantage. 



Hydrocyanic Acid Gas. This was the most successful, as 

 well as the most satisfactory, remedy we tried. Statements 

 had been made that this gas could not be used to advantage 

 without injury to the greenhouse crops. To determine the 

 truth of this a considerable number of tests were made, the 

 results showing that the adult white-flies are very easily killed 

 by an exposure too short to injure the plants. The more 

 important of these experiments are recorded below : 



On July 15, 1902, a vacant section of the greenhouse con- 

 taining 2,833 cubic feet of space was stocked with tomato and 

 squash plants and a swarm of white-flies. The gas treatment 

 was given at 10 a. m., the day being clear and hot. Twelve 

 ounces of strong sulphuric acid and 12 ounces of potassium 

 cyanide were added to one gallon of water in an earthenware 

 jar. The house was kept closed for 15 minutes, at the end of 

 which period the adult white-flies were dead. The plants were 

 uninjured, except a few leaves of a lily that had been growing 

 under a greenhouse bench. In later experiments it was found 

 that the white-flies began to fall off the plants within two min- 

 utes after the gas was formed. 



Later, during the same day, the main section of the house 

 was treated. This section was filled with fruiting tomatoes 

 and cucumbers on which the white-flies were exceedinglv 

 abundant and destructive. At 4:30 p. m. the gas was made 

 by adding 6 ounces of potassium cyanide to 1 pound of water 

 to which 1 pound of strong sulphuric acid had been added. 

 Nine minutes later the house was opened, when the earth and 

 floors were seen to be covered by myriads of the tiny flies that 

 had succumbed to the deadly gas. Apparently all the adult 

 flies were killed. No plants were injured. 



On July 28 and August 12 this treatment was repeated with 

 equally successful results. 



It seems evident from these experiments that the white-fly 

 can be controlled in greenhouses by this gas treatment. The 

 frequency of treatment and the number of treatments required 

 to subdue the pests in a badly infested house will need to be 

 determined by further observations. 



