DIPPING AND FUMIGATION OF NURSERY STOCK 9 



control of the scale on nursery stock and up to the present time it is most 

 widely used. 



Some nurserymen prefer to dip their trees in a miscible oil rather than 

 fumigate and this method has, to a great extent, been successful. 



In fumigating with hydrocyanic-acid gas the trees are dug in the fall or 

 early spring, all excessive moisture allowed to dry from the tree and then 

 placed in an air tight box or room. The gas is generated in the room and 

 the trees are left exposed to it for from 45 minutes to one hour. Special 

 preparations must be made, such as building an air tight box or house, in 

 using hydrocyanic-acid gas. The gas is very poisonous, the chemicals are 

 costly and it is easy to make a mistake in mixing them. Under certain 

 conditions the gas is likely to injure the stock, especially the more tender 

 species. This is also true in using liquid dips. 



Hydrocyanic- acid gas first used. Hydrocyanic-acid gas has been used 

 in collecting jars for years by entomologists to kill insects, but was first 

 used for the destruction of scale insects by D. W. Coquillett in the orange 

 groves around Los Angeles, Cal. His first work with hydrocyanic- 



acid gas was in September 1886, and was carried on at this time for the 

 control of the cottony cushion scale on citrus trees. Such means as tobac- 

 co smoke, sulphur fumes, concussion from gun powder, heat, muriatic acid, 

 carbonic acid gas, chloroform, arsenic, bisulphide of carbon and other 

 fumes and gases were tried, but none was so successful as hydrocyanic-acid 

 gas. Dr. F. W. Morse of the University of California also began studying 

 the control of the cottony cushion scale in 1887 and as a result that uni- 

 versity gave to the public, in bulletin 71, the first knowledge of the use of 

 hydrocyanic-acid gas. In doing this first work, a tent was thrown over the 

 tree and the gas generated beneath the tent by putting together in one ves- 

 sel sulphuric acid, water and dry potassium cyanide. 



All of this work done in California was upon citrus trees, which were in 

 full foliage and a great deal of burning and injury resulted. However, the 

 method of using hydrocyanic-acid gas has been so well perfected that at 

 the present time it is comparatively safe to fumigate citrus trees which are 

 infested with white fly or scale. 



The California agricultural experiment station, under the direction of 

 Morse, conducted experiments with other gases as insecticides with special 

 reference to the white scale (Icerya purchasi). The following is a summary 

 of the results obtained, as set forth in bulletin 70 of the California agricul- 

 tural experiment station. 



Chlorine, carbon bisulphide, sulphuretted hydrogen, ammonia, carbon 

 menoxide, aloxic acid, carbolic acid and hydrocyanic-acid gas were tried and 

 it was found that hydrocyanic-acid gas was the only one that produced suf- 

 ficiently fatal effects as to warrant a more thoro determination of the time 

 of exposure and quantities of material which would produce the best results. 



Hydrocyanic-acid gas was not used upon deciduous trees until 1894, 

 when the San Jose scale was found upon deciduous fruit trees in Charlottes- 

 ville, Va. and Coquillett was detailed by the United States Department of 

 Agriculture to conduct experiments with hydrocyanic-acid gas on these 

 infested trees. The results of the first experiments were so satisfactory 

 that the work was continued. 



