2 



plants than other forms. Attention, therefore, was required on 

 this point in order to avoid injury to the plants. 



In the use of arsenicals for the destruction of insects it is 

 necessary to get a finely divided condition of the arsenical, 

 which is at the same time relatively insoluble. The small undis- 

 solved particles then remain on the surface of the leaves until 

 washed off or eaten by insects. Soluble arsenicals are extremely 

 injurious to vegetation and, therefore, have to be avoided in 

 spraying useful plants. There is thus a complete contrast in the 

 ideas aimed at in spraying for insects and in spraying to kill 

 vegetation. Arsenical insecticides must be as insoluble as it is 

 possible to obtain them, while for killing weeds the most soluble 

 form of arsenic is desired. 



CHEMICALS USED FOR WEED DESTRUCTION. 



In experiments previously carried on at this Station 1 carbon 

 bisulphide was applied to a number of weeds which grow in the 

 garden and fields. This chemical was found to be very effective 

 in the destruction of some of our hardiest weeds, both of herba- 

 ceous and shrubby nature. When applied on a large scale, how- 

 ever, and in long-continued operations, there is some danger 

 from the injurious effects of the fumes of carbon bisulphide upon 

 the workmen and the chemical itself is rather expensive. It 

 seemed, therefore, desirable to carry on experiments with other 

 chemicals which could be applied more easily and at less ex- 

 pense. Arsenite of soda has been extensively used on the main- 

 land of the United States, in Australia and elsewhere for the 

 destruction of a variety of weed plants. Stone 2 found arsenite 

 of soda exceedingly effective against herbaceous weeds when used 

 at the rate of one part in 66 parts of water. Gillette 3 in using 

 arsenicals against insects found that the soluble portion of the 

 insecticides exercised a much greater burning effect upon the 

 foliage in the presence of dew and direct sunlight. Ivilgore 4 



1 Haw. Sta. Press Bui. No. 25. 



2 Mass. Rept. 1908, pt. I, pp. 62-72. 



3 Iowa Sta. Bui. 10, p. 419. 



4 N. C. Sta. Bui. 77b. 



