106 MARYLAND AGRICULTURAL EXPERIMENT STATION. 



the trees when used in very warm weather than it is when the weather is 

 cooler. 



The very poisonous character of the potassium cyanide itself and of 

 the hydrocyanic acid gas must be strongly impressed upon those who un- 

 dertake to use this treatment for the first time. The cyanide must be 

 kept where children and animals can not get at it ; it must be kept in 

 tightly closed vessels, and must be plainly labelled "Poison." During the 

 process of treatment every care must be taken to pi-event human ])eings 

 or domestic animals from inhaling the gas. 



Judging from the experiments at Charlottesville, Va., under the 

 auspices of Mr. Co(|uillett, and from those which were conducted so 

 extensively in Montserrat on limes, even this gas treatment fails to 

 destroy all the eggs where the insects are at all thickly crowded on the 

 tree, so that a single fumigation can hardly be depended upon to be per- 

 fectly effective in extermination. I learn also from Mr. Howard tliat 

 the pear trees seem to have materially suffered in the cracking of the 

 l)ark, as a consequence of this gas treatment. 



Summer and Winter Washes. — It so hai)i)ens, also, that the 

 different insecticides vary in their efficacy according to climatic and other 

 conditions, and there is yet a wide field for careful experimentation 

 bearing on these differences. In an address recently given before the 

 American Pomological Society at its late session (Jan. 30, 1895,) in Los 

 Angeles, Cal., I commented on these facts in tlie following language, 

 which may be repeated in this connection : 



"A\'e must know the effect of a specific insecticide, or a certain mix- 

 ture, upon the vitality of a given plant at a particular time of day, at a 

 particular season, with a particular sun exposure, and under certain 

 conditions of the plant, both as to vigor and exposure. "We must know 

 how different species of plants are affected under the same conditions. 

 AA'e have much to learn yet as to the possibilities of combining a fungi- 

 cide and an insecticide. In some cases this combination may be made 

 with great advantage, in other cases it is just as clear that no advantage 

 has been derived. But what I wish particularly to call your attention 

 to, is that all these different insecticides will act somewhat differently 

 according to the varying conditions indicated, aiul that experience 

 between the states east of the Ivocky :\Iountains and the Pacific Coast is 

 more particularly confiictijig. The kerosene-emulsions, and especially 

 the whale-oil kerosene-emulsions, have proved of the utmost value in the 

 warfare against the scale-insects of the East; Avhile some of the washes, 

 especially your winter washes, Avhich have, according to the best of evi- 

 dence, given satisfaction to you, have i)roved, by contrast, of much less 



