132 INSECTICIDES, FUNGICIDES, AND WEED KILLERS. 



them from plant lice ; the acid may, however, be 'used without fear 

 on Davallia, Adiantnm, Coleus, Viola, Rosa, Dianhis, Vitis, and 

 Ly coper siciun. It is necessary to try each variety of plants to see if 

 this method of treatment can be successfully adopted against the 

 caterpillars without risk to the plants themselves. 



Experiments made in July on Adveiititious Plants. — The experi- 

 mental ground was a square with side of 50 centimetres and contained 

 young plants about fifteen days old {Mercurial is annua, Polygonum 

 aviculare, Amarantus hlitum, Borago officinalis, and Erodium cicutar- 

 iutn) ; 500 milligrammes of potassium cyanide, dissolved in 400 cubic 

 centimetres of water, say a solution of 0-125 per cent, was distributed 

 in five deep holes of 15-20 centimetres. In the morning all the young 

 plants were dead. After six days nothing remained alive but two 

 stocks of Borago and one stock of Erodium. 



Action on Fungi. — Few trials have been made. Hitchcock and 

 ('arleton immersed the uredospores of Puccinia graminis in a 0*1 per 

 cent and a O'Ol per cent solution. The first solution prevented 

 germination. The second had no effect. 



Action on Insects. — Potassium cyanide, also the prussic acid given 

 off from it in moist air, are insecticides of extraordinary power, an in- 

 finitesimal dose killing insects in a few^ minutes. Its action is more 

 energetic than that of sulphocarbonate of potash, sulphuretted hydro- 

 gen, and ammonia. It acts on insects by respiration and through the 

 stomach. Entomologists use it to kill the insects which they catch. 

 Mouillefert submitted diiferent insects to the action of this poison, 

 especially the phylloxera, which he specially examined. One gramme 

 of potassium cyanide was placed in a 250 cubic centimetre flask. The 

 following are the results obtained with the different insects introduced 

 successively into the flask : A butterfly died in four minutes ; a dragon- 

 fly in ten minutes ; an earwig in ten minutes ; a plant louse is annihi- 

 lated in less than two minutes ; a stag beetle in four minutes ; and a 

 grasshopper in less than two minutes. 



In America potassium cyanide is a specific for the destruction of 

 certain plant lice ; it is to be observed, however, that their eggs with- 

 stand doses usually sufficient to kill the adult insects. According to 

 CoquiJlet it is chiefly the Diaspines, amongst which must be counted 

 Aspidiotes, Diaspis, Lecanium, Cerojjlastes, Cochineal, which are most 

 sensitive to prussic acid. Whilst almost all insects succumb when 

 they respire a weak doze of prussic acid, there are some which are 

 indifferent to this poisonous gas : these are certain Coccinella, the red 

 spider {Acarus telarius) of Linneus {Tetranchus telarius, L.) ; the 

 Schizoncura lanigera, Hausm. (woolly aphis) ; and some flies of the- 

 family of ProctotrupidcB of the genus Alaptus. The danger of the 

 prussic acid treatment is an obstacle to its becoming general, the more 

 so as there are less dangerous substances, which, in the same conditions, 

 are of equal service. There are, however, certain cases where prussic 

 acid is superior to these products. When it is a question of destroying 

 one of those plant lice covered with a chitinous carapace, ordinary 

 insecticides which do not reach the insect must be set aside, because 

 they cannot reach it. Emulsions of petroleum, amyl alcohol, benzine,. 



