8 INSECTICIDES, FUNGICIDES, AND WEED KILLERS. 



Traps fulfil the same purpose as shelters, they draw the insects to 

 a certain point and render them more accessible to destruction. When 

 a polyphagous (omnivorous) insect has a marked predilection for a 

 plant," trap-plants are sown between the lines of the crop. The insects 

 prefer to seek the trap-plant, on which it is easy to collect them or 

 destroy them by energetic chemical means, destroying both the insects 

 and the trap-plant at the same time. The larvae which ravage the 

 plants in the soil may be destroyed by analogous methods. Fleshy 

 roots or tubers are buried in the soil between the rows of the 

 cultivated plant which are removed when the parasites have chosen 

 a domicile there. Nematodes and grey worms, which are polyphagous 

 but have a marked predilection for certain tubers, are destroyed in 

 this way.'^ Lantern-traps are used to destroy winged nocturnal para- 

 sites, butterflies, and coleoptera. They form luminous fires which 

 attract the moths at night. Acetylene lamps fitted with a reflector 

 and surrounded with a plate coated with birdlime retains the nocturnal 

 visitors. This process is used in viticulture in which it helps to 

 lessen the Pyralis and the Cochylis. Therapeutical surgery is there- 

 fore chiefly used to combat animal parasites. 



Chemical Treatment. — Cryptogamic diseases require chemical 

 ti'eatment, for it is a case of overcoming organisms so infinitely little 

 that the eye can often only see them with difficulty. 



Curative Treatment. — The chemical treatment consists in placing 

 the parasites in contact with substances which have an injurious 

 effect on them. 



Insecticides are used to kill insects ; ayiticryptogamics or fungicides 

 to combat parasitic fungi. To get the best results from the use of 

 chemical reagents, it is desirable to know the properties of the curative 

 agents and the right method of using them. 



Examination of Curative Agents. — The chemical products 

 utilized in the struggle against parasites ought to respond to the 

 following different requirements : (1) To destroy the pal-asite or. arrest 

 its evolution. (2) To be more poisonous to the parasite than to the plant. 

 (3) To preserve their poisonous properties for a certain time and to 

 adhere sufficiently to the organ of the plant. (4) To enter into inti- 

 mate contact with the parasites or their elements of propagation. 



Action of Chemical Products on Parasites. — Most of the 

 chemical agents employed against parasites act chemically on their 

 vital substance. The most active are in general those which form 

 inei^t derivatives with it, which precipitate the albumen or which 

 modify the plasma ; such are corrosive sublimate, formol, copper salts, 

 phenols, etc. They thus arrest, temporarily or definitely, the evolution 

 of parasites or their elements of propagation. In the case of bacteria 

 the phenomena of intoxication may be more easily observed. It is 

 then observed that their evolution and reproduction are arrested by 

 the formation of an inert layer around them. It suffices often by 

 prolonged washing of these bacteria with appropriate liquids to re- 

 move the immobilizing layer and to allow them to resume under normal 



Translator's Note. — As many as 150 wire-worms have been trapped by rape 

 cake, etc., 2-3 inches underground close to one hop hill by Whitehead. 



