REMEDIAL MEASURES. 165 



With regard to the alleged immunity from de- 

 vouring insects of certain poisonous plants, it has 

 been pointed out that Pangiuni edule, which 

 abounds in prussic acid, is infested with a grub; 

 and ivy is occasionally eaten by caterpillars. 



Another point as regards insect pests is the well- 

 known destructive effect of a cold, wet spring on 

 the young larvae. The use of cyanide of potassium 

 requires especial care, but has been described as 

 easily carried out with success in greenhouses. 



It seems probable that lady-birds, the larvae of 

 wasp-flies and lace-wings, and ichneumon-flies 

 as well as wrens can keep down aphides. 



For an example of the treatment of a complex 

 case of "chlorosis" with mineral manures, the reader 

 may consult the Gardeners Chronicle, 1899 (July), 

 p. 405. Many similar cases have been recorded, 

 but it should not be overlooked that very com- 

 plex inter-relations are here involved. 



Charlock has been successfully dealt with by 

 applying 5 lbs. of copper sulphate in 25 gallons 

 of water to each acre of land while the weeds are 

 young. 



In all these cases the guiding idea is derived 

 from accurate knowledge of the habits of the 

 insect, fungus, or pest concerned, and obviously 

 the procedure must be timed accordingly. It is a 

 particular case of the struggle for existence, where 

 man steps in as a third and (so to speak) 

 unexpected living agent. 



It is clear from our study of the factors of an 

 epidemic that one of the primary conditions which 



