i64 DISEASE IN PLANTS. 



The introduction of lady-birds into districts 

 infested with Coccideae and similar pests which 

 they devour, is also recorded as successful, as also 

 the importation of birds into forests plagued with 

 caterpillars. It must not be over-looked, how- 

 ever, that man's interference with the existing 

 balance of events in the natural struggle for 

 existence is occasionally disastrous, as witness the 

 results of importing rabbits into Australia, goats 

 into the Canary Islands, and sparrows in various 

 countries. Darwin's well-known illustration of the 

 inter-relations between clover, bees, field-mice, 

 and cats {Orig. of Species, 6th ed., 1876, p. 57), 

 which shows the astounding probability of the 

 dependence of such a plant on the number of 

 cats in the neighbourhood, well illustrates the 

 situation. 



Mere mention must be made of other special 

 treatments. 



Caterpillars and larger animals are often picked 

 by hand or their natural enemies e.g. birds, are 

 encouraged in forests. Locusts are caught in nets, 

 trenches, etc., and buried. Woodlice, slugs, etc., 

 are often trapped by laying attractive food such 

 as carrots and overhauling the traps daily : simi- 

 larly with earwigs. Rings of tar round tree stems 

 have been employed to prevent caterpillars creep- 

 ing up them. 



American Blight has been treated by rapidly 

 flaming the stems. Syringing with hot water has 

 also been employed for vines affected with mildew, 

 mealy bug, etc. 



