168 BIOLOGICAL PROBLEMS 



was selected in the neighbourhood of Manchester 

 where the pest was known to be prevalent, and in one 

 of the experiments four parallel rows of cauliflowers 

 were set, with 233 in each row. Two alternate rows 

 were protected by disks, and two were left unprotected. 

 In August of the same year it was found that of the 

 cauliflowers in the protected rows 5.1 per cent had 

 been destroyed by the root maggot, and in the un- 

 protected rows no less than 63 per cent had been 

 destroyed. In the case of the cabbages, which are 

 in this respect hardier plants, the results were not so 

 striking, but still of the protected cabbages only 0.2 

 per cent in comparison with 13.2 per cent of the un- 

 protected cabbages were destroyed. 1 



These results do not take into consideration cases 

 of partial infection in which the plants recovered from 

 the attack, and only suffered a check in their growth 

 and full development. 



In this method of control we are dealing directly 

 with the insect that causes the damage, by interfering 

 artificially with the conditions that are essential in a 

 critical phase of its life-history. But it is advisable 

 in all these cases of direct attack to study also the 

 possibility of an indirect attack by way of the protec- 

 tion or encouragement of the natural enemies of the 

 insect. Nearly all the insects that have been carefully 

 studied are known to be destroyed in large numbers 



J O 



by their natural enemies. Insects are so prolific that 

 without these checks which are provided by nature 

 the world would soon be overwhelmed with insect life. 

 And it seems very probable that the variations we 

 observe in the severity of the attacks of insect pests in 

 different seasons are due in large measure to the varia- 

 tions in the numbers of their natural enemies in the 

 preceding season. 



1 J. T. Wadsworth : Annals Applied Biology, III, 1917. 



