HOW TO PROFIT BY EXISTING CONDITIONS. 87 



been used with great value in the freeing of apple orchards in Europe 

 from the apple-bud weevil (Anthonomus pomorum). The boll weevils 

 can not pass through a 16-mesh or 18-mesh wire screen, while the 

 parasites can do so, and therefore the release of these enemies will 

 be constantly increasing the proportion of parasites against the 

 weevils. Even if a 14-mesh wire screen is used, only a small pro- 

 portion of the weevils can escape through it and some gain is effected 

 by the release of the parasites. In order to demonstrate numerically 

 just how this would happen, three hypotheses are presented. In the 

 first case squares are collected and put in a 16-mesh wire cage, and 

 in the second case squares are collected and put in a 14-mesh wire 

 cage, and in the tliird case no squares are collected. The average per- 

 centage of control which follows as a result easily demonstrates the 

 advantage which will be almost immediately gained. 



It has been contended and proven that many weevils escape 

 through the ordinary wire screen. 



I. — Given 10,000 developing stages in a 1-acre field. 



(A) Collect squares containing 50 per cent of the stages and place in 16-mesli 



screened cage 5, 000 



Normal parasitism is 5 per cent 250 



Ants and heat kill 40 per cent 2, 000 



Mortality 2, 250 



Breed 2,750 



There escape through 16-mesh screen — 



10 per cent weevils 275 



90 per cent parasites 225 



(B) There remain in the field squares containing 50 per cent 5, 000 



Normal mortality as above, 45 per cent 2, 250 



Breed 2,750 



Some weevils escaped from cages 275 



Total weevils at end of generation 3, 025 



Parasites reared 250 



Parasites escaped from cages 225 



Parasites present in field 475 



There is 1 parasite to every 6.3 weevils. 



II. — Given conditions as above, squares in 14-niesh screened cage. 



(A) From collected squares breed 2, 750 



There escape tlirough 14-mesh screen — 



40 per cent weevils 1, 100 



All parasites 250 



(B) Breed in field 2,750 



Escaped from cages 1, 100 



Total weevils at end of generation 3, 850 



