A FOREIGN INVASION OF ENGLAND 311 



same cycle in future. More than nine-tenths of 

 the Hessian fly grubs hatched out in America 

 are thus destroyed by parasites before they reach 

 maturity ; and it seems likely that the surest 

 way of fighting in- 

 sect plagues like the 

 Hessian fly is by 

 encouraging the in- 

 crease of such natu- 

 ral destroyers. 



At first sight, to 

 be sure, it may seem 

 improbable that man 

 could do anything 

 to " encourage " the 

 reproduction of such 

 very small creatures ; 

 but that is not really 

 so* All that is ne- 

 cessary is to keep 

 the straw in which 

 the parasitic grubs 

 abound, and so allow 

 the two hostile kinds 

 to fight it out among 

 themselves for the Na J 5 WILY KNEMY LAYING HER EGGS 



IN THE LARVA. 



farmer s benefit. 



Mr. Knock mentions 



an instructive case of this sort from America, 



where the Calif ornian orange -growers were 



almost being ruined by the depredations of 



the scale-insect, a queer little beast which you 



