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 '5 WILY ENEMY LAYING HER EGGS 



f e ., IN THE LARVA. 



farmers benefit. 



Mr. Knock mentions 



an instructive case of this sort from America, 



where the California!! orange -growers were 



almost being ruined by the depredations of 



the scale-insect, a queer little beast which you 



