THE BALANCE OF NATURE. 139 



by that dpstructive little insect, "the tly," a kind 

 of green blight or a[)liis, whenever they adjoin u 

 garden with plum-trees ; and late researches liave 

 conclusively shown that the insect in question 

 passes its early larval stnge on the leaves of jduins, 

 and oidy later on in life takes to preying on the 

 ripening hop-vines. Thus, strange as it may 

 seem, the price of beer in England is in the last 

 resort unfavorably affected by the number of 

 plum-trees in Kent and Surrey. The year after a 

 wet summer, in fact, is particularly bad for the 

 liop-gardens ; fly then usually abounds in prodig- 

 ious numbers. And the reason is that a wet 

 summer prevents lady-birds from laying their eggs 

 in peace and quiet; and it is the grub of the 

 lady-bird that chiefly kee[)S down " the fly," on 

 which it feeds as its natural prey. Once more, 

 that gaudy, yellow weed, the charlock or wild 

 mustard, that often makes golden the wheat-fields 

 on slovenly American farms, appears to be inju- 

 rious enough in its own way to the wheat; but 

 why on earth should it be accounted dangerous 

 to the clean-kept turnip-fields? Simply because 

 charlock is the native food-plant of the dreaded 

 turnip-fly, which spreads from the little patches 

 on the border of the fields to the long rows of 

 neighboring turnips. Just so the terrible ''[)()tato- 

 bug," or Colorado beetle, the pest and horror of 

 American farmers, fed originally on a wild weed 



