46 ECONOMIC ENTOMOLOGY 



to that of parasite and host might be given. Certain ants attend 

 certain plant-Hce with the object of feeding upon the nectar excreted— 

 and not of feeding upon the plant-hce as some ignorantly suppose. 

 Forbes has shown that the corn-root plant-louse is actually dependent 

 upon the brown ant, Lasitis niger americanus, for its existence, for its 

 transfer to suitable winter quarters, to suitable weeds in early spring, 

 and j&nally to the corn itself. 



In bumble-bees' nests one may often find a related bee, Psithyrus, 

 living as a guest and fed by the worker bumble-bees. This guest bee 

 is not content to live quietly in the nest; she often destroys the Bombus 

 queen and gets "the poor workers to rear her young instead of their 

 own brothers and sisters" (Sladen). 



Ants' nests or formicaries often contain a motley crew of other 

 insects, among which are rove-beetles, pill-beetles, fly larvae, small 

 crickets, thieving ants, and parasites — the majority being thieves and 

 robbers. 



Again, no satisfactory explanation has yet been given for the 

 preferences many insects exhibit in their feeding habits. As ex- 

 amples, we are at a loss to know why in some districts the wheat 

 midge does more damage to spring wheat than it does to fall wheat; 

 why the Hessian fly injures certain varieties of wheat more than 

 others; why the grape blossom midge injures the early varieties of 

 grapes most; why' the Leconte and Kieffer pears are practically 

 immune from the San Jose scale and the white peach scale; why 

 the Northern Spy apple is not troubled with the woolly aphis; why 

 the Red Dutch cabbages are free from the cabbage root maggot; 

 why the spiny elm caterpillar and the European elm scale prefer the 

 American elm to the imported English elm; why the European elm 

 saw-fly and the elm leaf beetle prefer the European elm to the 

 American; why the forest caterpillar attacks the sugar maple in 

 preference to the soft maple; why the maple scale prefers the soft 

 maple to the sugar maple; why the apple maggot is more injurious 

 to sweet and sub-acid summer varieties than to fall and winter-acid 

 varieties; why the brown mite is seldom seen in quince and apricot; 

 and why the phylloxera is more injurious to the European vine than 

 to the native American species. 



Long-continued observations show that there are "all grades of 

 association between plants and insects from most casual contact to 



