250 NEW YORK STATE MLSEUM 



Value of alternate food plants. The aliility of plant lice to hold their 

 own is much increased by the habit possessed by certain species of livint,'^ on 

 alternate food plants. This means that a usually somewhat definite number 

 of generations is produced on one food plant and then migration occurs to 

 another and a second series of generations is passed followed by a return 

 migration to the original food plant. This latter may occur the same year 

 or the following season, and after the development of one or more gcnera- 

 ti(ins completes the life c\cle. This migration is untloubtedly of value to 

 the species since it affords, in the case of those completing the life cycle in 

 a single year, an opportunity Ui feed on practically fresh foliage three times 

 during the \ear. This occurs when the stem mothers hatch from the 

 winter eggs and attack the presumably vigorous foliage in early spring. 

 The second opportunity is when the insects forsake the original food plant 

 and establish themselves on the other, which presumably has not suffered 

 materially from insect e-nemies earlier in the season, and the third comes 

 after the nlurii migration, the original food plant having had an oppor- 

 tunity to rejuvenate itself while its enemies were feeding on the secondary 

 host. This change is undoubtedly of \alue and another factor is that at 

 each migration th(! aphitls esca])e fur a time; from such natural enemies as 

 hub bugs, s\rphi(l (lies and aphis lions and in the case of forms breeding so 

 rapidly as do our plant lice, this is of considerable importance and affords 

 the s|jecies an op])ortunity to reestablish itself in numbers before its 

 enemies discover it in the new location. This latter method of escaping 

 from natural enemies would in all probability be accomplished ecjually well 

 if the insects migrated simply to uninfested plants of the same species, but 

 the chances of finding such at hand are not nearly so great as are those of 

 securing an entirely different food plant practically free from aphids. 



Alternation of generations. Breeding on alternate food plants is some- 

 times accompanied by a true alternation of generations, as is exhibited in 

 two species, the life histories of which have been worked out by Mr Theo- 

 dore I'ergande of the United States Department of Agriculture, Bureau of 

 b'ntomology. The life histories of these two species is given in detail 



