250 NEW YORK STATE MUSEUM 



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

 own is much increased by the habit possessed by certain species of living 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 genera- 

 tions completes the life c\'cle. This migration is luidoubtedly of value to 

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

 a single year, an o]:)portunity to feed on practically fresh foliage three times 

 during the year. 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 enemies earlier in the season, and the third comes 

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

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

 ht)st. This change is undoubtedly of value and another factor is that at 

 each migration the a|)hids escape for a time from such natural enemies as 

 ladybugs, syrphid Hies 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 species an opportunity 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 equally 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 Pergande of the United States Department of Agriculture, Bureau of 

 Entomology. The life histories of these two species is given in detail 



