228 



their larval and adult stages, and Syrphid flies, while many species 

 are destroyed in countless numbers by Hymenopterous parasites, 

 of which the Braconids are the most important. The gall-forming 

 species, such as Eriosoma americanum on the elm and E. lanigerum 

 on the apple, are destroyed by Capsids. 



The green peach aphis, Myzus persicae, is generally distributed 

 wherever peaches or phims are grown. Early in the season it attacks 

 the leaves, causing them to curl and turn yellow, and also injures the 

 young peaches, which shrivel and drop. A few of the young stem- 

 mothers acquire wings and migrate, a large proportion of the third 

 generation doing the same, and usually by the middle of June nearly 

 every individual has developed wings and gone in search of some 

 herbaceous plant on which it can feed until about the first week of 

 September. Of about seventy food-plants recorded in comiection 

 with this insect in Colorado, the following are the most important : 

 cabbage, cauliflower, rape, lettuce, tomato, potato, beet and radish. 

 In the autmnn the migrants return preferably to the peach, on 

 the leaves of which the egg-laying females are deposited. By the 

 time these are about half-grown, the winged males that have developed 

 on the sunmier host also appear on the peach leaves. The summer 

 form of this species is sometimes found living over the winter in pro- 

 tected places on the green stems and leaves of herbaceous plants, 

 usually its summer hosts. The migration to a summer host of a 

 different kind from the winter one is useful to the species as a means 

 of eluding natural enemies, and the return to a woody plant for the 

 deposition of eggs is of great importance in the provision !.>f food for 

 the young individuals in early spring. 



Other Aphids which have this migratory habit, and which are 

 especially troublesome to fruit-growers, include : the green aphis 

 (Aphis avenae), which is common on apples early in the season, migrat- 

 ing to oats for the summer ; the rosy aphis (Aphis sorhi), which migrates 

 to plantains for the summer ; the clover aphis (Aphis bakeri), which 

 hatches on apple and thorn twigs and then goes to the clover, especially 

 red clover, for the remainder of the year ; the powdery plum and 

 prune aphis (Hyalopterus arundinis), which is abundant on the leaves 

 of these trees in early summer and then migrates to coarse grasses, 

 especially the reed grass (Phragmites sp.), on which it remains until 

 the autumn, when it returns to plums. 



Some of the species attacking shade trees and ornamental shrubs 

 also have migratory habits. The Aphid causing the leaf- cluster 

 gall on the American elm, when transferred to the apple, produces 

 colonies indistinguishable from those of the woolly aphis, Eriosoma 

 lanigervm, which have arisen from hibernating forms on the apple. 

 Though the woolly aphis is able to live fr-om year to year without 

 receiving migrants from the elm and apparently thrives in regions 

 where the elm is not grown, there seems to be a tendency to migrate 

 between these hosts to some extent. The snowball aphis (Aphis 

 viburnicola) migrates in the second generation to miknown hosts, and 

 returns in September to the snowball. Chermes cooleyi migrates 

 from the blue and Engelmann spruces to the Douglas fir in July, 

 while the form C. cooleyi var. coweni, which develops on the leaves 

 of the Douglas fir, returns to the blue and Engelmann spruces in the 

 same month, becoming the hibernating stem-mothers [see this Review, 



