THE MONTHLY BULLETIN. 61 



the host plant of the species. "While some species have long been known 

 to have this wonderful instinct, it is only in recent years that students 

 of this group have realized how general this habit is. Perhaps we can 

 not select a better example to illustrate this habit than the green peach 

 aphis, Myzus persicos, already referred to. This louse is generally 

 distributed in regions where the peach or plum is grown and is doubt- 

 less familiar to most growers of these fruits. Early in the season the 

 lice attack the leaves, causing them to curl and turn yellow. They 

 also attack the young peaches, causing them to shrivel and drop. A 

 few of the young of the stem-mothers acquire wings and fly away, a 

 large proportion of the third generation do the same thing, and by the 

 middle of June, as a rule, nearly every louse has developed wings, 

 1' st its appetite for the peach, and gone in search of some succulent 

 herbaceous plant which can serve it as food until about the first week 

 in September. Over seventy summer food plants of this louse have 

 been recorded about Fort Collins, Colorado, by the writer and his assist- 

 ants. Prominent among these plants are the cabbage, cauliflower, 

 rape, lettuce, tomato, potato, beet and radish. The return migrants 

 in the fall search out the pit fruits, seeming greatly to prefer the 

 peach, and upon the leaves of these trees they deposit the egg-laying 

 females. "When these are about half grown, winged males that develop 

 upon the summer hosts begin to gather upon the peach leaves also. The 

 sexes mate and later the females deposit their greenish-yellow eggs 

 about the buds of the small twigs. These eggs soon turn black in the 

 sunlight. This work of egg laying continues until the full quota has 

 been deposited or until severe freezes at night kill the egg-layers. The 

 egg-laying sexual females of this, and practically all other species of 

 our plant lice, are wingless. Here, as in the case of stem-mothers, 

 it is important that the females stay upon the plant where they are 

 born, that they may not wander to some other host upon which the 

 spring form, hatching from the egg, can not live. 



The stem-mothers and the egg-laying females of this species vary 

 from light pink to deep salmon in color, while other apterous lice of 

 the year are pale yellowish or greenish in color. One not knowing the 

 life history of this insect and its variations in form and color would 

 not suspect that these three wingless forms are of the same species. 



While this aphid has the alternating food habit, we have often found 

 the summer form living over winter in somewhat protected places upon 

 the green stems and leaves of herbaceous plants. In fact, I might say 

 that it is quite a common thing for these species of plant lice to con- 

 tinue indefinitely upon one of the food plants. This plant is usually 

 the summer host. 



The instinct to alternate food plants is doubtless of far greater 

 importance in getting away, or eluding enemies, than the simple habit 

 of flying from one food plant to another of the same kind, where it is 

 likely that a colony of the louse with its natural enemies already exists, 

 it is evidently a matter of importance, however, for some of the fall 

 brood to get back to a woody plant to deposit eggs where the opening 

 buds in the spring will furnish food for the young lice. 



Some of the other lice that are troublesome to the fruit grower and 

 that have this particular habit are: The grain aphis, Aphis avence, 

 which, in Colorado at least, is the most common green apple louse 



