2 BULLETIN 774, IT. S. DEPARTMENT OF AGRICULTURE. 



almonds are attacked also. In Europe grape, peach, and nectarine 

 are also attacked (Lowe). 



From the deciduous- fruit hosts mentioned the mealy plum aphis 

 migrates in early summer to plants of the genera Phragmites, 

 Typha, and Arundo, and in the late fail there is a return migration 

 to the fruit trees. This is the normal process of migration. Occa- 

 sionally the writer has seen generations of aphids persist on plum 

 until September, a habit that Lowe (3) records as not unusual in 

 New York State. 1 The migratory forms are winged aphids and 

 there are strong indications that they traverse long distances in pass- 

 ing from host to host, as summer colonies have been found in June 

 manv miles from the nearest winter hosts. Vast numbers of migrants 

 are produced both in spring and fall, and the production of consid- 

 erable numbers of summer migrants serves to distribute the species 

 among the summer hosts. In California Phragmites and Typha are 

 the known alternate hosts. 



INJURY. 



In the early summer months the aphids occur on the foliage, often 

 crowding together in great quantities. (PL I.) The lower surface of 

 the leaf is the preferred location, but the petioles and upper surface 

 are frequently infested. The young fruit is less common!}- attacked. 

 The infested leaves are generally curled and discolored and glisten 

 with honey dew deposits. (PL II.) The ground beneath the tree is 

 often sprinkled with the whitish shed skins dropped by the aphids. 

 The combined effect of myriads of aphids feeding simultaneously on 

 the tree produces fruit of small size and an early drop. In the years 

 1915 and 1916 an unusual midsummer apical cracking of green 

 prunes developed in California. Morris (4), after making observa- 

 tions in the Santa Clara Valley in 1915, opined that this cracking 

 was due to aphis action. In 1916 the writer made some observations 

 in Contra Costa County. He found that whereas on the whole crack- 

 ing was more general on trees that previously in the season had been 

 heavily infested with the aphis, it occurred also in other cases on trees 

 which had escaped infestation. He could not conclude otherwise 

 than that the aphis was not more than a contributing, or at least not 

 the sole cause of the apical cracking in the prunes. 



1 Blake- (1), conducting his observations at Redditch, England, found no certain mi- 

 gration from the winter hosts, the aphids remaining the year around on fruit trees. In 

 this connection it is of interest to note that he found the active cycle (from hatching of 

 stem mother to oviposition) to extend in England from the beginning of May to the 

 middle of October, 5.5 months, whereas in California the writer found the cycle to cover 

 a period of 9 months (Mar. 1 to Nov. 30). The longer growing period enjoyed by the 

 trees in California is of course responsible for this condition, and it is possible that the 

 much dryer climate of California is concerned in the summer migration of the species to 

 plants of a semiaquatic nature. The writer has observed that the aphids tend to remain 

 on the fruit uves later into the suniiner in the more humid than in the more arid 

 localities of California, while the occurrence on May 15 of large colonies on a summer 

 host at Salton Sea (a very arid region ) suggested that the aphids were living on this 

 host the year around. 





