THE PEA APHIS WITH RELATION TO FORAGE CROPS. 9 



numbers only 15 years ago is evidence that it is of exotic origin, 

 but further evidence of the fact that it is an introduced species 

 may be obtained by a study of the origin of its host plants. 

 Either the sexual forms or eggs of pisi have been found on 

 alfalfa (Medicago sativa), M.falcata, red clover (Trifolium pratense) , 

 everlasting pea (Lathyrus sylvestris), Lathyrus angustifolius , and 

 L. latifolius. All of these, according to De Candolle, 1 originated in 

 Europe, Asia, or northern Africa; indeed, all of the known cultivated 

 hosts of pisi had their origin in one or the other of these continents, 

 and from what can be learned from other writers the uncultivated 

 host plants as well are of exotic origin. Very probably the original 

 host of pisi was one of the perennials, either Medicago sativa or 

 Trifolium pratense, or perhaps Onobrychis sativa, if the aphis occur- 

 ring on this plant should prove to be pisi. Of these three hosts 

 M. sativa is supposed to be the oldest in cultivation, for, according 

 to De Candolle, it has been cultivated for more than 2,000 years, 

 while the other two have been in cultivation less than 2,000 years. 

 From the fact that M. sativa is not universally and commonly at- 

 tacked by pisi, while T. pratense is, we can with reasonable certainty 

 assume that the latter was the original host of this legume aphidid. 

 De Candolle has shown us that T. pratense is a native of Europe, 

 Algeria, and western temperate Asia, while M. sativa is a native of 

 western temperate Asia, and Onobrychis sativa originated in temperate 

 Europe, south of the Caucasus. Speaking further of red clover he 

 says: 2 " Trifolium pratense is wild throughout Europe, in Algeria, on 

 the mountains of Anatolia, in Armenia, and in Turkestan, in Siberia 

 toward the Altai Mountains, and in Kashmir and the Garhwal." 

 Of alfalfa he says (p. 103): "It has been found wild, with every 

 appearance of an indigenous plant, in several provinces of Anatolia, 

 to the south of the Caucasus, in several parts of Persia, in Afghanistan, 

 in Beluchistan, and in Kashmir." 



From these we may assume with a fair degree of accuracy that 

 Macrosiphum pisi originated in Europe or Asia, most probably in 

 western temperate Asia or southeastern Europe. 



FOOD PLANTS. 



The pea aphis commonly feeds on the clovers — especially red and 

 orimson clover — garden, grass, Canadian field, and sweet peas, vetch, 

 and, as will be seen later, not infrequently on alfalfa. Shepherd's- 

 purse {Bursa bursa-pastoris) has been repeatedly mentioned as a 

 host, but experiments conducted by Mr. E. H. Gibson of the cereal 

 and forage crop insect investigations, and our own tests, have given 

 negative results. Further the writer has examined a number of 

 different collections of Macrosiphum from this host, invariably 



1 Candolle, Alphonse de, Origin of Cultivated Plants, p. 4e8. London, 1884. 2 Loc. cit., p. 105. 



98034°— Bull. 276—15 2 



