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 
alfidfa (Medicago sativa), M.falcata, red clover (Trifoliuin lyratense), 
everlasting pea {Lathyrus sylvestris), Latliyrus angustifolius, and 
L. latifoJius. All of these, according to De Candolle/ originated in 
Europe, Asia, or northern aVfrica; indeed, all of the kno^^^l cultivated 
hosts of 2^si 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 
Trifoliuin pratense, or perhaps OnohrycMs sativa, if the aphis occm-- 
ring on tliis plant should prove to be pisi. Of these three hosts 
M. sativa is supposed to be the oldest in cultivation, for, according 
to De Candplle, 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 21. sativa is a native of 
western temperate Asia, and OnohrycMs sativa originated in temperate 
Europe, south of the Caucasus. Speaking further of red clover he 
says:- " Trifolium pratense is wild tliroughout Europe, in Algeria, on 
the mountains of Anatoha, 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 \\ald, vdili every 
appearance of an indigenous plant, in several provinces of Anatoha, 
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 m Europe or Asia, most probably in 
western temperate Asia or southeastern Europe. 
FOOD PLANTS. 
The pea apliis 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 uifrequently on alfalfa. Sheplierd's- 
purse (Bursa hursa-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 o^^^l tests, have given 
negative results. Further the writer has examined a number of 
diftVrent collections of Macrosiphum from this host, invariably 
1 CandoUe, Alphome de, Origin of Cultivated Plants, p. ^tN. I^ondon, 1S.S4. 
98034°— Bull. 276—15 2 
* Loc. cit.. p. 105. 
