June, ’18] DAVIDSON: ALTERNATION OF APHID HOSTS 291 
with a long list of hosts comprising most of our cultivated plants, and 
Aphis prunifolice , principally a pest of grains and corn. These two 
have been mentioned previously. Also there is the Clover-apple Aphis 
(Aphis bakeri Cowan) wintering on apple and summering on clovers, 
and Phorodon humuli alternating between plum and hop, chiefly 
injurious to the latter. 
In a group containing those species whose summer hosts alone are 
of economic importance there occur several species; the Potato Aphis 
(.Macrosiphum solanifolii Ashmead) is a pest of tomato, potato, egg¬ 
plant, cotton and lettuce; the Grain Aphis ( Macrosiphum granarium 
Kirby) infests grains and grasses; both these aphids migrate to roses 
in the fall in temperate climates, but in the sub-tropical zone they may 
occur the year around on their summer hosts. In California they ap¬ 
pear to feed and reproduce in the winter season both on rose and on 
the summer hosts. Aphis cerasijolioe Fitch is a species which passes 
the winter season on choke-cherry and the warm part of the year on 
grains and grasses, and may be synonymous with A. padi Linn. Aphis 
rumicis which attacks many varieties of beans is said to winter on 
Euonymus in temperate climates. The parsley Aphis (Rhopalosiphum 
caprece Kaltenbach) sometimes a pest on umbelliferous crops winters 
on willows: the Eastern Grape Aphis ( Macrosiphum illinoisense 
Shimer) has been found to migrate in the fall to Viburnum opulus , 
whereon the eggs are deposited, to be followed in spring by a return 
migration to grape. In this group belong also the woolly aphids of 
beet, apple and pear, previously mentioned. 
A third group contains those species whose winter hosts alone are 
economic plants. There are two wintering on saxifragaceous and six 
on rosaceous plants. The former are Rhopalosiphum lactucoe Kalten¬ 
bach, which migrates back and forward between Currant, Gooseberry 
and Sow-thistle ( Sonchus ), and Myzus ribis Linn, wintering on currants 
and migrating to and from Stachys and Leonurus of the Menthacece. 
The others are well-known pests of fruit trees; the Rosy Apple Aphis 
(Aphis malifolioe Fitch) migrates between apples and rib-grass and 
plantains; the Black Cherry Aphis (Myzus cerasi) between Cherry and 
Lepidium; the remaining four winter on stone fruits, the Rusty Plum 
Aphis (Aphis setarioe Thomas) passes the summer on grasses, the 
Mealy Plum Aphis (Hyalopterus arundinis Fabr.) on reeds, the Red¬ 
dish-Brown Plum Aphis (Rhopalosiphum nympheoe Linn.) on a num¬ 
ber of water plants, chiefly liliaceous, and the Green Plum Aphis 
(Aphis cardui Linn.) on thistles. 
Of the twenty-one species noted above at least fourteen, possibly 
fifteen or sixteen, are common to Europe and America. 
Strictly European economic species with alternate host habits have 
not been discussed in this paper. 
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