416 



fuiigus, though present, bemg a negligible factor in control. The 

 practical absence of the fungus, due to the summer heat and drought, 

 was a matter of small moment, since Aphid infestations at that time 

 of vear are of no commercial importance. 



Davidson (W. M.). Alternation of Hosts in Economic Aphids.— 



Jl. Econ. Entom., Concord, N.H., xi, no. 3, June 1918, 

 pp 289-294. 



True alternation of generations among Aphids implies a summer 

 host or hosts, supporting only actively feeding generations through 

 the summer and autumn, and a winter host or hosts harbouring 

 the egg or dormant stage. Frequently there is more than one simimer 

 host and these are not necessarily closely related botanically, while 

 on the other hand winter hosts are generally few, and if more than 

 one, closely related. Thus Phorodon hmnuli, Schr. (hop aphis) alter- 

 nates between hop and plum^ and Myzus cerasi, F. (black cherry 

 aphis) between cherry and Lepidkmi, a small crucifer. Again the 

 summer forms of Myzus persicae, Sulz. (spinach aphis) are equally 

 at home on lettuce (Compositae), turnip (Cruciferae) or parsley 

 (Umbelliferae) ; while Aphis rumicis, L. (bean aphis) feeds alike on 

 Leguminosae and Ohenopodiaceae in simimer. 



In many species complications arise through the insects living all 

 the year round on the summer host or hosts, a habit common to 

 all the root-inhabiting species. Thus Pemphigus betae, Doane (beet 

 aphis) normally winters on Populus, but colonies may appear on beet 

 or other roots at any time of the year ; similarly Eriosoma lanigerum, 

 Haus., and E. pyricola, B. & D., pass the winter in a dormant state 

 on elms, and in an active state on apple and pear, the normal summer 

 hosts respectively. In certain districts of California where both these 

 species abound, the spring forms of E. lanigerum (apple woolly aphis) 

 are rarely found on elm, the reverse being true of the spring forms of 

 E. pyricola (pear woolly aphis). 



In the south and south-western parts of the United States the 

 semi-tropical climate allows of Aphids feeding and reproducing through 

 the winter months, and thus several species that in the north hibernate 

 in the egg-stage on their winter hosts in the south pass the whole 

 year on their summer hosts without any pause in reproductive activity. 

 Thus Myzus persicae in the north hibernates in the egg-stage on stone 

 fruits, which suffer much damage from the resultant spring generations, 

 but which, in the south, escape injmy owing to the Aphids feeding 

 all the year round on vegetable crops and weeds without migrating. 

 Ajihis prunifoliae, Fitch (oat aphis) is another such species that winters 

 in the north on apple, l3ut in the south completes its life-cycle on 

 grasses and grains, apples in the semi-tropical zone thereby escaping 

 injury. However, owing to their viviparous reproduction throughout 

 winter, both these species occur on the summer hosts in greater abmi- 

 dance in the south than in the north, the heavier infestation of vegetable 

 and grain crops thus counterbalancing the absence of injury to fruit 

 trees. 



Both the above-mentioned species belong to that group in which 

 both the winter and summer hosts are plants of economic importance, 

 others being A. bakeri, Cowan (clover-apple aphis), wintering on apple 



