664 INSECT PESTS OF FARM, GARDEN AND ORCHARD 



fall they return to the plum, where the winter eggs are laid. This 

 species is known to occur in Germany, England, Australia, and 

 New Zealand, and seems to be widely distributed over the United 

 States. It occurs here on plum and prune and in Europe is said 

 to infest grape, peach, apricot, and nectarine, according to Lowe. 



The Hop Plant-louse * 



This species is best known as a pest of hops (see p. 275) during 

 the summer and rarely does very serious damage to the plum, 

 though often quite abundant on it. The wingless aphides are 

 light green or yellowish green without any noticeable markings. 

 The winged forms have the same body color, with the head, thoracic- 

 lobes, and a few dashes on the abdomen black. The species 

 may be readily distinguished by the prominent tubercle which 

 projects from the head on the inside of the base of each antenna, 

 and a less prominent tubercle on the basal segment of each 

 antenna, as shown in Fig. 204. According to the studies of Dr. 

 C. V. Riley and his assistants, the third generation in the spring 

 migrates from the plum to hops in late spring and in fall winged 

 viviparous females give birth to a few young which develop into 

 egg-laying females which mate with winged males which have devel- 

 oped on hops, the winter eggs being laid on the plum and other 

 species of Prunus. In California Clarke has been unable to find 

 any evidence of the species on plum or other vegetation outside 

 of the hop yards, where he finds the true sexes occurring in the 

 fall, but no evidence of eggs. Hops are often seriously damaged 

 by l)eing reduced in size and weight and from the loss in aroma 

 due to the presence of the aphides in them. The species is of 

 European origin, where it is a well-known enemy of hops, and has 

 become widely distributed in the United States and Canada. 



Control. Where it oviposits on plum it may be best con- 

 trolled by spraying as for the other plum aphides in the spring. 

 After it becomes established on hops it may be controlled by 



* Phorodon humuli Schrank. Family Aphididoe. See page 275 above. 

 See C. V. Riley, Report U. S. Dept. Agr., 1888, p. 93; W. T. Clarke, Bulletin 

 160, Cal. Agr. Exp. Sta. 



