256 



FRUIT INSECTS 



The Hop Plant-louse 



Phorodon humuli Schrank 



This notorious hop pest usually passes the winter and spring 

 on the plum. It is a native of Europe and was introduced into 

 New York about 1863, but now occurs throughout the country 

 wherever its host plants are grown. 



The black shining oval eggs are 

 deposited around the buds on the 

 terminal twigs of the plum in autumn 

 and hatch soon after the buds open 

 the following spring. The light green 

 lice cluster on the leaves and tips 

 of the tender branches, sometimes 

 severely injuring the crop. Usually, 

 there are about three generations of 

 wingless viviparous females (Fig. 225) 

 on the plum and then winged forms 

 are produced which migrate to the 

 hop, although some of the lice have 

 been known to remain on the plum 

 until cold weather. On the hop the 

 winged migrants start colonies of wingless viviparous females. 

 These forms reproduce asexually until the approach of cold 

 weather, when winged females are produced which migrate back to 

 the plum and there give birth to a small number of young that 

 become wingless egg-laying females. The latter are fertilized 

 by male migrants from the hops and then deposit the winter 

 eggs on the terminal twigs. It has been shown in California 

 that this plant-louse frequently, if not normally, remains on the 

 hop plant throughout the year. The wingless forms are a 

 uniform yellowish-green, while those having wings have black 

 markings on the thorax and abdomen. 



Fig. 225. — The hop plant- 

 louse, a wingless viviparous fe- 

 male of the third generation. 

 Redrawn after Riley. En- 

 larged. 



