Aphides. 



179 



persistent visitation. As fast as the hop growers washed off 

 the insects fresh swarms appeared, and, where spraying or 

 washing," as it is called in the hop districts, was neglected, 

 the leaves turned black, and the cones were black and full of 

 filth. The hop aphis {Phorodon humuU) is distinguish- 

 able by the lowest joint of the antennae being toothed, and 

 each of the frontal tubercles is also furnished with a tooth. 

 A remarkable feature in the life history of this aphis is 

 that it migrates from trees of the Prunus tribe to the hop 

 plants in the spring, returning to the former to lay eggs 

 aipon them, in which state the winter is passed. Clouds of 

 winged aphides may be seen in September leaving the hop 

 grounds on their way to the plum, damson, and sloe trees in 

 the neighbourhood, and, in the spring, flights of winged 

 females are again seen coming from the plum and damson 

 trees. It was long ago believed that there was a migration 

 of these aphides from plum and damson trees to the hop plants, 

 and Professor Riley determined this by close observations made 

 near Maidstone in 1887. There is little doubt that besides 

 the hibernation of this insect in egg-form upon trees of the 

 Prunus tribe, there are some wingless viviparous females, 

 which hibernate in the ground close to the hop hills, if the 

 weather is not abnormally severe, and produce young 

 upon the first approach of spring. In many hop districts, 

 notably in Kent, where there are numbers of damson trees, and 

 where the plantation of these trees is gradually extending, 

 aphis blight regularly recurs, and with increased intensity^ 

 entailing enormous expense in washing the plants, which 

 •operation frequently has to be done five and even eight 

 times. 



But this hop aphis — which comes from plum and damson 

 trees in the spring and returns to them in the winter, when 

 ^g"§"s, to be seen easily with a pocket lens, are laid on 

 the twigs and branches — must not be confounded with the 

 plum aphis [Aphis primi). This insect, figured on next page,* 

 remains upon plum and damson trees throughout the year, 

 and is quite a distinct species from the hop aphis. It makes 



* Tliis figure, and those of the currant and cherry aphides, have been reproduced 

 by the kind permission of Mr. Buckton from his monograph on British Aphides, 



2 



