HOP APHIS. 
85 
likewise was good enough to write me the main points of his investi¬ 
gations in Kentish Hop-yards during his stay in England, which, it 
will be seen, clearly prove egg-deposit of the Hop Aphis taking place 
on Plum in autumn. 
Prof. Riley reported before the British Association as follows*:— 
“ Phorodon Humuli liybernates in the winter egg-state, this egg 
being fastened to the twigs (generally the previous year's growth) of 
different varieties and species of Prunus, both wild and cultivated. 
The egg is difficult to detect, because it is covered with particles which 
resemble the bark in colour and appearance. It is usually laid singly, 
and when freed of disguising particles is seen to be ovoid and 0*04 mm. 
long.” 
From Prof. Riley’s report it appears that the annual life-cycle is 
begun on Plum by the female, the mother of the coming tribe being 
hatched from the winter egg. She is described as stouter than the 
individuals of any of the other generation, and as having legs, horns 
and honey-tubes relatively shorter, and the tubercles between the 
horns as hardly observable. The colour pale green, with bright red 
eyes and faint dusky feet. 
Three generations are stated to be produced on the Prunus or 
Plum, of which the third becomes winged, “and instinctively abandons 
the Plum and migrates to Humulus ,”—that is to the Hop. Here 
Prof. Riley makes the important observation, “ The habit of moving 
from plant to plant after giving birth to an individual , and thus scattering 
the germs of infection on Humulus, is ivell marked in this winged generation. 
In the observations reported by Prof. Riley which were made in 
America (on growing plants and in vivaria at Washington, and checked 
by others made at the same time in Hop-yards at Richfield Springs in 
New York State) it was considered to be the case that “ during the 
development of the three Plum-feeding generations the Hop is always 
free, and subsequently, until the return of migration, the Plum 
becomes more or less fully free from infestation by this species.” 
[In the above point I incline to think that there is a difference 
between the condition of Hops in America and England, as regards 
freedom from infestation until it comes from Plum on the wing, and 
this for reasons to be presently given. —Ed.] 
The existence of many generations on the Hop during summer we 
are all well acquainted with, but during Prof. Riley’s stay in Europe, 
and more especially in England, he personally observed the point 
not previously worked out, of the autumn migration of the Hop Aphis 
* The above extracts are taken from the “full abstract of a paper read before 
Section D of the British Association for the Advancement of Science, Manchester, 
Sept. 3rd, 1887.” 
