290 Appendix. 



upon prunus, the third being winged. This last is * * * called the migrant, 

 and it instinctively flies to the hop plant, which is entirely free from attack during 

 the development of the three generations upon the plum. A number of partheno- 

 genetic generations are produced upon the hop until in autumn, and particularly 

 during the month of September, winged females are again produced. This is the 

 return migrant, and she instinctively returns to the plum. Here she at once 

 settles, and in the course of a few days, according as the weather permits, produces 

 some three or more young. These are destined never to become winged, and are 

 true sexual females. Somewhat later on the hop the true winged male, and the 

 only male of the whole series, is developed, and these males also congregate upon 

 the plum, on the leaves of which toward the end of the season they may be found 

 pairing with the wingless females, which stock the twigs with the winter eggs." 



POSSIBILITIES OF WINTER TREATMENT. 



Prof. Riley suggests that if the eggs of the species upon the plum trees were 

 treated by sprays strong enough to kill them the hops could be thoroughly pro- 

 tected. The conditions that appear to exist in the hop-growing regions of Cali- 

 fornia, as detailed above, are such that this method of control does not offer any 

 promise of success. These field studies during the past two years have shown 

 that here the life-history as given by Prof. Riley is not at all followed out, since 

 ■careful search of plum trees, both near to and distant from hop yards, has failed 

 :at all times with us to disclose the presence of the hop aphis (Phorodon humuli) 

 or its eggs. We are forced to conclude that, probably because of climatic conditions, 

 this aphis has in this State a life-history much at variance with that given by 

 Prof. Riley and generally accepted as normal with the species. 



It does not seem, therefore, that in this State any treatment of trees of the 

 ■varieties of prunus (plums, prunes, green gages, etc.) for the destruction of the 

 liibernating hop aphids. or their eggs, would be of value, since they are not there 

 to he destroyed. The treatment of the soil of the hop yards during the winter 

 with the idea of destroying the possible aphid eggs might accomplish more, but 

 we do not think that there is much ground for hope of success, because of the 

 practical difficulty of making such a treatment thorough enough to be effectual. 



RESUME. 



To sum up, then, it seems tliat under our California conditions the hop aphids 

 appear first in the spring as wingless parthenogenetic females (stem-mothers) 

 upon the staminate (male) vines. They reproduce rapidly here, and occasional 

 winged individuals soon appear. These migrate to near-by vines, and the aphids 

 are thus spread through the affected yard. They confine their attack to the leaves 

 •of the vine until young liop cones are formed, and then attack these also. The 

 damage done is twofold, consisting of a reduction in the size and weight and 

 tilso a loss of value to the hops, because of the poor aroma, due to the actual 

 presence of the aphids in them. Experiments with sprays made up of mixtures 

 of kerosene emulsion and tobacco decoction, and of whale-oil soap and quassia 

 extract, were entirely effective in controlling the aphids. Other materials experi- 

 mented with were not nearly so satisfactory in their effect as were the two 

 described. The hop aphis is quite readily identified. The wingless and winged 

 parthenogenetic females are followed late in the year by wingless oviparous 

 females and winged males. These are always found upon the late-growing male 

 vines and in no other situations. From tlie evidence at hand it would seem 

 that hibernation of the aphids took place in the egg form in the hop yards. This 

 does not agree with the history of the insect elsewhere as published by Prof. Riley, 

 .and tliere seems to be no possibility of a successful winter treatment for the liop 

 ^iphis under our California conditions. Indeed, spraying with either of the washes 

 ■fdescribed in this paper so completely controls the hop aphis that the insect 

 need not be considered a menace to the crop and the necessity of winter spraying 

 as removed. 



