The Hop Aphis. 285 



THE HOP APHIS. 



{Phorodon Uumuli, Schrank.) 



By Warren T. Clarke, University of California Agricultural Experiment Station. 



In certain hop-growing sections of California the hop aphis is at times a 

 serious menace to the industry. Owners of hop yards dread the appearance of 

 these minute insects on their vines, well knowing that if they increase unchecked 

 great losses will result. Indeed, instances are known in this State where the 

 value of the hop crop has been reduced fully one-half, because of the presence 

 of the aphis. It is known and feared in all parts of the world where hops are 

 grown, and entomologists on the continent of Europe, in England, and in this 

 country have devoted much study to this pest. Studies of the hop aphis here in 

 California, however, seem to show that the results obtained in these other sec- 

 tions do not fully apply here, owing probably to climatic differences. This will 

 be more fully brought out in the course of this discussion of the insect and its 

 activities in this State. 



SPRING APPEARANCE AND DISTRIBUTION. 



Under the California conditions of climate and soil the first hop plants to 

 begin growth in the spring are those bearing staminate flowers only, the males 

 (variously called "He Hops," "Bulls,"' "Los Toros," etc.). Leaves and runners 

 appear upon these from ten days to two weeks before growth begins with the 

 female or pistillate-flowered plants from which hops are gathered, and they 

 remain green for some time after these female plants have become dry and un- 

 succulent. The staminate-flowered or male plants are scattered about the hop 

 yard usually in the proportion of one of these to from one hundred and fifty 

 to two hundred of the pistillate plants, and by them pollination of the hop is 

 accomplished. It seems that in the hop yards oithis State the aphids invariably 

 appear first upon the under side of the leaves of the male plants, and they can 

 usually, in affected fields, be seen upon them from two to three weeks before any 

 can be discovered upon the female plants. On May 2, 1903, we found upon the 

 leaves of male hop plants in the Pajaro Valley wingless parthenogenetic* female 

 hop aphids and their young. In one instance the mother aphid had clustered 

 about her seven of her offspring. The plant upon which these were found had 

 grown out about one foot and the aphid colony was upon the lowest leaf. On 

 the date in question and on the following day (May 3. 1903), a number of 

 hop plants, invariably staminate ones (for indeed the pistillate plants had not 

 yet begun to grow), were found to have aphids upon the lower leaf or leaves. 

 Inquiry developed the fact that in the yards under observation the attack had 

 always previously begun in the localities where these were found, the infection 

 spreading from these points until finally large areas of the yards were affected. 

 These starting points of the hop aphis attack in these yards in 1903 were care- 

 fully noted, and the development, and course of the trouble observed through 

 the year. In from fourteen to twenty-one days after the first wingless partheno- 

 genetic female aphids were observed, an occasional winged female, also partheno- 

 genetic, was developed. These winged aphids passed to the female hop plants and 



* Insects propogated without sexual reproduction. 



