20 



THE HOP APHIS IX THE PACIFIC REGION. 



FIRST APPEARANCE OF THE HOP APHIS IN THE SEASON. 



The first wingless viviparous aphides of the season at Santa Kosa, 

 Cal., at Independence, Oreg., and at Agassiz, British Columbia, 

 were invariably observed upon hop vines near shrubby growth 

 bordering a watercourse or fence, near a sheltering tree, or near 

 buildings. At Santa Rosa and Agassiz, where the writer made 

 observations during the early part of the season, the aphides were 

 most numerous near shrubbery or buildings, the numbers decreasing 



as the center of the 



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field was approached. 

 This condition is 

 shown diagrammati- 

 cally in figure 7. In 

 fact, at Santa Rosa 

 on May 30, 1911, and 

 June 6, 1912, the 

 aphides occurred in 

 numbers only near the 

 brush, trees, or build- 

 ings, the other parts 

 of the field being al- 

 most entirely free. 



FAVORABLE AND 

 UNFAVORABLE 

 CONDITIONS FOR 

 THE APHIS. 



Moderately warm, 

 moist seasons with an 

 occasional rain but 

 with little strong wind 

 are the most favor- 

 able for the develop- 

 ment of the hop aphis, 

 and the most severe infestations occur during seasons of such 



weather. 



A hot, dry wind is very unfavorable to the aphides and in some 

 sections,' when followed by dry, warm weather, will materially 

 check infestation. 



EFFECT OF HEAT. 



Some careful observations on the condition of the hop aphis after 

 continued hot weather, and especially when the hot weather was 

 accompanied by a north wind, were made at Perkins, Cal. 



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Fig. 7.— Diagram showing relative number of aphides in different 

 parts of a single hopyard, indicating the increased infestation 

 near Santa Rosa Creek (a). (Original.) (The numbers repre- 

 sent approximate counts of aphides which were present on the 

 vines June 1, 1911. On September 1 these vines were grossly 

 infested. The row nearest the creek (a) is row 1; the next is 

 row 2; and after that the numbers were taken from every fifth 

 row. Each number represents the number of aphides found on 

 the hill in that location.) 



