NATURE OF DAMAGE. 21 



It was noticed that in every case where the aphides were reduced 

 by the effect of the heat, some small ones remained and upon ma- 

 turing produced the following generation. In some cases not a 

 single full-grown aphis was found after the hot weather had ceased, 

 but many of the young aphides were present upon the vines. A 

 similar observation was made at Santa Rosa, Cal. 



FOOD PLANTS. 



Phorodon Jiumuli feeds principally upon the hop. It has, however, 

 some alternate food plants on which the sexual forms develop. (See 

 p. 14.) 



It is a common belief that the aphides found upon the shrubs and 

 trees growing near the hopyards (see PI. II, fig. 2) are hop aphides 

 and that they later migrate to the hops. Specimens of aphides were 

 taken from various plants growing near the hopyards at Agassiz, 

 British Columbia, Independence, Oreg., and Santa Rosa, Cal., and 

 identified. In no case was Phorodon Jiumuli found among the aphides 

 collected. Even though aphides may be extremely numerous upon 

 such near-by plants, they do not in the least menace the hop crops; 

 hence their destruction, from the standpoint of hop-aphis control, is 

 unnecessary. 



NATURE OF DAMAGE. 

 GENERAL EFFECT OF APHIDES UPON HOPS. 



The hop aphis injures the crops in two ways: By extracting the 

 plant juices it prevents the normal growth of the plant, and by the 

 excretion of honeydew, on which grows the black-smut fungus, 

 Cladosporium sp., it injures the quality of the crop. 



In one hopyard at Santa Rosa, Cal., May 31, 1911, several vines 

 were found which were severely infested. Later in the season these 

 vines were observed to have made little growth. The few hop cones 

 which had formed were very small, some being only slightly larger 

 than the burrs. Plate III, figure 1, shows some of these small cones 

 compared with normal cones which were taken from near-by unin- 

 fested vines. The relatively small growth of the infested vines com- 

 pared to that of the uninfested vines is well illustrated in Plate III, 

 figure 2. The vines in the foreground were severely injured by the 

 aphides, while those farther back were only slightly infested until 

 late in the season and made a very fair growth. 



Some vines that were only slightly infested were observed through- 

 out the season. These vines grew well and bore a fine crop of hops, 

 but just before the harvest the aphides entered the cones, extracted 

 their vitality, and covered the scales with honeydew, in which the 

 black-smut fungus soon established itself. These cones were so 



