82 BULLETIN 903, U. S. DEPARTMENT OF AGRICULTURE. 



a layer of wet sand placed in the bottom of the jar was conducive 

 to the production of migrants. When moisture was applied peri- 

 odically to filter papers, the production of migrants was greater 

 the more frequent the applications. 



What effect, if any, temperature has upon the production of 

 migrants can not be shown except that they are produced during the 

 hottest months of the year. Contrasting the hot summer of 1913 

 with the cooler one of 1914, it was found that the production was 

 about equal each year. 



Migrants are produced in greater numbers in soils which retain 

 moisture than in those which dry out rapidly. Otherwise no further 

 influence traceable to soil conditions has been noticed. Although the 

 general behavior of phylloxera differs considerably in relation to 

 different types of soil, as between these different types the production 

 of migrants does not appear to change. 



In the season 1914, 12 vinifera vines were growing in cages. These 

 were inoculated in the spring, and six of them later treated through- 

 out the summer and autumn with fertilizers applied in liquid form 

 periodically. These fertilizers — nitrogen, potash, phosphoric acid, 

 and magnesium — were combined in a normal fertilizer and also 

 used in combinations in which one element was in marked excess. 

 The fertilized vines produced noticeably larger nymphal infestations. 

 In 1915 other potted vines were treated likewise, except that all 

 the fertilizer was mixed with the soil at the time of planting, and 

 the vines were not inoculated until a month later. In this series the . 

 number of nymphs was no greater or less on the fertilized vines than 

 on the unfertilized. 



Migrants formed part of radicicole generations 2 to 5, those of the 

 third generation being the most abundant. It was never observed 

 that any of the first generation (direct progeny of the hibernants) 

 became winged. 



NYMPHICALS OR INTERMEDIATE FORMS. 



The insects of the nymphical type are intermediate in form between 

 the winged migrant and the wingless radicicole. In their adult 

 stage they vary largely. Grassi (11) has figured and described sev- 

 eral individuals which represent stages in the variation. His speci- 

 mens varied from a type which differed only from the radicicole in 

 the possession of two or three extra eye facets and in longer append- 

 ages to one which superficially resembled a nymph in that it had well- 

 developed compound eyes and noticeable wing pads. This last type, 

 however, upon close examination, differed from the nymph as follows : 

 (l)The antennae (fig. 8; compare Avith fig. 9, antenna of nymph) 

 frequently bore two sensoria, as in the winged insect, but the basal 

 sensorium was less developed than in that form; (2) the wing pads 



