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



shade temperature of 90° F., eggs exposed to sunlight were killed 

 in 20 minutes. At a shade temperature of 76° F., 40 minutes' exposure 

 to direct sunlight killed all aphids, but when placed in the shade 

 the eggs resisted the maximum test of 60 minutes' exposure. 



It is therefore apparent that eggs can resist the sun's rays to a 

 considerable extent. The extent of their resistance to atmospheric 

 temperatures in the shade can not be estimated, though it is of course 

 greater than their resistance to direct sunlight. The eggs utilized in 

 these tests were selected at random, and therefore were in various 

 stages of embryonic development. 



Experiments with the submersion in water of active newly hatched 

 larvse are detailed under the heading " Diffusion," which follows. 



DIFFUSION OF PHYLLOXERA. 



In European countries four natural means of diffusion are recog- 

 nized: (1) By the winged insect; (2) by newly hatched wandering 

 larvae issuing from the soil; (3) by newly hatched wandering larvae 

 traveling through the soil; (4) by the gall-inhabiting form. To 

 these there should be added casual means, as follows : Cultivating 

 instruments, vine supports and picking boxes, plants between the 

 vines, man and domestic animals, water, cuttings and rooted vines, 

 phylloxerated land, and old stumps. 



DIFFUSION BY FLIGHT. 



Comparing the slower diffusion of the phylloxera in California 

 with that of certain European vine-growing sections, it was from the 

 first doubted that the winged form was a common diffusing agency, 

 in spite of the fact that its production is often abundant in California 

 vineyards on the roots of vines the second and third years after the 

 initial infestation. This doubt became strengthened by (1) lack of 

 leaf galls in nature and failure to discover winter eggs on a large 

 number of vines of different varieties known to have been infested 

 by migrants, or to have been close to vines thus infested; (2) the fact 

 that, in confinement, during five years, thousands of migrants were 

 utilized and only 72 sexual forms were secured, and, in turn, no 

 normal winter eggs. On comparing the researches of European 

 observers it is found, however, that in most cases they were unable 

 to raise the sexual forms in confinement in any numbers, so this 

 second point is inconclusive. 



Grassi (11, p. 138-148) and his colleagues demonstrated that the 

 insect which hatches from the winter egg always settles on the young 

 vine leaf and becomes the gall-making stem mother (gallicole). 

 They also found (11, p. 274-280) that there occurred a nymphlike 

 form which deposited parthenogenetic eggs from which issued root- 



