124 BULLETIN 903, U. 8. DEPARTMENT OF AGRICULTURE. 



which vary in size, but the majority are about one-fifth of an inch 

 in diameter. They are frequently very abundant, and two or more 

 may coalesce. The bark of the root often cracks longitudinally, and 

 a chain of swellings arises from phylloxera punctures. As long as 

 these swellings remain fresh, the health of the vine is not much im- 

 paired, but as soon as they decay the vine is injured, and when thej^ 

 decay in numbers the roots are frequently destroyed, causing first the 

 stunting and subsequently the death of the vine. 



BIOLOGY. 



The grape phylloxera was named in 1855 by Fitch in America 

 from the gall-inhabiting form, and in 1868 by Planchon in Europe 

 from the root-inhabiting type. In 1870 Riley and Lichtenstein 

 proved that the two forms were two separate phases of a single 

 species; consequently, Fitch's specific name vitifoliae must be con- 

 ceded priority. 



In its native region, eastern North America, the insect has a very 

 complicated life cycle, which includes an aerial gall-inhabiting 

 form. In California the gall form has been observed only once and 

 that in the year 1884. 



The California life cycle (fig. 10), as indicated by research, is 

 much more simple than that which obtains in the East, and as far 

 as the economy of the insect is concerned, is purely parthenogenetic. 



Winter is passed in the form of the hibernant larva. Virtually 

 all hibernants are newly hatched larvae which settle down to hiber- 

 nate immediately after hatching from the egg in the autumn, but 

 a few hibernate in an older stage. Coincident with the first flow 

 of sap in spring, these hibernants commence to feed, and mature on 

 the average five and a half weeks later. The hibernant larva is light 

 brown in color, and is about one-third millimeter long and half as 

 wide. The mature hibernant is about 0.75 mm. long and 0.40 mm. 

 wide, and does not differ from the adult radicicole of any other 

 generation. On the average, it takes the hibernant six months to 

 mature, the period ranging from four and a half to seven and a 

 half months. The mature hibernant gives rise to a number of genera- 

 tions — as many as eight — of root-feeding phylloxera? throughout the 

 summer and autumn. Although somewhat arbitrary, April 15 to 

 October 15 best indicates the period of the active half-year of the 

 insect, the period October 15 to April 15 being the dormant or 

 hibernating season. 



All forms of the phylloxera are oviparous. The average number 

 of eggs per adult radicicole is about 110 and the average egg-laying 

 period about 45 days. Incubation varies with temperature and lasts 

 from 5 days in midsummer to over 30 days in December. The eggs 



