126 BULLETIN 903, U. S. DEPARTMENT OE AGRICULTURE. 



The winged form is produced from the middle of June until No- 

 vember. It is more abundant in the coastal districts than in the in- 

 terior valleys. In their first two instars the larvae of the winged 

 form do not differ from the corresponding stages of the wingless 

 form, but in the third and fourth stages they differ structurally, and 

 in these stages are termed, respectively, prenymph and nymph. Both 

 these forms are elongate in shape and are light greenish-yellow or 

 yellowish-brown. The nymphs have two pairs of grayish-black wing 

 pads. The winged insect is orange in- color with grayish-black head 

 and thorax and two pairs of scantily veined wings. 



The nymphs transform in most instances near the surface of the 

 soil and the winged migrants issue on the surface and fly about in 

 the vineyard and neighboring regions. 



The winged insects deposit eggs of two kinds, viz, male and 

 female, and the insects which mature from these eggs are the true 

 sexes. These forms are unable to take food, and under normal con- 

 ditions mate upon reaching maturity and the female forthwith de- 

 posits a single egg under the bark of the vine. This egg hatches in 

 spring and gives rise to a series of generations of gall-inhabiting and 

 gall-making wingless aphids. A certain percentage of larva? born in 

 the galls, however, migrate to the roots before taking food, and in 

 this way the species returns to the soil. 



In California, under natural conditions, it is doubtful whether 

 any sexes mature and still more doubtful whether any winter eggs 

 hatch. Laboratory experiments indicate that the sexes mature in 

 about 12 days. 



In the late autumn, along with the nymphs are found curious 

 forms intermediate in appearance between adult radicicoles and 

 nymphs. These are called intermediates or nymphicals. They are 

 not abundant and all those whose progeny have been observed were 

 parthenogenetic. 



The diffusion of the phylloxera is effected in nature by the wan- 

 dering newly hatched larva? of the radicicoles during summer and 

 autumn. These pass from vine to vine, either on the surface of the 

 soil or through subterranean cracks or pathways. They may also be 

 borne by the wind or on vineyard material, such as picking boxes. 

 Probably water is responsible for some diffusion in hilly or irrigated 

 vineyards, and cultivating instruments by picking up pieces of in- 

 fested roots may effect fresh infestations. The phylloxera is easily 

 introduced into a vineyard or section by the practice of planting 

 infested rooted vines to make up for cuttings which did not succeed 

 in previous years. 



