During August and September, the phylloxeræ invest the rootlets in countless numbers,
and are so abundant as to entirely obscure the colour of the roots, and to cause them to appear yellow from the enormous number of their minute organisms.
The effect of the attacks of the parasite upon the rootlets is to give rise in it to the formation of a number of little tumefactions or enlargements. These in course of time decay, and their destruction results in the death of the plant.
Exposure to air and sunlight acts fatally to the Phylloxera, shrivelling and drying it up. Hence its instinct of self-preservation, no less than its search after its food, leads it to bury itself beneath the surface of the soil. But, as the insect does not possess an organisation that fits it for burrowing, the character of the soil has a great deal to do in affording facilities or the reverse favorable to its existence.
If the soil be of such a nature, that it splits easily into fissures or cracks, which better lead to or serve to expose the vine roots, it will of course afford a much more easy means of access to the parasite than if it be compact or close.
Hence it is that clayey and chalk soils, from their liability to split up on the surface, afford much more congenial habitats for the Phylloxera than sandy ones, which, being dry and
closely-knit, afford a much more impenetrable barrier to the entrance of the insect, or to its subterranean movements.
a, healthy root; b, root on which the lice are working, showing the knots and swellings caused by their punctures; c, root deserted by them, on which the rootlets have begun to decay; d, d, d, lice on the larger roots, natural size; e, female pupa, dorsal view; f, winged female, dorsal view, greatly enlarged.
These statements are borne out by the fact that where the disease has shown itself it has been found to vary in extent and intensity in proportion as the soil of the vineyard is more or less clayey; and many instances are known