Why is the larva of the weevil prized in India ?

Because it feeds on the pith of sago, and is eaten itself as a delicacy.

Why are weevils so destructive to wheat in granaries ?

Because of their fecundity, and the extraordinary manner in which they are produced. Thus, the female perforates a grain of wheat or barley with the jaws placed at the end of her long proboscis, and deposits a single egg within it; and when the young grub is excluded from the egg, it thus finds provided a fit dwelling and a store of proper food. The parent insect thus deposits its eggs in five or six grains every day, for several successive days. In about seven days' time, the larva is excluded from the egg, and, after feeding its accustomed time, changes into the chrysalis within the grain, and, in about a fortnight afterwards, comes forth a perfect weevil. The parent insect, after depositing its eggs in situations where there would be a supply for the sustenance of its offspring, does not die, but, according to Leuwenhoek, they live throughout the summer and winter; and they feed voraciously on the interior of the grain, both in the larva and perfect insect state. Kirby states, that a single pair of these insects may, in one year, produce above 6,000 descendants.

Why is it recommended to establish a colony of ants near a granary attacked as above?

Because the ants being continually engaged in searching for food, would soon find their way into the interior of the granary, and feed upon the larvae of the weevils.