The Pamphlet Collection of Sir Robert Stout: Volume 14
Entomologists have not absolutely decided as to the manner in which the life of this aphis is carried on through the winter; but there is every reason to believe that it is continued by eggs deposited in the autumn by wingless females, as well as by hibernating aphides, at least in suitable winters. Wingless (apterous) females invariably produce the eggs, and wingless females are, it is believed, also the direct products of the eggs. These are important facts leading to valuable suggestions of means of prevention, for they prove almost conclusively that the egg is placed upon or near the plant upon which its depositor has been nourished. In the case of the hop plant which is cut down nearly close to the ground in September, the aphis deposits its eggs in the ground hard by, or upon the short pieces of bine that are left upon the hop stocks, and upon the bines that are subsequently carried away for litter.
The larva extruded from the egg is nearly as large as the full grown larva, or louse as it is commonly called in the hop districts (Fig. I.1), but it presents certain differences of form, and particularly in respect of its cornicles. It is very active, having an enormous beak or rostrum, which it uses at once, and if conditions of food and temperature suit it begins to propagate its species alter the extraordinary manner of reproduction of successive broods of larva; or lice, like itself, without sexual coition and influence of the male. Professor Owen writes of this as follows::-"This larva, if circumstances of food and warmth be "favourable, will produce a brood and indeed a succession of "broods of larvae like itself, without any connexion with the "male. In fact no winged males have appeared at this season. "If the virgin progeny be also kept from any access to the male "each will again produce a brood of the same number of aphides; "and carefully prosecuted experiments have shown that this pro-"creation from a virgin mother will continue to the seventh, "ninth, or eleventh generation, before the spermatic virtue of the "ancestral coitus has been exhausted." Reaumur has stated that one aphis may be the mother of 5,904,700,000 individuals during the month or six weeks of its existence. With this amazing power of increase it is not wonderful that the hop plantations are devastated, nor that the hop planters anxiously seek information, and methods of prevention and remedies against these attacks.
A general distribution of aphides is made throughout the hop plantations by means of winged females carried by the wind. These are perfect insects (Figs. I., 1,2) which are viviparous, and page 11 they appear from time to time in circumstances and from some causes not clearly ascertained, among the broods of wingless larva upon the leaves. Buckton remarks that a change takes place in the larva of the aphides. Swellings occur on the sides of the mese-and meta-thorax with which the wings of this future image or perfect insect, are developed. These altered forms constitute the pupa which often shows considerable difference both in markings and colour.
It is certain that when food begins to fail upon aphis infested plants, batches of winged aphides appear and fly away to fresh fields and pastures new.
There is a regular general migration, or movement of winged females, early in the season, between the 20th and 31st of May generally, which can hardly be caused by a failure of food, and a large migration in the autumn when all the generations of viviparous larvæ are exhausted. The male comes on the scene, always in winged form, towards the autumn and pairs with the wingless oviparous female from which coition the innumerable swarms are generated, to blight and ruin the hop crop of the next season.
Miss Ormerod is of opinion that a variety of hop aphis Aphis mahaleb also infests the hop plants equally with the sloe and damson trees, and that in blighting seasons winged females of this variety migrate from these to the hop plants. Miss Ormerod points out that these are very slight varieties of the common species, and are so similar in habit as regards injury to hop plants that for all practical purposes they may be considered one. This adds very much to the chances of blight, as damson trees are very largely cultivated in all parts of Kent, and in Herefordshire and Worcestershire, on all sides of the hop plantations.