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The Pamphlet Collection of Sir Robert Stout: Volume 14

Mode of Attack and Results

Mode of Attack and Results.

The hop aphis appears upon the hop plants generally about the beginning of May, and if the conditions of temperature and of the plants are favourable it propagates with astonishing rapidity. The never-ending still-beginning swarms live entirely upon the sap of the plants, and suck it up by a kind of pumping process with their monstrously long beaks, attacking first the youngest and smallest leaves of the leading shoots, which are more succulent than the older leaves. After a week or two the growth of the plants is checked and they struggle in vain to reach the tops of the poles. Their juices are exhausted by the continuous uckings of these insects, and the respiratory action of the leaves is stopped as to their under surfaces, upon which the aphides always congregate and feed, by their filth and exuvi æ, and upon their upper surfaces by the "honey dew," a peculiar glutinous sweet secretion ejected from the bodies of the aphides; this falling upon the leaves effectually prevents them from absorbing oxygen into their tissues. After this, which, as a rule, happens from three weeks to a month after the appearance of the insects, the plants give up, the leaves turn black and fall off, and all chances of a crop are lost. Heavy thunder showers often give renewed vigour to the plants at this stage by cleansing the leaves and partially restoring respiration, and this makes country folks say that lightning kills the aphides.

Sometimes it happens that aphides do not appear upon hop plants, or at least, not in dangerous numbers, until the cones are formed; in this event they get inside the cones and increase with greater rapidity than ever, blackening and disintegrating these so that they cannot be picked. This is the most feared kind of attack, as no remedies can be applied when the cones are formed, nor can the insect enemies of the aphides get at them easily.

With regard to the liability of hop plants to be blighted by aphides, there is no doubt that arrested growth and sudden checks from change of temperature predispose them, as vines and other plants are predisposed, to receive insect attacks, as well as various disorders. It is deemed expedient, therefore, by practical planters, as by practical vine cultivators, not to dress or cut the plants too early in the spring, because young shoots, especially, as Dr. Sachs says, when the parts of the plant are of small size and have a large hairy surface, as is the case with the leaves and internodes of the hop plants, are particularly liable to be injured by radiation in the clear cold mornings of the spring season.

In 1882 the advantages of late dressing were apparent. Severe white frosts in the beginning of April much injured the page 10 forward bines and made them stunted and brittle, while those dressed late and therefore untouched by the frosts, escaped injury to some extent, and in several remarkable instances were comparatively unharmed by the blight.