The Bean Aphis, Or Black Dolphin. Aphis Fabæ. Kirby and Spence. A. Rumicis. Linn.

Fig. XX.

1. Part of bean plant with aphides in situ; 2, male, mag.; 3, length and wing-breadth; 4, wingless female, mag.

Bean plants are often noticed to be swarming at their tops with black insects some time before they come into flower. Frequently these are so numerous as to prevent the plants from developing flowers, and if the flowers do struggle forth they produce but few beans, and these of a small stunted description. If the heads of the bean plants in fields badly attacked are examined they will be found covered with black aphides, whose beaks are thrust into the tissues of the stalks and leaves, from which they are sucking out the juice. The leaves and stems below them are covered with a viscous fluid. After a time this becomes black from the admixture of the excretions of the numerous insects. This filthy composition hinders, or absolutely checks, the respiration of the plants. With their sap exhausted by the myriad suckers, and their leaf and stem tissues choked up, the plants soon languish and die.

In the last season—1885—the crop in many bean fields was almost ruined by these aphides. The beans were few and small, and the haulm short and almost useless. A sickly odour went up from the infested plants, such as is smelt oftentimes in badly blighted hop gardens. It may be remarked here that almost every species of aphis was plentiful and unusually destructive in 1885. Plants of corn, fruit, hop, and vegetables, flowers, and page 66 shrubs, trees, and weeds were all more or less attacked and injured by their peculiar aphis pests. The circumstances of the winter and spring seasons appeared to suit their hibernation and propagation; while the weather of the spring with its more than usual variation of temperature rendered their plant food particularly pleasant to their tastes.

From the quantity of saccharine matter in the honey dew, or secretions of the aphides, it seems that a large or abnormal quantity of sugar in the composition of the sap of plants is necessary to encourage and sustain their attack. Alternations of temperature tend to increase the amount of sugar in the juices of plants. The more delicate and susceptible plants are more quickly, and in a greater degree, affected in this respect, and become infested with aphides whose progenitors have migrated from less attractive quarters. Thus the bean aphis, which is common to the dock (from whose Latin term Rumex it takes its name) as well as to the broom and furze, forsakes these plants and flies to the bean plant, and if the bean plant is in a suitable condition it remains and multiplies upon it.