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Jumping Plant-lice or Flea-lice — Part 2

Life History of the Hop-Plant Louse

(Phorodon humuli Schrank.)

This insect which we have just mentioned as an extremely injurious species is commonly known as the hop-plant louse and sometimes as the hop blight insect. It is a native of Europe, is a great drawback to the success of hop-growing in England, was for many years known in the hop plantations in central New York and in southern Wisconsin and has within the last ten years made its appearance in the extensive hop-growing regions in Washington, Oregon and northern California. All through the Middle and Southern States occasionally hop plants are grown in door-yards and this plant-louse is found practically every season upon these plants. The hop is an annual plant, dying down to the ground with the first frost in the autumn. It is obvious, therefore, that the plant-louse is to be found upon some other plant during the late fall, winter, and early spring. This alternate food plant as it is called is the plum. Just why plum trees are nearly always found in the immediate vicinity of hop yards is a mystery. The winter egg of the louse is found upon the plum tree usually at the base of the buds and sometimes under the scales of a bud. From these eggs in the spring hatches the first generation which is composed entirely of virgin females and the individuals of this generation are known as the stem-mothers. In two or three days after hatching, having migrated to the minute leaves bursting from the buds, they begin to give birth to living young, these also all being females. Every day of her existence the stem-mother gives birth to several young varying in number from two to seven or eight. Each of these after reaching the age of about eight days begins in its turn to give birth to living young, so that the stem-mother may live to see her grandchildren of the fourth or fifth generation. The third generation acquires wings, although all are still females. By the time the winged generation makes its appearance the hops will have begun to come up in the fields and the lice fly to the hops, settle upon the leaves, and begin to give birth to the fourth generation which is composed like the second of wingless parthenogenetic females and the same process is repeated until possibly eleven or twelve generations in all have been produced. By the time the twelfth generation makes its appearance the month of September has come and hop-picking is well along. All of the lice at this time acquire wings. Some of them may be of the twelfth generation, others through the fact that their ancestors have been late born through a series of generations may be only of the fifth generation. The first to acquire wings in the autumn are always females and these fly back to the neighboring plum trees. Later individuals of this generation and frequently all of the individuals of an additional generation on the hop are true males, the male thus making its appearance for the first and only time in the life round of the species. By the time they have developed the first issuing females will have settled upon the plum trees and will have given birth parthenogenetically as before to a generation of wingless individuals which comprise the true females--not the virgin females as before--but the true females which must be fertilized by the males. So that, by the time the winged males have developed from the hop and fly back to the plum we have this generation of wingless sexual or true females awaiting them. Impregnation then takes place, the males die, and these wingless sexual females give birth to the winter eggs.