Abstract
Data are given relating to the emergence, number of larvae per infested grain, larvae overwintering > 1 yr., alternative host plants, degree of infestation of wheat, and relative parasitism on the midges Contarinia tritici and Sitodiplosis mosellana as they occur on the field of permanent wheat at the Rothamsted Exp. Station. The date of emergence of the midges is correlated with the development of the wheat grain and with the harvesting-dates. Corresponding cycles of 5 yrs. occur in the abundance of both spp. Peaks in numbers of midge larvae are followed by yrs. of heavy parasitism, and yrs. of low numbers are followed by low parasitism in both spp. Likewise heavy parasitism accompanies low winter survival of midges and parasites and vice versa. The % of midge-infested grain is negatively correlated with the yield of the wheat. Manuring does not affect the intensity of midge infestation. One year''s fallowing a field reduces the infestation, but this effect disappears by the 3d successive crop. One year''s fallowing also increases the number of grains of wheat in the 1st succeeding crop and to a lesser extent in the 2d crop. During yrs. when the number of blind spikelets of wheat is high, the numbers of both actual and possible grains produced per head is low.

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