Field Observations on Adults of the Wheat Bulb Fly (Leptohylemyia coarctata (Fall.)).

Abstract

A study of adult populations of Wheat Bulb Ply, Leptohylemyia coarctata (Fall.), has been carried out in the field by routine sweeping at Rothamsted. It has been observed that the males emerge slightly before the females and that the emergence period may cover at least three weeks in late June and early July. Although the number of males may exceed the number of females at first, the females predominate later in the season due to the shorter life span of the males.The numbers of flies on the wheat have been found to fluctuate appreciably throughout the day. During the first week of the emergence period the number of flies taken increased steadily throughout the day. After the date of population peak, however, the maximum numbers occurred in the crop in the very early morning and the late evening, which suggested a daily flight dispersion followed by a general or localised return of the flies to the crop. Further study of the data showed that the daily temperature rhythm was only partly responsible for this daily flight dispersion, and that there appeared to be an active return flight to the crop in the evening. Generally the males were more active than the females and did not settle so deeply in the crop.The temperature threshold for flight was observed to be 12 to 13°C. Winds up to 8 m.p.h. did not appear to affect flight activity, but higher winds, e.g., 15 m.p.h., markedly reduced flight, the flies remaining deep in the crop near ground level. Gale-force winds, however, were observed to produce a permanent depletion in the number of flies infesting Broadbalk field, indicating that the population was probably localised.Although portions of the populations dispersed fairly rapidly from the emergence sites, recaptures of radioactive flies labelled with 32P indicated that the extent of the dispersions was not very great. The females dispersed more than the males, and were influenced to some extent by the occurrence of wheat in flower. Frequently the flies were found to have congregated on the lee edge of the crop, but other preferred regions have been observed which could not be attributed to the influence of the wind.