Development of wheat bulb fly (della coarctata fall.) larvae and pupae at different temperatures

When eggs of wheat bulb fly were added to young winter wheat plants in pots and kept at 5°, 8°. 10°, 12°, 15° and 20°, only 14%-44% of the eggs added produced larvae that succeeded in invading plants, but when newly hatched larvae were added directly 54%-92% were successful. At temperatures from 5° to 25°, the number of days required for larvae to complete their development 111 well-grown plants ranged from 11-14 days at 25° to 55-80 days at 5°. This corresponds to an accumulated temperature of approximately 286 day degrees C above the developmental threshold of 0.5°. The daily rate of development of larvae was marginally most rapid between 12° and 18°. When larvae were feeding in small, poorly growing plants, development was delayed and more shoots were needed before pupation than for healthy plants. The pupal stage which develops in the field from the end of April to June needs approximately 400 and 420 day degrees for completion in males and females (threshold 5°). Total day degrees required for the hatching of the egg to adult are about 700 (males) and 720 (females). Records of maximum and minimum temperatures from a meteorological screen I m above the ground can be used to calculate the number of day degrees accumulated from 1 May to I2 June, from which the dates of peak emergence of wheat bulb fly can be predicted. If 350 day degrees or more accumulate during this period, the peak emergence occurs near 20 June and with 200 or fewer day degrees it occurs near 11 July. The amount of damage to wheat by wheat bulb fly larvae in 1953, 1954, 1965 and 1966 depended largely on the number of eggs laid, the date of sowing, and also on the rate at which temperature accumulated in the autumn and winter; in all years, late sown crops would have had little opportunity to grow beyond the susceptible stage by the time they were attacked.