The Onion Fly (Delia antiqua (Mg.)) was bred at Wye, Kent, under insectary and laboratory conditions to obtain further information on its biology and reactions to various temperatures.

A comparison of the times of emergence of overwintering generations of Onion Fly and Cabbage Root Fly (Erioischia brassicae (Bch.)) in an insectary showed that the former emerged consistently about a month later than the latter. This supported the observations of previous workers that the Onion Fly was not active in the field until late May.

A diet containing protein was necessary for oviposition. The length of the preoviposition period was affected by temperature. Flies maintained at 25°C. laid eggs in 10–24 days, with an average of 15 days. At spring and summer laboratory temperatures (12–29°C.) the preoviposition period was 10–25 days, with an average of 15 days; and in autumn and winter (11–20°) it was 11–46 days with an average of 21 days. Flies maintained at 6–8° did not lay eggs.

Flies in captivity laid up to 123 eggs at the rate of 1–45 per day. Intervals between batches of eggs varied from 1–9 days, and the oviposition period was 7–25 days. Flies lived up to 6 days after oviposition.

The incubation period depended on the temperature. Eggs at 25°C. hatched in 2 days; at 20° in 3–4 days; at 15° in 4–6 days; and at 7–10° in 9–14 days.

The temperature also affected the duration of larval and pupal stages. At 25°C. the successive larval stadia required 2, 2 and 5–9 days, respectively. Lowering the temperature increased the time for development until at 9–11° the three larval stadia required 8–12, 8–18 and 24–37 days, respectively. At 25° the pupal stage lasted 12–17 days; at 20° it was 16–25 days, and at 15° it was from 35 days to over a year. At 25–30° the development from egg to adult was completed in 25 days; at 25° it required 28 days; at 22–27°, 30 days; at 19–24°, 33 days.

At 25°C. the Onion Fly bred continuously without diapause. When larvae were reared at temperatures below about 18° pupal development was often retarded. Pupae formed from larvae reared at 12–18° and maintained at approximately that temperature, required 94–442 days to complete their development. By raising the temperature of pupae formed from larvae reared below 18°, it was demonstrated that a true diapause had been induced. Of 54 pupae raised to 25°, 8 (15%) completed their development in the expected time (13–16 days) and 46 (85%) required 94–208 days. Of 322 pupae formed from larvae reared in an insectary in May and June at 10–18° and exposed to July temperatures of 13–20°, 37 (11%) emerged in 25–35 days and 252 (78%) required 234–294 days. Similarly, of 283 pupae formed from larvae reared at 12–21° and exposed to a temperature of 15–23°, 30 (11%) completed their development in 21–31 days while 253 (89%) required 118–341 days. Exposure to low temperatures was not necessary to terminate pupal diapause.