Why do onion flies, Delia antiqua (Diptera: Anthomyiidae), avoid midday adult eclosion?

Abstract

The onion fly, Delia antiqua Meigen, 1826 (Diptera: Anthomyiidae), pupates underground and emerges as an adult in the early morning. This dipteran insect actively avoids adult eclosion in the middle of the day using the temperature amplitude as modulator of the circadian timing system. Daytime emergence is likely disadvantageous for newly eclosed D. antiqua adults. Therefore, in this study, we investigated whether high sand temperatures during the day prevented adult emergence from the sand. First, the upper thermal limit of the activity of newly eclosed adults was determined to be approximately 42 °C. Next, the newly eclosed adults were buried in heated sand in the laboratory to examine whether high sand temperatures inhibited their emergence to the sand surface. The flies failed to emerge to the sand surface when the temperature of the sand reached 43 °C. Finally, the newly eclosed adults were buried in sand in the field at different times of the day. All flies buried in the morning hours, when the sand temperature at a depth of 5 mm remained below 42 °C, successfully emerged from the sand. However, those buried at noon, when the sand temperature exceeded 42 °C, did not emerge to the sand surface. These results suggested that the temperature of sun-heated sand during the hottest times of the day is a limiting factor determining adult eclosion timing in D. antiqua.