Edaphic factors such as soil temperature and moisture influence soil-dwelling insects, whose most vulnerable stages typically are eggs and young larvae. In this study, the survival of eggs and first-instar larvae of the cabbage maggot, Delia radicum L., was measured under laboratory conditions after exposure to a range of soil temperatures and moistures. When eggs were exposed to constant temperature (20–29°C) and humidity (5–200% [wt:wt]), temperature had no significant effect on survival, whereas humidity <25% [wt:wt] caused egg mortality. The gradual exposure of eggs to high temperatures resulted in low mortality below 33°C, but <5% of eggs survived at 40°C. When first-instar larvae were exposed to constant temperature (17–29°C) and humidity (5–100% [wt:wt]), both factors as well as their interaction had a significant effect on larval survival, which was nil at 5% (wt:wt) for all temperatures but increased from 21.9 to 42.8% at 17°C and from 34.1 to 55.0% at 29°C, for soil moisture contents of 15% and 100% (wt:wt), respectively. Eggs of D. radicum are resistant to low soil moisture and high temperature conditions. Larval survival tends to increase with an increase in soil temperature and moisture. It is suggested that soil temperature be integrated into insect development simulation models instead of air temperature, to build more effective models for cabbage maggot management.