Model construction and field validation for the stage transition of seedcorn maggot (Diptera: Anthomyiidae)

The seedcorn maggot (SCM), Delia platura (Meigen) (Diptera: Anthomyiidae), is highly polyphagous species that damages cucurbit plants worldwide, as well as garlic and onion, and occurs on all continents except Antarctica. We estimated the basic thermal parameters of SCM to develop its stage transition (ST) model: linear and nonlinear development models (DM) and distribution models (DSM). Previously published datasets for SCM immature stages were collected and used for the parameter estimation of DMs. The DSM parameters were estimated using the standard deviations and mean development times. In linear models, the lower threshold temperature (LT) and thermal constant (degree-days) for each stage were estimated as 4.1 °C and 33.6 DD for eggs, 6.6 °C and 160.0 DD for larvae, and 5.2 °C and 217.8 DD for pupae, respectively. We fitted the data to a total of 17 nonlinear development equations, and the best-fit model was used to develop the ST models by combining them with DSMs, which simulate the proportion of individuals shifted from one stage to the next. In field validation conducted in a caged environment, ST-model showed good performance fitting with actual field observations: a discrepancy of an average of 0.5 d in the September cohort (2024), 3.2 d in the November cohort (2024), and 1.6 d in the March cohort (2025), when measured at 10%, 25%, 50%, 75%, and 90% cumulative adult emergence. Our stage transition models for the eggs, larvae, and pupae of SCM will help develop a population phenology model for this fly in the future.