A field study was conducted to calculate the functional lower developmental threshold and cumulative required degree-days for first emergence of adult Apthona nigriscutis Foudras, a classical biological control agent of the exotic weed leafy spurge Euphorbia esula L. The study was conducted in Wyoming, Montana, and Minnesota from 1991 through 1999 and made use of the computer program, CALFUN (version 2.0), to estimate the thresholds and required degree-days. Five different methods were used to calculate degree-days, and two kinds of estimates (mean and median) were made for functional lower developmental threshold and required degree-days. In all, 10 phenology models were developed. When we tested these for their accuracy in predicting adult A. nigriscutis first emergence, those that used the historical or sine wave methods for calculating degree-days, and those that used median functional lower developmental thresholds and required degree-days, provided the best predictions. Based on these findings, we suggest using the median functional lower developmental threshold from the historical (−2.56°C) or sine wave (0.14°C) methods to calculate degree-days. Then, when cumulative degree-days approach 1,189.2°C (historical method) or 898.5°C (sine wave method), the onset of adult A. nigriscutis emergence should be imminent. A computer program was written for use by spurge managers so they can easily determine when first emergence of adult A. nigriscutis is likely to begin.