

How to Manage PestsDegreeDays: Estimation Methods
Procedures for estimating degreeday accumulations are frequently used in the place of the more accurate method of calculating degreedays from hourly temperature data, and the UC IPM Web degreeday calculator uses several of the common estimation methods. However, the use of these methods requires that consideration be given to the time of year, geographical location, and biology of the organism under study. Roltsch et al. analyzed seven methods of estimating degreedays for two years of air temperatures collected at each of nine California locations. The estimates, based on a lower temperature threshold of 50 F (10 C), were then compared by month to degreeday values calculated by hourly summation from the same locations and time periods. The findings are summarized below. Summary of ResultsDifferences among similar methods. Degreeday estimates using seven methods were compared to hourly summations. The methods were averaging, single sine (1, 2), double sine, single triangle, double triangle, and corrected sine and triangle (these latter two were developed inhouse at UC IPM).
Differences based on time of year. Degreeday estimates, based on a lower threshold of 50F (10C), were totaled for each month and compared to hourly sums for the same periods.
Differences among climatic zones. Hourly air temperature data for nine stations in the California Irrigation Management Information System weather network were used to calculate the degreeday totals used as the standard for comparison. The stations were located in Brawley, Riverside, Salinas, Castroville, Watsonville, Five Points, Davis, Camino, and McArthur.
Cutoff methods. High temperature threshold cutoff techniques were assessed to characterize their compatibility with the triangle and sine degreeday approximation methods. Horizontal, vertical, and intermediate cutoff methods, using thresholds of 80 F (26.7 C) and 89.6 F (32 C), were compared.
* Summary of results of a study reported in Roltsch, W. J.; Zalom, F. G.; Strawn, A. J.; Strand, J. F.; Pitcairn, M. J. 1999. Evaluation of several degreeday estimation methods in California climates. Int. J. Biometeorol. 42:169176. ReferencesAllen, J. C. 1976. A modified sine wave method for calculating degree days. Environ. Entomol. 5:388396. (Single sine method and double sine method) Arnold, C. Y. 1960. Maximumminimum temperatures as a basis for computing heat units. Am. Soc. Hort. Sci. 76:682692. (Averaging method) Baskerville, G. L.; Emin, P. 1969. Rapid estimation of heat accumulation from maximum and minimum temperatures. Ecology 50:514517. (Single sine method) Lindsey, A. A.; Newman, J. E. 1956. Use of official weather data in spring time temperature analysis of an Indiana phenological record. Ecology 37:812823. (Single triangle method) Roltsch, W. J.; Zalom, F. G.; Strawn, A. J.; Strand, J. F.; Pitcairn, M. J. 1999. Evaluation of several degreeday estimation methods in California climates. Int. J. Biometeorol. 42:169176. (Analysis of seven methods) Sevacherian, V.; Stern, V. M.; Mueller, A. J. 1977. Heat accumulation for timing Lygus control measures in a safflowercotton complex. J. Econ. Entomol. 70:399402. (Double triangle method) 