Relating Degree–Day Accmuulations to Calendar Dates: Alfalfa Weevil (Coleoptera: Curculionidae) Egg Hatch in the North Central United States

Abstract

We present a technique for determining dates of alfalfa weevil, Hypera postica (Gyllenhal), egg eclosion by correlating degree-day accumulations with calendar dates. This technique uses historical climate data from various locations to calculate the median date a degree-day accumulation has been reached. ClimProb 3.1, a computer software program and weather database, was used to assess these correlations. We used 165 locations in 12 midwestern states for the analysis. Sixty years (1931-1990) of continuous daily climate data (maximum temperature, minimum temperature, and liquid precipitation) were used for each location. We began degree-day accumulations on 1 January of each year and used a minimum developmental threshold of 8.9°C (48°F). Degree-day accumulations were determined using a sine-wave method. The relationship between the date when the accumulated degree-days reached a threshold and the location (expressed as degrees north latitude) was quadratic from 36° N to 48° N. The technique gives approximate dates when egg hatch is likely to occur throughout the north central United States, and these dates can be used to augment existing decision criteria for initiating sampling programs for alfalfa weevil. This technique can be used for other pest management programs that depend on estimating pest development.