Model Description

The Oklahoma Alfalfa Weevil Model is based on three factors:

• The growth stage of the alfalfa weevil (modeled via degree days),
• The growth stage of the alfalfa plant (obtained through scouting), and
• The population levels of weevil larvae (also obtained through scouting).

Like other insects, the alfalfa weevil requires a minimum, or threshold, temperature for growth and development to occur. For the alfalfa weevil, the threshold is 48ä F. This model utilizes the base-48F degree-day accumulation (since January 1) to estimate weevil development. The Oklahoma Mesonet, the state's unique automated weather station network, provides the daily temperature data from which these degree-day accumulations are calculated.

In addition to the degree-day information provided by the Mesonet, the model requires scouting information after 150 base-48F degree days (since January 1) have accumulated. The grower is asked to collect a 30-stem sample (ideally from 30 evenly-spaced intervals across the dry interior portions of the field). After 30 stems have been collected, they should be placed in a 2-3 gallon container, then beaten vigorously against the inside of the container for 10-20 seconds. The number of larvae which fall out should be counted and recorded. Additionally, the grower should randomly select 10 stems and record their average length to the nearest inch. If fewer than 150 degree days have accumulated, it is not necessary to provide the scouting information to the model.

Note that the model calculates two sets of variables which relate to the temperature thresholds associated with some mortality for alfalfa weevil eggs (10F) and larvae (20F). Beginning in December, the model calculates (for each site) the number of hours since December 1 that temperatures were at or below 10F, the last date on which such temperatures occurred, and the number of hours of such temperatures occurring on that date. Beginning in February, the model also calculates (for each site) the number of hours since February 1 that temperatures were at or below 20F, the last date on which such temperatures occurred, and the number of hours of such temperatures occurring on that date. All of this information can be found under Current Output or Seasonal Output.

The model is more fully described in the OSU Extension publication CR-7177, "Scouting for the Alfalfa Weevil in Oklahoma" (PDF format: 478 KB).