Beet Armyworms (Spodoptera exigua) are a common and widespread pest on a hosts. They can be very challenging to control, and different insecticide products may target different life cycle stages (eggs, nymphs, adults) and not affect the pest at the other life cycle stages. For this reason, predicting when different life cycle stages will occur, based on current observations and weather data, can be very helpful. Researchers have developed a growing degree day model for Beet Armyworms to help predict when different life cycle stages will occur.

Calculating Growing Degree Day Units

The Beet Armyworm growing degree day model uses a lower threshold of 54F for calculating growing degree day units. The growing degree day units can be calculated from daily maximum and minimum temperatures using these thresholds, or by using the Pest Prophet app, which features this model and is easy to set up and use.

Setting the Biofix date

In order to begin growing degree day accumulation, first a “biofix” date must be set which is the first date, after which growing degree day units will begin to accumulate. In the Pest Prophet app, the biofix date can be adjusted for any observed life cycle stages.

For the Beet Armyworm model, the biofix date should be set when eggs or adults are first observed, in order to predict the emergence of next generation larvae, and then adults. Approximately 95 GDD(F) after the biofix date (when eggs are observed), larvae will be observed. Approximately 883 GDD(F) after the biofix date, Adult moths will emerge.

For best results the biofix should be re-adjusted for each generation, based on scouting observations. This will improve accuracy of later crop stage predictions.

Sources:

Hogg, D. B., and A. P. Gutierrez. 1980. A model of the flight phenology of the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), in central California. Hilgardia: 48: 1-36.