Egyptian Cotton Worm (Spodoptera littoralis) is a common and widespread pest on cotton. 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 Egyptian Cotton Worm to help predict when different life cycle stages will occur.

Calculating Growing Degree Day Units

The Egyptian Cotton Worm growing degree day model uses a lower threshold of 53F and an upper threshold of 95F 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 Egyptian Cotton Worm model, the biofix date should be set to January 1st for the Northern Hemisphere, in order to predict the emergence of adults from overwintered larvae, and the following generations. Approximately 433 GDD(F) after the Jan. 1 biofix date the first adults from overwintered larvae will be observed. Peak egg hatch and larvae emergence will occur at approximately 798 GDD(F). Approximately 1264 GDD(C) after the biofix date, the next generation of 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:

Baker, C.R.B. and G.W. Miller. 1974. Some effects of temperature and larval food on the development of Spodoptera littoralis. (Lep., Noctuidae). Bull. Entom. Res. 63:495-51.