Sweetpotato (Silverleaf) Whitefly is a common pest on a variety of hosts, including 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 Sweetpotato / Silverleaf Whitefly to help predict when different life cycle stages will occur.

Calculating Growing Degree Day Units

The Sweetpotato Whitefly growing degree day model uses a lower threshold of 50F and an upper threshold of 90F 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 Sweetpotato Whitefly model, the biofix date should be set when eggs are observed, in order to predict the emergence of adults. Approximately 570 GDD(F) after the biofix date (when eggs are observed), adults will be observed.

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:

Zalom, F.G., E.T. Natwick, and N.C. Toscano. 1985. Temperature regulation of Bemisia tabaci (Homoptera: Aleyrodidae) populations in Imperial Valley Cotton. J. Econ. Entomol. 78: 61-64.