The Imported Cabbageworm (Pieris rapae) is a common and widespread pest on a variety of cole and cruciferous crop hosts. They can have several generations each season, and can be active throughout the years where hosts are present. Different treatment options can effect different lifecycle stages on the Imported Cabbageworm. 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 Imported Cabbageworms to help predict when different life cycle stages will occur.

Calculating Growing Degree Day Units

The Imported Cabbageworm growing degree day model uses a lower threshold of 9.4C 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 Imported Cabbageworm model, the biofix date should be set when eggs or adults are first observed, in order to predict the emergence of next generation larvae. Approximately 195 GDD(C) after the biofix date (when adults are observed), larvae will be observed and may be targeted for treatment.

Sources:

Tatchell, G. M. 1981. The effects of a granulosis virus infection and temperature on the food consumption of Pieris rapae (Lepidoptera: Pieridae). Entomophaga 26: 291-299.