Walnut Husk Flies are an important economic pest on walnuts in a variety of areas. They can be challenging to control. Researchers have developed a growing degree day model for Walnut Husk Fly that can help predict when adult emergence from overwintered pupae, as well as egg laying and when larvae exit the walnut husks.

Calculating Growing Degree Days

The Walnut Husk Fly model uses a lower threshold of 41F. It can be calculated with daily high and low temperatures, using the sine model to estimate growing degree day units, or the Pest Prophet app can be used to calculate GDD accumulation from hourly temperature data for a specific field.

For this model, the “biofix” date, or date from which to start accumulating growing degree days, should be set to March 1st for the Northern Hemisphere.

First adult emergence occurs at approximately 1980 GDD (F) after the biofix date. Egg-laying will begin at 2480 GDD(F) after the biofix date, with the first egg-hatch/ larvae at 2700 GDD (F) and first larvae exiting husks at 3461 GDD(F) after the biofix date.

Sources:

Mitchell, R. G., and L. L. Sower. 1988. Phenology and heat accumulation as field guides for the seasonal history of the western pine shoot borer (Lepidoptera: Olethreutidae).