The San Jose scale is a soft scale is a major pest on many fruit and nut trees in the western United States. It can be difficult to control once populations are built up. The San Jose Scale Growing Degree Day Model can help to predict the timing of different life cycle stages.

San Jose Scale Life Cycle Stages

San Jose Scales overwinter in the black-capped crawler stage. Males emerge first and are able to fly around and mate immediately with females, who remain under the scale. Females then produce crawlers which are able to move around the plant before feeding and then forming their scale covering.

The periods of crawler emergence is key for insect prevention, if necessary. Researchers have created the San Jose Scale Emergence Growing Degree Day Model to better predict when the adults will emerge.

Calculating Growing Degree Day Units

The San Jose Scale Emergence Growing Degree Day model uses a lower threshold of 51F and an upper threshold of 90F. 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.

Using Jan. 1st as a default “biofix” date, or starting point for accumulation. Male emergence occurs approximately at  200 GDD(F).  First instar females will emerge at approximately 973 GDD(F). This can be used an approximate target for beginning treatments, or for beginning scouting procedures.

Source:

Jorgensen, C. D., R. E. Rice, S. C. Hoyt, and P. H. Westigard. 1981. Phenology of the San Jose Scale (Homoptera: Diaspididae). Can. Ent. 113: 149-159.