Spider Mites such as the Two-Spotted Spider Mite, Pacific Spider Mite and Strawberry Spider Mite are widespread pests that can affect a variety of hosts. They can be very challenging to control, and different insecticide products may target different life cycle stages (eggs, nymphs, adults) and not affect mites 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.

Two-spotted Spider Mites, Pacific Spider Mites and Strawberry Spider Mites, have growing degree day models that have been developed to help predict when different life cycle stages will occur.

Calculating Growing Degree Day Units

The Two-Spotted Spider Mite growing degree day model uses a lower threshold of 53F for calculating growing degree day units. The Pacific Spider Mite uses 52F as a lower threshold and the Strawberry Spider Mite uses 54F. 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 all three of these models.

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 example if predominantly eggs are observed, this can be used, after which growing degree days will help predict approximately when nymphs will be the dominant life stage. If primarily adults are observed, this can be used in order to predict egg-laying and nymph emergence of the next generation.

For best results the biofix should be re-adjusted periodically, based on scouting observations. This will improve accuracy of later crop stage predictions.

Source:

Carey, J. R., and J. W. Bradley. 1982. Developmental rates, vital schedules, sex ratios, and life tables for Tetranychus urticae, T. turkestani and T. pacificus (Acarina: Tetranychidae) on cotton. Acarologia 23: 333-345.