Blue Alfalfa aphid is a pest on alfalfa and other crops. It can be very difficult to control once a population has been established.

Researchers have developed the Blue Alfalfa Aphid growing degree day model, available on the Pest Prophet app, in order to help predict when different life stage events will occur, based on the weather.

Calculating Growing Degree Day Units

TheBlue Alfalfa Aphid growing degree day model uses a lower threshold of 38.2F  and an upper threshold of 80.8F 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 Blue Alfalfa Aphid model, in order to predict the emergence of second generation aphid, the biofix date should be set once the first generation has been consistently observed in the field.

If using the model for aphids in Apterous (wingless) form, approximately 286 GDD(F) after this biofix date, next generation aphids will emerge and this is the optimum time for insecticide treatment.

If using the model for aphids in Alate (winged) form, approximately 330 GDD(F) after this biofix date, next generation aphids will emerge and this is the optimum time for insecticide treatment.

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:

Summers, C.G., R.L. Coviello, and A.P. Gutierrez. 1984. Influence of constant temperatures on the development and reproduction of Acyrthosiphon kondoi (Homoptera: Aphididae). Environ. Entomol. 13: 236-242.