The Sunflower Stem Weevil, are major pests of sunflowers in some areas. Sunflower stem weevils overwinter as adults, usually in field trash, or other secluded areas and emerge in spring, (or in late winter in warmer climates.) Soon after the adults emerge, females begin to lay eggs into sunflower stems. There is only generation per year. Researchers have developed a growing degree day model to help predict when life stages will occur based on that year’s weather.

Calculating the Growing Degree Day Units

The Sunflower Stem Weevil growing degree day model uses a lower threshold of 5C, and an upper temperature threshold of 32C. These units 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 growing degree day models, there needs to be a “biofix”, a set date after which growing degree day units will accumulate. For the Northern Hemisphere, January 1st is the recommended biofix date.

The overwintered adult weevils will typically emerge at 421 GDD (C) with 10% emergence at 536 GDD (C), 50% emergence at 671 GDD (C) and 90% emergence at 866 GDD (C).

Sources:

Charlet, L. D. 1987. Emergence of the sunflower stem weevil, Cylindrocopturus adspersus (Coleoptera: Curculionidae), relative to calendar date and degree-days in the northern great plains. J. Kan. Ent. Soc. 60:426-432.