Sunflower Stem Weevil
Sunflower stem weevil pupation, adult formation and emergence in the field can be predicted using accumulated degree-days. The stem weevil development increases at a nearly constant rate as the temperature rises above 41°F (5°C.).
For the sunflower stem weevil, degree-days are accumulated after 1 January for each 24-hour period in which temperatures exceed 41°F (5°C). 90% pupation of sunflower stem weevil larvae occurred when 585 degree-days have accumulated.
On the other hand, 90% adult formation will occur in dry old sunflower stalks when 984 degree-days have accumulated in your area.
It has been predicted that:
- 50% of adult weevils have emerged from stalks to infest new crop when 1004.4 degree-days have accumulated. Scouting for this insect should be initiated around this time.
- 75% adult weevil emergence occurs at about 1215.0 degree-days accumulation and
- 90% emergence follows 1431.0 degree-days accumulation.
In years of unusually warm or cold springs, weevil development/emergence may occur earlier or later than normal.
One can calculate degree days using the steps as indicated:
1. Begin accumulating degree-days on March 1 (the developmental threshold for this insect is 41° F).
Degree-days = (Daily Maximum temp. + Daily Minimum temp.) – 41°F
2. Add each day’s accumulation to the previous running total to determine accumulated degree days (DD).
3. Compare the running total to the degree-days that correspond with recommendations for initiation of sampling.