Sunflower Stem Weevil
Adult stem weevil
Adult stem weevil

There are two stem weevils. The sunflower stem weevil Cylindrocopturus adspersus is the stem weevil of concern. There is only one generation per year. This weevil is 1/8 to 3/16 inch long and is grayish-brown and white spots on the wing covers and the thorax. The sunflower stem weevil is most common in the High Plains region and is often an insect of concern for that region. The black stem weevil Apion occidentale has been determined to be of no economic consequence.
stem weevil larva closeup
stem weevil larva closeup

Life Cycle: There is one generation of the sunflower stem weevil. Emergence is late May to early June in the High Plains and mid to late June in the northern plains. Eggs are deposited on the lower stem. Upon hatching the larvae begin to feed in the pith area of stem and in August they descend to the lower part of the stem and construct an overwintering chamber.
Damaged sunflower field
Damaged sunflower field
due to SF stem weevil

Damage: The sunflower stem weevil is often responsible for weakened stems and lodging. This is especially the case in drought affected sunflower. Under severe damage conditions a significant yield loss can occur. This insect is also a vector for Phoma black stem and charcoal rot infections.

Economic Thresholds: One adult per three plants is the threshold level. It is important to note that the adults are difficult to scout. When disturbed they drop to the ground and blend into the soil.

Scouting Method: The standard X pattern is recommended. The High Plains region recommends that scouting should begin at ‘600 degree days' and continue to mid-July. In the Northern Plains scouting should begin in the 10 leaf stage and continue to mid-July

Management: Sunflower stem weevil can be controlled by foliar insecticides when the threshold level is reached. The challenge in using foliar insecticides is scouting. Delayed planting can be successful. In the High Plains planting after 500 degree days has been reached is recommended. In the North Dakota late May to early June planting window is suggested while South Dakota the window extends to mid-June. Delayed planting, however, can result in potentially lower yields due to immaturity at harvest. Reducing the plant population to gain thicker stalks is another management scheme. The thicker stalk may withstand lodging. Tillage is another option but that door is quickly closing with recognized negatives with tillage. There are hybrid differences and seed companies may recommend one or more hybrids with ‘tolerance' to the stem weevil.

Research: USDA ARS Sunflower Research Unit along with Kansas State University has been field testing for resistant germplasm during the last four years. Differences have been noted and the NSA has funded a post-doctoral scientist for the 2008-11 seasons to accelerate this work. Researchers are looking for germplasm that will be resistant to several insects such as the stem weevil as well as the Dectes and possibly the head moth.

Photos: Visit the Photo Gallery.

Crop Surveys: The Sunflower Crop Survey is conducted bi-annually prior to harvest. Volunteers from all levels of the sunflower industry visit sunflower fields to survey the crop condition. Teams survey for yield and production practices, weeds, insects, diseases and bird damage.

Another resource about Insects can be found in the Archive section of The Sunflower magazine.

Source: NDSU Extension Bulletin 25 Sunflower Production, NDSU Extension Service, September 2007 and High Plains Production Handbook June 2005
 
Additional Resources
High Plains Sunflower Production Handbook  file size: 1554 kb

NDSU Extension Bulletin 25 - Revised 9/2007 file size: 5591 kb

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