Influence of Cultural Practices on the Sunflower Seed Weevil in South Dakota

Joe V. Gednalske


The two species of sunflower seed weevils found in South Dakota, Smicronyx fulvus and Smicronyx sordidus have a wide distribution, which extends from the western Appalachian highlands across the plains region to the Pacific mountain system. Anderson reported S. fulvus specimens in Fall River County. Kirk and Balsbaugh reported that specimens of S. fulvus and S. sordidus had been collected throughout South Dakota on Helianthus species. Oseto and Braness observed S. fulvus adults feeding and mating on wild Helianthus annuus L., H. maximilani S., H. petiolaris N. and H. tuberousus L. They recovered larvae from H. annuus, H. maximilani, and H. petiolaris. The weevils apparently moved readily to domestic sunflowers when production began on a large scale basis in the 1970’s. Seed weevils have attained economic levels in South Dakota since 1978. Aerial application of parathion has been the most frequently used treatment and is a highly toxic chemical to insects and mammals. Published information on the biology and control of seed weevils has been limited. Attempts at chemical control were initiated as early as the mid 1930’s by Satterthwait. He experimented with calcium arsenate, lead arsenate, sodium fluoride, pyrethrum, tale, hydrated lime, and gypsum, applied with a hand bellows. He reported unfavorable results for all treatments. Additional chemicals were evaluated by Muma et al, also with poor results. More recently Oseto and Braness reported success in reducing seed weevil damage with treatments of phosmet, endosulfan, fenitrothion, and methidathion. Although he reported no results, Satterthwait suggested that since seed weevil larvae overwinter in the soil, fall plowing may afford control. Satterthwait also screened several varieties of sunflowers for resistance to seed weevils. In his studies for varietal resistance, he indicated that earlier blooming varieties had a smaller percentage of the seed infested by weevils than later blooming varieties. The purpose of this study was to evaluate cultural methods for sunflower seed weevil control. Procedures examined included the influence of tillage methods, crop rotation, date of planting, hybrid maturity, and degree-day units on emerging seed weevil populations. Tillage practices done as both fall and spring treatments including disking, chisel plowing, noble blading, and moldboard plowing were examined for their effect on larval position in the soil, time of adult weevil emergence, and the number of adults emerging from the soil. Crop rotations were investigated for possible influence on weevil mortality and time of adult emergence. Sunflower planting date and hybrid maturity were compared to seed weevil ovipositional patterns. Degree-day accumulations were utilized for potential predictive values in assessing adult weevil emergence.