The guild of stem-infesting insect pests of cultivated sunflower, Helianthus annuus L., within the central Plains is a concern to producers, chiefly due to losses caused by plant lodging from the sunflower stem weevil, Cylindrocopturus adspersus (LeConte) (Coleoptera: Curculionidae), and Dectes texanus texanus LeConte (Coleoptera: Cerambycidae). The incidence of a root boring moth, Pelochrista womonana (Kearfott) (Lepidoptera: Tortricidae), also has increased. Experiments were conducted in Kansas during 2000–2001 to investigate the effect of irrigation timing and intensity on densities of C. adspersus, D. texanus, and P. womonana larvae within cultivated sunflower stalks. Supplemental soil moisture provided by irrigation during the growing season increased both seed yield and oil content, and it reduced insect densities of the sunflower stem weevil and P. womonana in the sunflower stalk. Results showed that ensuring adequate moisture during the growing season can assist in reducing stem-infesting insect densities, revealing an additional advantage of crop irrigation beyond improved sunflower productivity.