Plant population and weeds influence stalk insects, soil moisture, and yield in rainfed sunflowers

Insect infestation, soil moisture, and yield were examined in populations of ≈ 33 140 plants/ha (low) and ≈ 40 340 plants/ha (high) of an oilseed sunflower, Helianthus annuus L, cv.‘Triumph 660CL’with two levels of weediness. Less weedy plots resulted from the application of herbicide combination of S‐metolachlor and sulfentrazone, whereas more weedy plots resulted from application of sulfentrazone alone. Among the 12 weed species recorded, neither plant numbers nor biomass differed between crop plant densities. Larvae of the stalk‐boring insects Cylindrocopturus adspersus (Coleoptera: Curculionidae) and Mordellistena sp. (Coleoptera: Mordellidae) were less abundant in high density sunflowers, ostensibly due to reduced plant size. However, the same effect was not observed for Dectes texanus (Coleoptera: Cerambycidae) or Pelochrista womanana (Lepidoptera: Tortricidae), two other stalk‐boring insects. Soil moisture was highest in low density and lowest in the high density sunflowers that were less weedy. Stalk circumference, head diameter, and seed weight were reduced for sunflower plants with short interplant distances (mean = 20 cm apart) compared to plants with long interplant distances (mean = 46 cm apart). These three variables were greater in less weedy plots compared with more weedy plots and positively correlated with interplant distance. Yields on a per‐hectare basis paralleled those on a per‐plant basis but were not different among treatments. The agronomic implications of planting density are discussed in the context of weed and insect management.