Thermal biology and overwintering behavior of the red sunflower seed weevil (Coleoptera: Curculionidae)

Authors: PANTZKE, SHAWNA - North Dakota State University; FERGUSON, BETH - Former ARS Employee; Rajamohan, Arun; Rinehart, Joe; PRISCHMANN-VOLDSETH, DEIRDRE - North Dakota State University; Prasifka, Jarrad

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2023
Publication Date: 4/25/2023
Citation: Pantzke, S., Ferguson, B., Rajamohan, A., Rinehart, J.P., Prischmann-Voldseth, D., Prasifka, J.R. 2023. Thermal biology and overwintering behavior of the red sunflower seed weevil (Coleoptera: Curculionidae). Environmental Entomology. https://doi.org/10.1093/ee/nvad041.

Interpretive Summary: The red sunflower seed weevil is an important pest of the sunflower crop in North America. Though weevil larvae feed on sunflower seeds, this insect spends most of each year overwintering in the soil. Because so many weevils appear to die while in the soil, experiments were conducted to determine if weevils are being killed by freezing and whether they move vertically in the soil, a behavior that could reduce their exposure to very cold temperatures. In laboratory tests, seed weevil larvae froze at very low temperatures (less than -4°F). Weevil larvae placed onto soil outdoors appeared to move small distances (< 1 inch) during the winter, but most larvae were still found within about 2-3 inches of the soil surface. Because temperatures where the weevils overwinter are not cold enough for them to freeze, an additional experiment tested whether the stress of cold temperatures that do not freeze the weevils may still result in their death; this experiment showed cold periods of a week or more at or below 18°F could cause some larvae to die. The shallow location of most larvae also suggests routine crop management like tillage and herbicide incorporation could help farmers by lowering the numbers of seed weevils surviving each winter.

Technical Abstract: Most natural mortality of the red sunflower seed weevil, Smicronyx fulvus LeConte (Coleoptera: Curculionidae), occurs while larvae overwinter in the soil. To test the hypothesis that S. fulvus mortality is related to low temperatures, experiments were used to (i) evaluate the temperature at which larvae freeze (= supercooling point [SCP]), (ii) assess possible vertical movement between entry into the soil in fall and adult emergence in summer, and (iii) determine if realistic soil temperatures could explain patterns of overwintering mortality. Average SCP for groups of S. fulvus larvae differed between years and months, but only ranged from -20.93 to -22.68°C. Most overwintering larvae were found within 6 cm of the soil surface, but larvae appeared to move 1–2 cm deeper between pairs of successive sample dates (September to January, January to April). Significant larval mortality that occurred between January and April 2021 was tentatively attributed to a period in February where daily minimum soil temperatures ranged from -8 to -12°C. When overwintering under control conditions (constant 4°C) was interrupted with week-long exposure to -4, -8, or -12°C in a cold bath, significant S. fulvus mortality was seen for temperatures at or below -8°C. Combined results suggest that mortality of overwintering S. fulvus is likely caused by continuous exposure to low temperatures that may not be cold enough to freeze larvae. Additionally, the shallow overwintering by S. fulvus supports the idea that routine farm management, including tillage and herbicide incorporation, may help limit populations of this sunflower pest.