Cold tolerance of the bean weevil (Acanthoscelides obtectus) (Chrysomelidae: Bruchinae): A pest of field and stored beans

The bean weevil, Acanthoscelides obtectus (Coleoptera: Chrysomelidae: Bruchinae), is a pest of bean seeds both in storage and in fields. Its presence in stored beans can be problematic when exporting seeds to countries where it is a quarantine pest. Treatment of bean seeds that may contain bean weevils are done using chemical fumigants, typically phosphine gas. However, the use of cold temperatures could be a viable option. This study investigates the cold tolerance of the bean weevil. Two strains of were used, one from Kansas, USA, and one from southern Ontario, Canada. Insect life stages inside bean seeds were assessed using 2D X-ray. The survival time at -5 °C and the supercooling point of eggs, early instar larvae, late instar larvae, pupae, and adults was assessed. For the assessment of survival at -5 °C, each life stage was placed at -5 °C for: 0 (control), 1, 2, 3, 4, 7, and 14 days. Half the old instar larvae tested in this experiment were placed 2 weeks at 15 °C followed by 2 weeks at 10 °C to acclimate them to cold. Results show that the most cold hardy stages were old larvae and adults (adult mean survival: -14.0 ° ± 0.3 SE, n = 52). There were no differences between the two strains tested. Although insect mortality was higher in the cold acclimated groups, some cold acclimated individuals survived 14 days at -5 °C while none of the non-cold acclimated individuals survived up to 7 days at -5 °C.