Artificial selection to a nonlethal cold stress in Trogoderma variable shows associations with chronic cold stress and body size

Authors: Gerken, Alison; Abts, Shelby; Scully, Erin; Campbell, James - Jim

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2019
Publication Date: 4/14/2020
Citation: Gerken, A.R., Abts, S.R., Scully, E.D., Campbell, J.F. 2020. Artificial selection to a nonlethal cold stress in Trogoderma variable shows associations with chronic cold stress and body size. Environmental Entomology. 49(2):422-434. https://doi.org/10.1093/ee/nvz162.
DOI: https://doi.org/10.1093/ee/nvz162

Interpretive Summary: Cold treatments are often used as a pest management tactic to reduce stored product insect infestations of grain and grain products, including bulk stored and packaged products. However, while some stored product insect species are cold tolerant and can survive some cold treatments, it is not known if they can undergo further adaptations to survive colder temperatures or longer exposure times. The ability of the warehouse beetle, an important and cosmopolitan stored product pest, to adapt to cold temperatures was assessed by exposing them to non-lethal temperatures and selecting individuals that were slow or rapid in their speed of recovery. By repeating this process over multiple generations, we demonstrated that cold tolerance quickly emerged in this laboratory population of warehouse beetle. Use of this artificial selection process to produce changes indicated that selection in response to cold had a genetic basis and was highly heritable. Additionally, populations that had adapted tolerance to these short-term exposures to cold also tended to have improved survivorship after a longer exposure to cold (48 hrs). Beetles selected for cold tolerant also tended to develop larger body sizes compared to groups that were selected for susceptibility to cold, suggesting that body size may be linked with cold tolerance. Results suggest that even after 30 years as a laboratory colony, this warehouse beetle population can adapt quickly to cold. This ability could lead to expansion of the areas where it currently occurs into more temperate climate zones and suggests that they may be able to develop tolerance to cold treatments. This study suggests that managers must make sure to use appropriate cold temperatures of sufficient duration in order to reduce the evolution of tolerance to cold treatments in the field.

Technical Abstract: Extreme temperature has been employed as an alternative to chemical treatments for stored product pests for years. Resistance to heat or cold treatments has not been documented in stored product insects, but repeated use of ineffective treatments could lead to adaptive tolerance. Trogoderma variabile (Dermestidae) is a common pest of stored products and the larval stage is highly resistant to cold and destructive. We artificially selected populations by inducing chill-coma at 4 different cold temperature treatments: 3 and 5 hours at -10 deg C and 3 and 5 hours at 0 deg C. Recovery time was highly heritable after selection for 7 generations for decreased recovery time (cold tolerance) and increased recovery time (cold susceptibility) at all time and temperature combinations. Three replicate populations for each time and temperature combination varied substantially, suggesting different mutations in each population were likely responsible for selected phenotypes. Body size decreased in populations selected for cold susceptibility compared to those selected for cold tolerance and survivorship to long-term cold stress increased in the cold tolerant populations compared to the susceptible populations. After the cessation of the selection experiment, cold tolerance dissipated within four generations from the populations at -10 deg C, but was maintained in populations exposed to 0 deg C. Our results suggest that warehouse beetles can adapt to cold stress quickly, but in the absence of cold stress, the proportion of cold tolerant/susceptible individuals is quickly reduced, suggesting that some of the mutations responsible for these phenotypes may be associated with fitness costs under normal conditions.