Objective
Characterize the overwintering physiology of striped and crucifer flea beetles (FBs) and develop and initial temperature-based model to predict their spring emergence from diapause and onset of feeding on the Canadian prairies. Will collect FBs from Alberta (ARA collaborators) in each of 4 seasons, and then lab test:
1) Supercooling Point (temperature just before they freeze)
2) Cold Tolerance Strategy (do FBs avoid or tolerate freezing)
3) Lower lethal temperature limit
4) Temperature threshold for FB diapause emergence
Will monitor FB populations in AB (ARA collaborators and UofC/UofA) and in SK (AAFC collaborators).
Project Description
Despite flea beetles’ economic importance, little is known about their overwintering physiology. Prairie winters should be harsh for flea beetles due to extended exposure to sub-zero temperatures and lack of food. Yet, flea beetles appear to survive the winter by entering a state of hibernation called diapause and consistently emerge in the spring to inflict damage on canola crops. Most temperate pest insects enter diapause to survive harsh winter conditions. Diapause is cued by exposure to short-day, autumn-like conditions and is accompanied by a suite of physiological changes that lead to developmental arrest, low metabolic rates (to save energy when food is unavailable), and an increase in cold tolerance to ensure their survival of extreme low temperatures. Diapause is often terminated after a pre-determined amount of time has elapsed or by exposure to a certain environmental cue (i.e. prolonged cold exposure). Satisfaction of a certain chilling period is often required before insects can terminate diapause and resume their normal development. Once insects have terminated diapause, they must wait for an increase in ambient temperature to move, emerge from their overwintering sites, and behaviorally resume feeding. Understanding the thermal physiology underlying diapause emergence and feeding onset can help inform the timing of insect pest outbreaks. Insect thermal limits fluctuate with the seasons, and those in diapause have lower lethal limits of survival than their spring or summer-active conspecifics. Diapausing insects exhibit one of two cold tolerance strategies during the winter; freeze avoidance or freeze tolerance. Freeze-avoidant insects survive extended periods at temperatures below 0 °C as long as they remain unfrozen. At their supercooling point (SCP, the temperature at which their body fluids freeze), they will die. Thus, to survive sub-zero temperatures freeze avoidant insects use various mechanisms to depress their SCP so body fluids remain liquid at extremely low temperatures. Freeze-tolerant insects survive freezing of their body fluids and the extent to which they survive ice formation depends on how long they are frozen, the frequency of freezing, and their ability to recover from being frozen. Understanding thermal limits and cold tolerance strategies in pest insects can help predict the extent to which they survive after extreme cold events in the winter. There is currently no comprehensive understanding of diapause and cold tolerance in striped and crucifer flea beetle populations in North America. Understanding overwintering traits, such as cold survival limits, the temperatures that cue the end of diapause in the spring, and the minimum temperatures that permit flea beetle emergence and feeding on canola plants will improve predictive power to anticipate or more rapidly respond to flea beetle outbreaks. Historically, striped flea beetles were found in more northern regions of the Prairies, including the Peace and Aspen-Parkland regions. However, there has been a shift in populations and striped are being found at lower latitudes and, in some cases, in higher numbers than crucifer. Previous research has identified 15 °C as the temperature associated with peak crucifer flea beetle emergence in the spring. However, these data represent ground temperatures correlated with peak beetle emergence rather than the actual minimum temperature that cues the physiological and and behavioural processes required for flea beetles to finish their diapause, emerge from their overwintering sites and start to feed. Thus, there could be lower temperatures where flea beetles are active that have yet to be examined. Further, these data were collected only on crucifer flea beetles, thus there is a knowledge gap of specific emergence temperatures from striped flea beetles in the prairies. Finally, these data were collected 20 years ago, when climatic conditions likely differed. Indeed, a 2017 analysis of Alberta climatic trends between 1950 and 2010 showed increases in mean winter temperatures and decreases in days below -20 °C. These changes could be sufficient for local adaptations in thermal sensitivity of beetle diapause and cold tolerance strategies. There is a need to re-assess emergence temperatures and cold tolerance limits/strategies of both crucifer and striped flea beetles from a current and more precise physiological perspective.
