Biocontrol in the Freezer: Extreme cold induces greater mortality in EAB biocontrol agents than in EAB larvae
Summary written by Annette Evans, edited by Meghan Graham MacLean
Summary
The invasive Emerald Ash Borer (EAB) has devastated North American Ash trees since its accidental introduction in the early 1990s. Several biocontrol agents have been released in an effort to control the spread and impacts of EAB, including the parasitoid wasps Tetrastichus planipennisi and Spathius galinae, which attack EAB larvae. While both EAB and its biocontrol agents overwinter as larvae, it is unknown if or how extreme cold temperatures in the winter will affect the survival of both EAB and its biocontrol agents.
In this study, Duan et al. (2020) examined cold-induced mortality in EAB and biocontrol agents T. planipennisi and S. galinae in Michigan following a polar vortex that produced unusually cold temperatures in 2019. Despite significant variation among sites, Duan et al. found that overwintering mortality was higher in both biocontrol agents compared to EAB, suggesting that these biocontrol agents are more susceptible to cold-induced mortality than EAB. In Michigan cold-induced mortality in EAB ranged from 4.5% - 26% while mortality in the larval parasitoids was 8% - 35% for T. planipennisi and 18% - 50% for S. galinae. In contrast, overwintering mortality in Connecticut, which experienced more typical winter temperatures, was extremely low (< 2% for EAB and <7% for biocontrol agents). Tree size did not have a significant effect on mortality of either EAB or the biocontrol agents in Michigan or Connecticut.
In the short term the authors suggest that EAB populations may increase following extreme winter cold events as parasitism rates will likely be lower due to greater overwintering mortality of biocontrol agents. Given that extreme winter weather events are projected to increase with climate change, it is possible that these parasitoids will be less effective at managing EAB populations in the future. Therefore, future biocontrol work should focus on identifying and selecting new EAB biocontrol agents that are pre-adapted for extremely cold winter conditions.
Take home points
Extreme winter temperatures following a polar vortex event in Michigan caused an increase in overwintering mortality of both EAB larvae and the larvae of parasitoid wasps T. planipennisi and S. galinae.
In contrast, very low levels of overwintering mortality for either EAB or the two biocontrol agents were observed at sites in Connecticut that experienced typical winter temperatures.
Both biocontrol agents showed greater cold-induced mortality than EAB larvae in Michigan following the extreme cold period, suggesting that these biocontrol agents are less cold tolerant than EAB.
Morality levels varied between sites in both Connecticut and Michigan, but ash tree size did not affect overwintering mortality of EAB or the biocontrol agents.
Management implications
Parasitoid wasps introduced for biocontrol of EAB are less tolerant to extreme cold and winter temperature fluctuations than EAB, which may limit the effectiveness of these biocontrol agents, particularly in northern locations.
Climate change is projected to increase extreme temperature fluctuations, which may cause more frequent or larger winter mortality in current EAB biocontrol agents. This may limit the future effectiveness of these agents.
Additional EAB control measures will need to be employed following extreme cold events or rapid winter temperature fluctuations.
Future biocontrol efforts should focus on identifying and selecting species or strains of biocontrol agents that have greater cold hardiness, to improve overwintering survival in northerly locations.
Keywords
Emerald Ash Borer (EAB); Biocontrol; Climate extremes; Impact studies; Insect pests; Management efficacy