Cold tolerance and overwintering survival of Aphelinus certus (Hymenoptera: Aphelinidae), a parasitoid of the soybean aphid (Hemiptera: Aphididae) in North America

Authors: STENOIEN, CARL - University Of Minnesota; CHRISTIANSON, LINDSEY - University Of Minnesota; WELCH, KELTON - University Of Minnesota; DREGNI, JONATHAN - University Of Minnesota; Hopper, Keith; HEIMPEL, GEORGE - University Of Minnesota

Submitted to: Bulletin of Entomological Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2023
Publication Date: 6/26/2023
Citation: Stenoien, C.M., Christianson, L., Welch, K., Dregni, J., Hopper, K.R., Heimpel, G.E. 2023. Cold tolerance and overwintering survival of Aphelinus certus (Hymenoptera: Aphelinidae), a parasitoid of the soybean aphid (Hemiptera: Aphididae) in North America. Bulletin of Entomological Research. 1-13. https://doi.org/10.1017/S0007485323000196.
DOI: https://doi.org/10.1017/S0007485323000196

Interpretive Summary: The soybean aphid (Aphis glycines) was first detected in North America in the year 2000, and it quickly became one of the most economically damaging pests of soybeans. The main control measure has been the application of broad-spectrum insecticides, although biological control by resident natural enemies can greatly diminish population levels. One of these natural enemies is the accidentally introduced Eurasian parasitoid Aphelinus certus which has become abundant and widespread in soybean-growing areas of north-central North America. Despite the potential for substantial suppression of soybean aphid populations by this parasitoid, its impact appears to be limited by low rates of parasitism early in the growing season. We tested the hypothesis that A. certus might experience high overwintering mortality by measuring survival in laboratory and field settings. Our results show that A. certus is capable of overwintering in the region inhabited by soybean aphid, especially under snow cover, but may experience substantial mortality. Climate change is predicted to bring warmer, drier winters to the North American Midwest, with decreased depth and duration of snow cover, which may further reduce overwintering survival of A. certus. On the other hand, the higher survival at lower latitudes, both on the ground and in foilage, suggests that a northward shift in warmer winters could increase A. certus overwintering survival in Minnesota. Furthermore, survival could improved through changes in field management, in particular low or no-till farming.

Technical Abstract: The soybean aphid (Aphis glycines) was first detected in North America in the year 2000, and it quickly became one of the most economically damaging pests of soybeans. The main control measure has been the application of broad-spectrum insecticides, although biological control by resident natural enemies can greatly diminish population levels. One of these natural enemies is the accidentally introduced Eurasian parasitoid Aphelinus certus (Hymenoptera: Aphelinidae) which has become abundant and widespread in soybean-growing areas of north-central North America. Despite the potential for substantial suppression of soybean aphid populations by this parasitoid, its impact appears to be limited by low rates of parasitism early in the growing season. We tested the hypothesis that A. certus might experience high overwintering mortality by measuring survival in laboratory and field settings. In the laboratory, we used thermocouple thermometry and exposures to ecologically relevant durations of low temperatures to determine the cold-tolerance strategy of A. certus and found it to be freeze-intolerant with a median supercooling point of -28°C. When exposed to temperatures of 0°C for up to seven months, adults emerged only after a winter duration of at least 60 days and survival decreased with winter duration beyond 150 days. We conducted in-field studies at sites from northern Minnesota to southern Iowa to determine if diapausing A. certus could overwinter above and below the snowpack in wooded areas adjacent to Rhamnus cathartica, the overwintering host of soybean aphid. Survival was greater for parasitoids placed on the ground than 1 meter off the ground in woody vegetation and was negatively correlated with increasing latitude. A second iteration of this field study included soybean fields as a treatment and confirmed that mummies placed on the ground were much more likely to survive than those placed above the ground in the woody vegetation. This is likely due to the warmer and less variable temperatures characteristic of the subnivean microclimate. Our results suggest that A. certus is capable of overwintering in the region inhabited by soybean aphid but may experience substantial mortality even under ideal conditions. Climate change is predicted to bring warmer, drier winters to the North American Midwest, with decreased depth and duration of snow cover, which may further reduce overwintering survival. On the other hand, the higher survival at lower latitudes, both on the ground and in foilage, suggests that a northward shift in warmer winters could increase A. certus overwintering survival in Minnesota. Furthermore, survival could improved through changes in field management, in particular low or no-till farming.