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Browsing by Author "Christianson, Lindsey"

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    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, 2023-06) Stenoien, Carl M.; Christianson, Lindsey; Welch, Kelton; Dregni, Jonathan; Hopper, Keith R.; Heimpel, George E.
    Broad-spectrum insecticides are the main control measure of the invasive and economically damaging soybean aphid (Aphis glycines) in North America, although biological control by resident natural enemies can also greatly diminish population levels. One such natural enemy is the accidentally introduced Eurasian parasitoid Aphelinus certus (Hymenoptera: Aphelinidae), though 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. In the laboratory, we used thermocouple thermometry to measure the supercooling points of diapausing parasitoids and assessed parasitoid survival after exposure to ecologically relevant durations of low temperature. We found A. certus to be freeze-intolerant with a median supercooling point of -28 & DEG;C. When exposed to temperatures of 0 & DEG;C for up to 7 months, adults emerged only after exposures of at least 60 days and survival decreased with durations beyond 150 days. We also 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. Survival was negatively correlated with increasing latitude and was greater for parasitoids placed on the ground than 1 meter off the ground, likely due to the warmer and stabler temperatures 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.
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    Host influence on the cold hardiness of the emerald ash borer, *Agrilus planipennis* Fairmaire (Coleoptera: Buprestidae)
    (2014-12) Christianson, Lindsey
    The emerald ash borer, Agrilus planipennis Fairmaire, is an invasive insect in North America that has caused extensive damage to ash trees throughout its invaded range. The range of Fraxinus spp., specifically black ash (Fraxinus nigra) extends farther north than the known A. planipennis distribution. The ability of an insect to withstand cold, or its cold hardiness, is important in limiting its northern distribution. The cold hardiness of A. planipennis from green ash and black ash was assessed using two laboratory measures, supercooling points and lower lethal temperature, in laboratory- and naturally-infested A. planipennis larvae. Supercooling points of A. planipennis larvae from black and green ash, with medians ranging from 25.5 to 32.6°C and 24.0 to 34.2°C respectively, were not significantly different during the winters of 2012-13 and 2013-14, although the laboratory infested larvae tested in November 2011 had significantly warmer supercooling points. Kaplan-Meier estimates of the likelihood of freezing of larvae from black ash and green ash were not significantly different between tree species or between years. A high proportion of larvae died after being frozen, but freezing did not kill all larvae. Lower lethal temperatures for 50% of the population (LT50) were estimated after accounting for the proportion of larvae that would survive freezing. LT50s of larvae from green ash were 35.2°C (95% CI 35.9°C, 34.7°C) in the winter of 2012-13 and 33.4°C ( 34.1°C, 32.1°C) in the winter of 2013-14. Larvae from black ash had estimated lower lethal temperatures of 33.4°C ( 34.4°C, 32.3°C) in 2012-13 and 33.1°C (≤ 37.5°C, 35.5°C) in 2013-14. Agrilus planipennis larvae overwinter under the bark of ash trees, so larvae may not be experiencing winter air temperatures. To determine temperatures under the bark of ash trees, temperature probes were placed under the bark of both green and black ash at 1.4m high and at the base of the tree on the north and south faces. Weather stations recorded air temperature. Daily minimum temperatures under the bark of green ash ranged from 0.5 to 4.7°C warmer than daily minimum air temperatures, and temperatures were 1.2 to 5 degrees warmer under the bark of black ash. Temperatures at the base of the trees provided the most insulation, possibly due to any snow cover adding to the insulative effect of the bark. During the time in which we took measurements, the air temperature fell below 30°C for an average of 16 hours during the winter of 2012-13. Temperatures under the bark of green ash did not reach -30°C, and temperatures under the bark of black ash fell below -30°C, on average, for 2.6 hours. In 2013-14, air temperature was 30°C or colder for an average of 146.1 hours, 78.1 hours under the bark of green ash, and 118.7 hours under the bark of black ash. Because of the differences in air temperatures and temperatures under the bark of trees, we cannot use air temperature to directly predict A. planipennis mortality. Because under-bark temperatures do not reach the temperature required to kill 50% of the population of A. planipennis every year, further research should consider how the larvae survive sublethal temperatures for longer periods of time.