Browsing by Author "Heimpel, George E."

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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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    Density-dependent lifespan and estimation of life expectancy for a parasitoid with implications for population dynamics
    (Oecologia, 2020) Miksanek, James Rudolph; Heimpel, George E.
    Parasitoid lifespan is influenced by nutrient availability, thus the lifespan of parasitoids that rely on their hosts for nutritional resources (either via host feeding or by consuming honeydew) should vary with host density. We assessed the survival and reproduction of one such species, Aphelinus certus— a parasitoid of the soybean aphid, Aphis glycines —over a range of host densities using a laboratory assay. We found a positive, asymptotic relationship between host density and the lifespan and fecundity of A. certus that was supported by a traditional survivorship analysis as well as a logistic model. Parasitoids from this assay were also used to develop a wing wear index relating setae damage to parasitoid age. This index was used to estimate the life expectancy of field-collected parasitoids, which was shorter than the life expectancy of laboratory-reared female parasitoids. Finally, host-density-dependent parasitoid lifespan was incorporated into a coupled-equations matrix population model that revealed that decreasing the degree of host density dependence leads to higher equilibrium host densities and changes in the quality of equilibrium (e.g. stable limit cycles). These results detail the relatively unstudied phenomenon of host-density-dependent parasitoid lifespan and suggest that differences between laboratory- and field-determined parasitoid life expectancy have important implications for population dynamics and the biological control of insects.
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    A field-based assessment of the parasitoid Aphelinus certus as a biological control agent of soybean aphid in North America
    (Biological Control, 2020) Miksanek, James Rudolph; Heimpel, George E.
    Damaging outbreaks of soybean aphid continue to occur in North America despite the valuable biological control services provided by resident natural enemies. The adventive parasitoid Aphelinus certus Yasnosh (Hymenoptera: Aphelinidae) has recently established in North America and has been hypothesized to be a key component of the resident natural enemy community, but there have been few studies evaluating the efficacy of this parasitoid in suppressing soybean aphid populations. We used an exclusion cage study to quantify the effect of A. certus on soybean aphid population growth at four sites spanning western and east-central Minnesota from 2017 to 2019. There were minimal differences in soybean aphid population growth between experimental treatments that excluded natural enemies and control treatments, suggesting that parasitism of soybean aphid by A. certus did not have a strong impact on soybean aphid population growth during this study. Because, for example, A. certus larvae can reduce host reproduction prior to mortality (resulting in underestimates of effects in short-term studies), our results reflect the challenges of using exclusion cages to assess the effects of individual natural enemy species, especially those with complex life cycles.
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    Potential Causes of Declines in Minnesota’s Prairie Butterflies with a Focus on Insecticidal Control of the Soybean Aphid
    (Minnesota Invasive Terrestrial Plants and Pests Center, University of Minnesota, 2017-03-20) Runquist, Erik; Heimpel, George E.
    Minnesota is home to diverse prairie butterfly fauna, but several species have recently undergone drastic population declines. For example, 10 of the 15 butterfly species classified as Endangered, Threatened, and of Special Concern by the State of Minnesota depend exclusively on native prairies. The declines of two of these, the Dakota Skipper and the Poweshiek Skipperling, have been so precipitous that they are now exceedingly rare despite having been predictably common previously. Poweshiek Skipperling is now on the verge of global extinction. Multiple regional and local factors may have contributed to these declines, and those factors are expected to have interacted in various ways. However, a working hypothesis is that these butterfly declines are at least in part the result of insecticide drift related to management of the soybean aphid, which invaded Minnesota in 2000 and led to substantial increases in insecticide applications to soybeans. On November 15, 2016, the Minnesota Invasive Terrestrial Plants and Pests Center (MITPPC) in collaboration with the University of Minnesota Institute on the Environment held a workshop to assess hypotheses that could explain the butterfly declines. The workshop featured seven speakers from the US Fish and Wildlife Service, Minnesota Department of Natural Resources, Minnesota Zoo, University of Minnesota, and Environmental Protection Agency. The presentations, which MITPPC has posted online (http://z.umn.edu/mitppcbutterfly), focused on three themes: (i) biology and conservation status of the endangered butterflies, (ii) the possible role of soybean aphid management in affecting butterfly populations, and (iii) risk assessment and potential solutions via conservation measures. The workshop fostered collaboration between experts in prairie conservation and agricultural pest management and featured extensive discussion between participants following the presentations. Members of the conservation and agricultural communities are typically not afforded the chance to engage in conversations of this type and discussions were particularly fruitful for information sharing and relationship building. This document synthesizes the findings of the workshop and provides guidance to the MITPPC on a needed integrative research program studying the non-target consequences of invasive soybean aphid management on Minnesota’s endangered prairie butterflies. We conclude that skipper conservation will be advanced through a new research program that includes tests of the hypothesis that soybean aphid insecticides are contributing to skipper declines. Targeted research is needed to assess 1) the extent of insecticide exposure the butterflies may experience in the wild and 2) the biological consequences of those exposures using replicated controlled experiments. Ongoing drift studies conducted by the Minnesota Zoo and US Fish and Wildlife Service (see below) have provided important preliminary links to soybean aphid insecticides, but significantly more data is needed to really understand wild exposure dynamics. Controlled exposure experiments may soon be launched by Minnesota Zoo and University of Minnesota researchers, but those experiments are currently funding- and personnel-limited and cannot address the full range of needed tests. The MITPPC can play a leading role by supporting and expanding critical comprehensive research on potential non-target effects of management against an invasive pest.