Population Ecology Of Aphelinus Certus, An Adventive Parasitoid Of Soybean Aphid In North America, With Implications For Biological Control

Abstract

Parasitoids are excellent model systems in addressing fundamental aspects of biology and ecology while offering a high degree of economic and ecological value in regulating the population densities of their host species in both natural and agroecological settings. The single most important arthropod pest of soybean is the soybean aphid (Aphis glycines). Although not recommended for release due to its broad host range, the parasitoid Aphelinus certus has since been accidentally introduced into North America and has been hypothesized to be a key natural enemy of soybean aphid. However, the overall impact of A. certus in the biological control of soybean aphid is uncertain. The chapters in this thesis are united by broader theories and concepts addressing the role of A. certus in biological control. Chapter 1 presents a coupled-equations matrix population model parameterized by a series of laboratory bioassays outlining the life history of soybean aphid and A. certus; analysis of the model suggested that a parasitism rate of 0.21 d−1, which would be equivalent to at least 3.4% mummies, is capable of maintaining soybean aphid below economically damaging levels in 31.0% of simulations. Chapter 2 further explores the effects of host density on parasitoid lifespan and reproduction as well as establishes a quantitative wing wear index for estimating the age of field-collected parasitoids. Moving from theoretical to in-field efficacy, Chapter 3 evaluates the impact of A. certus on soybean aphid in soybean fields across central and western Minnesota (United States) from 2017–2019; contrary to the predictions of the model in Chapter 1, the results of this field experiment did not find a strong effect of A. certus in reducing the population growth rates of soybean aphid. Finally, Chapter 4 evaluates the prevalence of parasitism by A. certus on the native aphid species Aphis asclepiadis and Aphis monardae in prairie ecosystems; these results suggest that A. certus readily colonizes or is already established in natural habitats and poses a potential threat to endemic aphid populations. Lastly, I offer conclusions by more broadly discussing this work within the context of population dynamics, ecology, and biological control.