2020-21 Rapid Ag: How Parasitoid Mortality Limits Biological Control of Soybean Aphid
The parasitoid wasp Aphelinus certus is one of the most important natural enemies of soybean aphid in Minnesota. Recent analyses indicate that it is currently reducing the need for insecticide applications against soybean aphid by about 10%. Here, we ask whether A. certus has the potential to even further reduce insecticide use. In particular, we will determine whether the overwintering success of this parasitoid can be enhanced by the use of reduced-tillage practices. Our research to date has shown that A. certus primarily overwinters within soybean fields and we will determine the extent to which tillage practices cause parasitoid mortality. Our previous research has also uncovered the presence of various parasitoids which attack Aphelinus (‘hyperparasitoids’). These hyperparasitoids are currently causing low levels of parasitism and are thus likely not compromising biological control. However, we propose studies of their biology and distribution to determine whether they may be a threat in some areas of the state or increasing in prevalence. We will use information from research on both parasitoid overwintering and hyperparasitism to parameterize an existing model of soybean aphid population dynamics to more accurately estimate the value of A. certus as a biological control agent of soybean aphid.
The soybean aphid, Aphis glycines, remains a critical pest in Minnesota and the region. Since the aphid’s invasion from Asia in 2000, soybeans in the North Central United States have experienced massive increases in insecticide use comprised of both foliar applications and neonicotinoid seed treatments (Ragsdale et al. 2011; Douglas & Tooker 2015). Most of the insecticides used against the soybean aphid have broad-spectrum toxicity and therefore endanger insects that are beneficial and of conservation concern, as well as some vertebrate animals including humans (Ragsdale et al. 2011). Furthermore, the manufacture and application of foliar insecticides against soybean aphid leads to the emission of substantial amounts of greenhouse gases (Heimpel et al., 2013). For these reasons, finding alternatives to insecticides is a critically important component of developing sustainable soybean production in Minnesota and beyond.
One of the most promising strategies to permanently and sustainably control soybean aphid is biological control by parasitoid wasps. Renowned worldwide for controlling populations of pest aphids, aphid parasitoids are insects that deposit eggs into aphids, eventually killing them by larval feeding (Brodeur et al. 2017). Parasitoid-based biological control of soybean aphid in North America shows promise because soybean aphid is well-controlled by its native insect predators and parasitoids in Asia (Heimpel et al. 2004). One of these species of parasitoids, Aphelinus certus, has established in North America. Since its first detection in Minnesota in 2011, A. certus has spread throughout the state and proven to be an important natural enemy of soybean aphid (Kaser & Heimpel 2018). Recently developed mathematical models indicate that A. certus decreases the need to apply insecticides on approximately 10% of soybean acreage in Minnesota (J. Miksanek & G.E. Heimpel unpublished). One of the main limiting factors in this model is early-season parasitism; it is clear that increased early-season parasitism could greatly increase biological control and therefore decrease the need for insecticide applications. We have also found that A. certus is subject to parasitism itself – a process known as ‘hyperparasitism’. Hyperparasitism is well known in parasitoid ecology and can, in some cases, compromise biological control (Heimpel & Mills 2017). In this proposal we investigate these two potential limiting factors in biological control of soybean aphid – overwintering mortality and hyperparasitism – and propose experiments to reduce overwintering mortality.
Project Goals and Objectives:
Our over-arching goals are to determine whether parasitoid mortality is compromising biological control of soybean aphid and, if so, whether this mortality can be decreased. We propose the following three objectives as a means of accomplishing these goals.
- Evaluation of the effect of tilling on overwintering mortality of soybean aphid parasitoids.
- Evaluation of early-season migration and development of soybean aphid and its parasitoids.
- Evaluation of Aphelinus certus hyperparasitism.
Brodeur, J., A. E. Hajek, G. E. Heimpel, J. J. Sloggett, M. Mackauer, J. K. Pell, and W. Völkl. Predators, parasitoids and pathogens. 2017. in H. F. van Emden, ed. Aphids as Crop Pests – 2nd Ed. CABI.
Douglas MR & Tooker JF (2015) Large-Scale Deployment of Seed Treatments Has Driven Rapid Increase in Use of Neonicotinoid Insecticides and Preemptive Pest Management in US Field Crops. Environmental Science & Technology 49: 5088-5097.
Heimpel, G.E. & N.J. Mills. 2017. Biological Control: Ecology and Applications. Cambridge University Press. Cambridge, U.K.
Heimpel GE, Ragsdale DW, Venette R, Hopper KR, O'Neil RJ, Rutledge CE & Wu Z (2004) Prospects for importation biological control of the soybean aphid: anticipating potential costs and benefits. Annals of the Entomological Society of America 97: 249-258.
Heimpel GE, Yang Y, Hill J & Ragsdale DW (2013) Environmental consequences of invasive species: greenhouse gas emissions of insecticide use and the role of biological control in reducing emissions. PLoS One 8: e72293.
Kaser, J. M. and G. E. Heimpel. 2018. Impact of the parasitoid Aphelinus certus on soybean aphid populations. Biological Control 127:17-24.
Ragsdale DW, Landis DA, Brodeur J, Heimpel GE & Desneux N (2011) Ecology and management of soybean aphid in North America. Annual Review of Entomology 56: 375-399.