In experiments conducted over the 2014-2016 field season at the U of MN Agricultural Experiment Stations in Rosemount and St Paul, MN, and in two farmer’s fields in Northfield and Cottonwood counties, we evaluated introductions of the Asian parasitoid Aphelinus glycinis in the presence and absence of prairie vegetation and also evaluated the effectiveness of the resident Asian parasitoid Aphelinus certus at the St. Paul site.
Our results showed that A. glycinis could establish well during the field season but we could not confirm successful overwintering of this species although limited overwintering was found in an experiment conducted in 2014 (but not 2015). Further overwintering studies are in progress. Prairie vegetation did not have a significant positive or negative effect on soybean aphid parasitism by this other parasitoid species in our studies. However, the diversity of parasitoid species was higher adjacent to prairie species.
Parallel studies on the resident Asian parasitoid A. certus conducted in St. Paul during 2015 showed that this species was able to maintain soybean aphid at population densities below the economic spray threshold. Different studies in 2016 showed that both A. certus and A. glycinis species was negatively effected by neonicotinoid seed treatments. A study investigating the compatibility of both of these species and aphid-resistant soybean varieties in 2016 is currently being analyzed.
The soybean aphid, Aphis glycines, is the key pest of soybeans in the North Central U.S. We have been studying the effect of Asian parasitoids on soybean aphid pressure for the last decade and recently obtained permission from the USDA to release a specialized soybean aphid parasitoid known as Aphelinus glycinis against the soybean aphid in Minnesota. Simultaneously we have been studying the effect of native prairie plantings on biological control of soybean aphid by resident predators and parasitoids. In this research project we conducted releases of A. glycinis adjacent to these prairie plantings and compared resulting parasitism rates to control sites without prairie plantings. We also conducted releases of A. glycinis on two organic farms in Minnesota during the course of the study.
We conducted releases of A. glycinis during the summers of 2014, 2015 and 2016 at 2 experiment station sites and 2 farmer’s fields. We also conducted overwintering studies on this species over the 2014/2015 and 2015/2016 winters with a third round of studies in progress. During the summer of 2015 we conducted releases of A. glycinis adjacent to prairie vegetation and compared these releases to ones conducted at control sites that were within monoculture soybean fields.
Aphelinus glycinis parasitoids were reared by Dr. Keith Hopper of the USDA in Delaware and shipped to Minnesota for release and in excess of 75,000 parasitoids were released per site. At the organic farmer’s fields (in Northfield and Cottonwood) parasitoids were released in July of 2014 from 10 release tubs throughout the field and sampling was done throughout the rest of the season at 8 sites surrounding the releases (Fig. 1). Sampling was also done in adjacent soybean fields and buckthorn stands (the overwintering host for soybean aphid) during Spring and summer 2015. When sampling we noted the presence of A. glycinis as well as another Asian parasitoid, A. certus, which has become established in Minnesota since 2011.
Releases in 2015 were conducted in Rosemount MN at UMORE Park and they were done as part of a larger study investigating the effects of prairie plantings adjacent to soybean fields on biological control of soybean aphids. We released A. glycinis adjacent to 4 replicated prairie plantings (Fig. 2A) and 4 control sites (within soybean fields). Each release consisted of at least 10,000 parasitoids and they were conducted from specialized release tubs (Fig. 2B). As in the 2014 study, we evaluated parasitism by both A. glycinis and A. certus and also sampled the year after the release to evaluate overwintering success.
The parasitoid release studies were successful in showing that A. glycinis was able to establish during the summer field season, but we have not strong evidence of overwintering and there was no significant difference in the level of establishment between the prairie and control plots by this parasitoid species in our study at the Rosemount Ag. Experiment Station. The study also showed that A. glycinis were found at only very low levels at sites 10m from the release, which suggests a low dispersal rate. Another soybean aphid parasitoid, A. certus was also not found in greater numbers near the prairie versus control sites, but a third species, A. asychis, was found attacking soybean aphid almost exclusively adjacent to the prairie plots. We showed that another soybean aphid parasitoid, A certus is able to effectively suppress soybean aphids and that both A. certus and A. glycinis are negatively impacted by neonicotinoid seed treatments.
Prairie plantings adjacent to soybean fields enhanced soybean aphid parasitism by Aphelinus asychis in our study. A. asychis is native to Eurasia and was introduced into the Western U.S. to control the Russian wheat aphid in the late 1980s and has been sporadically reported attacking soybean aphid in the Midwest.
We do not have confirmation of overwintering by the introduced parasitoid A. glycinis in Minnesota.
Both A. glycinis and A. certus are negatively affected by the neonicitinoid seed treatment thiamethoxam.
We are not promoting further distribution of Aphelinus glycinis due to our finding of poor overwintering success by this species. However, our finding of strong suppression of soybean aphid by Aphelinus certus is being reported at extension events. Farmer behavior does not have to change in response to this but for conventional farmers it suggests that insecticide application will decrease to the extent that the spray threshold of 250 aphids/plant is adhered to. Organic farmers that use alternative spray tactics should also be able to spray less, assuming that they adopt an IPM approach that incorporates sampling and a spray threshold.
Educational & Outreach Activities
1/2014. Kaser, J.K., N. Padowski, J. Peterson & G.E. Heimpel. Soybean aphid research – Biological control using parasitoid wasps. MN AgExpo; Mankato MN. Poster presentation.
1/2015. G.E. Heimpel. Promoting sustainable biological control of the soybean aphid: biodiversity and releases of parasitoid wasps. Northern Plains Sustainable Agriculture Conference. Aberdeen, S. Dakota.
9/2015. Heimpel, G.E. Habitat Diversity and Biological Control of Insect Pests: Soybean Aphid as a Case Study. Dept. Seminar at Virginia Tech University, Blacksburg, VA.
6/2016. J. Kaser, J. Dregni, N. Padowski, R. Koch, G.E. Heimpel. Biological control ecology: lessons from introduced soybean aphid parasitoids. Symposium presentation. Entomological Society of America, North Central Branch meeting, Cleveland, OH.
Areas needing additional study
Research should focus on the role that A. certus is playing in suppressing populations of soybean aphids. We need to know how this varies with region of the state and with farming practices (e.g. organic vs. conventional). This parasitoid will only result in a reduction of insecticide application if farmers are adhering to IPM principles and a spray threshold. It is therefore important to know the extent to which these practices are being used.