Lots of buckthorn around? Try early soybean aphid scouting

Brooke Sonnek, Extension ANR Intern, Blue Earth & Le Sueur Counties
Reviewed by Shane Bugeja Extension Educator, Blue Earth & Le Sueur Counties and Robert Koch, Extension soybean entomologist

The soybean aphid is one of the greatest agricultural pests in Minnesota.⁸ When infestations grow out of control, management and insecticide applications can become very costly. Some populations have also become resistant to pyrethroid insecticides which can add additional expenses. Fields that are most susceptible to early aphid infestations often have abundant common (or European) buckthorn in their wooded borders. 

Buckthorn is the overwintering host for soybean aphids, and during the late spring/early summer these insects can make the short journey to nearby soybean plants.⁷ There is nothing worse than finding your field with high numbers of aphids much earlier than expected. Buckthorn removal has potential benefits to your wooded lot, but unfortunately buckthorn removal will not prevent aphid problems over the season. Still, identifying if your land is abundant with buckthorn might help you target fields for early aphid scouting.

Common Buckthorn Profile

Common buckthorn. Photo: Rick Carlino

Common buckthorn (Rhamnus cathartica) is an invasive species and a Restricted Noxious Weed in Minnesota. Buckthorn can be identified as a tall shrub which covers wooded areas in dense thickets of its spreading branches. Spread by birds into new areas, buckthorn crowds and shades out native plants and shrubs.³ Buckthorn does this by leafing out earlier and keeping its leaves later into fall.

Common buckthorn flowers. Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Soybean aphids on buckthorn. Photo: Christina DiFonzo, Michigan State University, Bugwood.org

Why implement earlier scouting?

As the overwintering host, nearby buckthorn gives aphids easy and early access to your soybean crop. This scouting should occur around early June as aphids can colonize plants as early as May 13 (Fig.1). Focusing earlier scouting efforts on fields with known nearby buckthorn can allow you to identify potential problem areas and facilitate more timely management of infestations. This helps eliminate the possibility of being surprised later in the season with infestations past the threshold of 250 aphids per plant.¹³ If necessary, speed scouting can be an acceptable, but a less accurate substitute for traditional whole plant counts. Regardless of what scouting method you choose, identifying when economic thresholds are met is critical.⁵

Fig. 1. Soybean aphid life cycle in Minnesota. Source: Minnesota Department of Agriculture

Buckthorn removal limitations

While seemingly simple, removing all common buckthorn (whether by herbicide treatments, physical removal, or goats) is not a practical solution to stop soybean aphids in Minnesota. Aphids can drift in from far away areas—traveling up to six miles per day⁴—and large buckthorn populations can be on others’ land and outside of our control.¹

Traditional Buckthorn Treatments

Most traditional removal techniques involve chemical treatment. Cutting the stems and applying herbicide to the cut area is very common. Other treatments exist, such as spraying herbicide on the bark or onto the leaves, but all are time consuming and labor-intensive. Deciding if buckthorn removal is right for you and your operations can be tricky.

UMN Extension produced several videos for planning and managing buckthorn removal that may be helpful.

Diving Deeper into Goat Grazing

Studies have shown buckthorn removal can be achieved through grazing animals such as goats.^10,11 Grazing is a non-chemical solution which can have less costs compared to expensive and labor-intensive means of traditional removal. This solution can be beneficial for creating positive relations and possible partnerships with neighbors who are woodland owners near your land. There are even tools such as the Cropland Grazing Exchange program to connect landowners in need of grazing with livestock owners. The Cropland Grazing Exchange map provides means for connections by showing locations of potential grazing partners and including details like type of land (cover crops fields, woodlands, etc.) or livestock, fencing needs, and other relevant potential partnership information. Whether it's your own herd of goats or working with a neighbor to have animals graze, buckthorn can be removed in a productive fashion.

Buckthorn Removal Can Have Extra Benefits Too

For woodland owners, removing buckthorn can also result in a beneficial resurgence of biodiversity. Woodlands are able to thrive and support wildlife when native species are allowed to grow. It can also greatly support long-term silvopasture for grazing by thinning out shrub cover.¹⁵ A recommendation along with creating silvopasture is to plant native tree species as an investment for timber harvesting.¹⁵

Replacing buckthorn with native or non-invasive species can even prevent buckthorn re-invasion.¹⁴ It can also combat local soil erosion and increase organic matter which is commonly poor with buckthorn presence.⁶ Investing in the removal of buckthorn can transform a woodland into a thriving habitat for plants, wildlife, and beneficial insects including pollinators.⁹ An increase of pollinators and pollinator habitats can benefit many crops, possibly including soybeans.^2,12

Buckthorn management can have many benefits to your land and community through cooperative efforts and even a revitalization of plant and wildlife diversity. However, it is important to note that removing nearby buckthorn will not eliminate aphid problems. Regardless of whether you choose to remove or not, identifying nearby buckthorn presence and exercising early aphid scouting could directly benefit your soybean crop.

Sources

1. Bahlai, C., Sikkema, S., Hallett, R., Newman, J., & Schaafsma, A. (2010). Modeling Distribution and Abundance of Soybean Aphid in Soybean Fields Using Measurements from the Surrounding Landscape. Environmental Entomology, 39(1), 50-56.


2. Garibaldi, L., Carvalheiro, L., Vaissière, B., Gemmill-Herren, B., Hipólito, J., Freitas, B., Zhang, H. (2016). Mutually beneficial pollinator diversity and crop yield outcomes in small and large farms. Science (American Association for the Advancement of Science), 351(6271), 388-391.
3. Gupta, A., Rager, A., & Weber, M.W. (2019). Common Buckthorn. University of Minnesota Extension. https://extension.umn.edu/identify-invasive-species/common-buckthorn


4. Jardine, D.J., & Tilmon, K. (2020). Soybean Aphid. Soybean Research & Information Network. https://soybeanresearchinfo.com/soybean-pest/soybean-aphid/?pdf_download=1


5. Kim, C., Schaible, G., Garrett, L., Lubowski, R., & Lee, D. (2008). Economic Impacts of the U.S. Soybean Aphid Infestation: A Multi-Regional Competitive Dynamic Analysis. Agricultural and Resource Economics Review, 37(2), 227-242.


6. Knight, K., Kurylo, J., Endress, A., Stewart, J., & Reich, P. (2007). Ecology and ecosystem impacts of common buckthorn (Rhamnus cathartica): A review. Biological Invasions, 9(8), 925-937


7. Koch, R. & Potter, B. Scouting for soybean aphid. University of Minnesota Extension, Southwest Research and Outreach Center. https://extension.umn.edu/soybean-pest-management/scouting-soybean-aphid


8. Koch, R. & Wold-Burkness, S. Soybean aphid. University of Minnesota Extension. https://extension.umn.edu/soybean-pest-management/soybean-aphid


9. Larkin, D., Steffen, J., Gentile, R., & Zirbel, C. (2014). Ecosystem Changes Following Restoration of a Buckthorn‐Invaded Woodland. Restoration Ecology, 22(1), 89-97.


10. Marchetto, K., Heuschele, D., Larkin, D., & Wolf, T. (2020). Goat Digestion Leads to Low Survival and Viability of Common Buckthorn (Rhamnus cathartica) Seeds. Natural Areas Journal, 40(2), 150-154.


11. Marchetto, K., Wolf, T., & Larkin, D. (2021). The effectiveness of using targeted grazing for vegetation management: A meta‐analysis. Restoration Ecology, 29(5).


12. Monasterolo, M., Musicante, M., Valladares, G., & Salvo, A. (2015). Soybean crops may benefit from forest pollinators. Agriculture, Ecosystems & Environment, 202, 217-222.


13. Minnesota Department of Agriculture. (2022). Recommended IPM Approach and Treatment Threshold for Soybean Aphid Control in Soybean. Minnesota Department of Agriculture. https://www.mda.state.mn.us/sites/default/files/docs/2022-06/ipmsoybeanaphid_0.pdf


14. Schuster, M., Wragg, P., Roth, A., Bockenstedt, P., Frelich, L., & Reich, P. (2022). Using plants to control buckthorn (Rhamnus cathartica): Improved biotic resistance of forests through revegetation. Ecological Engineering, 182, 106730


15. Zamora, D.Wyatt, G., Buttler, M., Ford, M., Magner, J., Reichenback, M., Burkett, E., Current, D., & Walter, D. (2017). Silvopasture: Establishment and Management Principles for Minnesota. University of Minnesota Extension.