Crystal Almdal and Alejandro C. Costamagna, Department of Entomology, University of Manitoba
Soybean aphids are an invasive species and major pest with the potential to reduce soybean yields up to 40%. The soybean aphid was first detected in North America in 2000 and has since spread throughout the United States and Canadian prairies. Since its introduction, it has occurred infrequently in Manitoba, but reached widespread outbreak levels in 2006, 2008, 2011, and more recently in 2017. Our previous work during 2012–2014 (low soybean aphid years) showed that natural enemies, including ladybeetles, green lacewings, brown lacewings, minute pirate bugs, damsel bugs, hoverflies and parasitoids provide strong control of soybean aphids. With the outbreak of soybean aphids in 2017, we were able to compare aphid suppression and natural enemy response between outbreak and non-outbreak years in Manitoba. Specifically, we asked:
- 1. Does an aphid outbreak impact aphid suppression by natural enemies?
- Do natural enemies respond to aphid outbreaks? and
- Where are natural enemies migrating from?
Answering these questions will allow us to gain a better understanding of how we can maximize pest suppression by natural enemies to reduce the need to apply broad-spectrum insecticides.
We conducted field experiments in 12 fields in 2017 and 11 fields in 2018 throughout southern Manitoba (Emerson, Morris, Gimli, Winkler, Oakville, Beausejour, Arnaud, Carman, Elm Creek, Steinbach, New Bothwell, St. Adolphe, Warren, Kleefeld, La Broquerie, Scanterbury). To determine the level of aphid suppression, we set up five aphid colonies on plants open to natural enemies and five aphid colonies on plants closed off to natural enemies in each field studied for a period of two weeks (Figure 1).
To monitor the movement of natural enemies, we set up bi-directional Malaise traps between soybean and the neighbouring field (either canola, alfalfa, wheat or woody vegetation, Figure 2). In addition to Malaise traps, we conducted sweep-net samples within soybeans to determine the abundance of natural enemies within our study soybean fields.
In 2017, we saw seven out of our 12 soybean study fields above the economic threshold of 250 aphids per plant. We witnessed high levels of winged aphids immigrating in soybeans, counting hundreds of winged and wingless aphids on plants, that at times were covered entirely in aphids (Figure 3). In contrast, 2018 was the complete opposite, where we saw very few aphids in our study fields, with counts nowhere near the economic threshold.
The high level of soybean aphid migration in 2017 resulted in aphids escaping natural enemy control on our experimental plants. Since our aphid colonies open to natural enemies were also open to winged aphids, we could not tease apart the impact of natural enemies from the impact of aphid immigration. Therefore, we only observed significant aphid suppression in one of the twelve study fields, where aphid immigration was comparatively low. In contrast, in 2018, we observed significant aphid suppression on all experimental aphid colonies, as soybean aphid immigration was very low. Therefore, when aphid populations within fields are low, which is the usual situation in Manitoba, natural enemies can suppress them, but during massive immigration of winged aphids, natural enemies seem to be overwhelmed and aphids develop outbreak populations that escape control.
Although suppression was not observed in 2017, we did see a four-fold increase in the abundance of ladybeetles, a six-fold increase in hoverflies, and a two-fold increase in damsel bugs compared to 2018. Natural enemies were responding numerically to soybean aphids in 2017. Each week we observed more aphids, but we also saw more natural enemies than in the previous sampling week, including hoverflies, ladybeetle larvae and adults (Figure 4), minute pirate bugs, damsel bugs, and green and brown lacewings. Additionally, during our last sampling week, we saw many black aphid mummies on soybean plants. Aphelinus certus parasitoids likely parasitized these aphids, but we are awaiting confirmation on its identification. We also saw brown aphid mummies on soybean, indicating the presence of another parasitoid species, but they were not as abundant as the black ones.
Our results suggest that ladybeetles are moving into and staying in soybeans to feed on aphids. We observed more ladybeetles migrating to soybeans during the high-aphid year than the low-aphid year. More ladybeetles were seen immigrating into soybeans than emigrating out. Green lacewings and hoverflies were found to move more between soybeans and canola than between soybeans and wheat, alfalfa or woody vegetation in 2017, and in 2018 hoverflies followed this same trend. That suggests canola is an important crop for hoverflies regardless of soybean aphid density. Canola is most likely an important habitat for both hoverflies and green lacewings because it provides them with pollen as an additional food source. Overall, we saw natural enemies moving between soybeans and wheat, canola, alfalfa and woody vegetation, suggesting they are all crucial contributors of natural enemies.
Natural enemies can control soybean aphids when aphid populations are low. An aphid outbreak does impact aphid suppression by natural enemies even though natural enemies are responding numerically to aphid outbreaks. Ladybeetles are moving into soybeans at a higher rate during an outbreak year than a low-aphid year and are found to be staying in soybeans. Green lacewings and hoverflies mostly move between soybeans and canola; therefore, canola serves as an essential crop for soybean aphid suppression by contributing natural enemies in soybeans. Planting a variety of crops in the landscape may promote pest control services in soybeans by providing different types of natural enemies that have different habitat requirements that would move into soybeans when aphid immigration occurs.
We conclude that natural enemy populations suppress soybean aphids during most years and have the potential to reduce soybean aphid outbreaks in years of high-aphid immigration. Our research is focusing on finding the best combination of natural habitats and crops that increase natural enemy populations in agricultural landscapes to achieve sustainable aphid control.