Soybean Aphid and Changing Pesticide Use in Iowa

by Hyewon Lee, Erin W. Hodgson, Doris M. Lagos-Kutz, and David A. Hennessy

Soybean Aphid (Aphis glycines) has been a major problem for US soybean producers since its emergence in the early 2000s. The outbreak in the Midwest was fast and successful because of the extensive soybean production and the wide distribution of its overwintering host, the common buckthorn (Rhamnus cathartica). Severe regional outbreaks in the mid‑2000s were followed by a rapid increase in foliar pesticides to protect yield (Ragsdale et al. 2011). Since 2010, an equilibrium has emerged where soybean fields now encounter low to moderate aphid pressure with intermittent local outbreaks. Using: (a) Iowa State University Research Farms; and, (b) American Midwest Suction Trap Network (STN) data, we report on temporal patterns in soybean aphid. For northern and central Iowa, we also discuss changing patterns in pesticide use targeted at aphids, neonicotinoid seed treatments (NSTs) targeting a broad range of insects, and agronomic practices such as planting period. These data allow us to evaluate how changes in aphid pressure, insecticide use, seed treatment adoption, and planting practices have interacted over time. Together, they help explain why pest pressure and pesticide use have moderated over time.

Aphid density data and their usage

We use two data sources to show trends in soybean aphid population dynamics. The first arises from insecticide efficacy evaluations conducted at Iowa State University. Iowa field data records the weekly aphid density (aphids per plant) in untreated soybean plots in northern Iowa (i.e., Kanawha, Nashua, and Sutherland). These observations are available by location from 2005 onward and reflect local population growth under field conditions.¹ Because aphids reproduce asexually and predominantly produce wingless morphs when conditions are favorable, these counts provide a direct measure of local infestation pressure. 

The other data source is best suited to monitoring regional dispersal. STN is a regional network that samples primarily flying insects of concern to the corn and soybean crops. The coordinated monitoring system is managed by the University of Illinois at Urbana–Champaign and funded by the USDA and North Central Soybean Research Program. The network includes suction traps that were set mainly on university research farms (and some private farms) in 12 Midwestern states.² Weekly records of soybean aphids per trap were collected from 2005 onward. These traps catch primarily sample winged morphs that arise when field populations crowd and aphids disperse to colonize new hosts. Together, the field and traps count document complementary phases of the aphid life cycle: field densities index local buildup, while trap catches index subsequent migratory flights.

In this article, we restrict both datasets to Iowa observations, particularly northern and central counties.³ To summarize pressure over the growing season, we calculate Cumulative Aphid Days (CAD) for field data and an analogous Cumulative Trap Catch (CTC) for STN data. CAD is the seasonal accumulation of soybean aphid activity over the growing season, as aphids can feed on plants for over 90 days. CAD is a common metric in entomology that relates feeding intensity to estimated yield losses.⁴ The measure seeks to summarize both exposure and intensity of infestation. CTC is not a standard entomological metric, but we define it here as the season-long sum of weekly STN trap catches to provide a transparent analog to CAD for comparing seasonal intensity across data sources. CAD and CTC capture complementary phases of soybean aphid biology (on plant populations vs. migratory flights) and are not expressed in the same units. We therefore compare patterns rather than magnitudes and interpret STN as a regional signal rather than field risk per se.

Soybean aphid pressure and pesticide use

Figure 1 shows aphid infestation pressure and pesticide use trends. The green line shows trap catch information, with large regional outbreaks in the mid-2000s, particularly in 2009. Afterward, the Iowa STN never again recorded high values. Field observations (blue) also capture elevated infestation pressure in 2009. Iowa field observations highlight localized outbreaks until the extreme outbreak in 2019, when CAD exceeded 35,000 (equivalent to 25.3% yield loss; Ragsdale et al. 2007). Based on field data from 2014 onward (weekly observations available beginning in 2014), only 15 weeks out of 204 exceeded the commonly advocated economic threshold of 250 aphids per plant. Notably, since the 2019 outbreak, the economic threshold has been exceeded only once. Despite differences in spatial and temporal coverage, the two data sources together yield valuable analytical insights. Importantly, aphid populations have declined markedly since 2020, with regional trap catches and field counts remaining at low levels.
Figure 1 also shows pesticide use data trends. The data are from Kynetic USA, a provider of on-farm crop input data based on grower survey responses. The pesticide use shown is noteworthy as it reflects only consumption reported by farmers who indicated that their pesticide application was specifically targeted at soybean aphids, possibly among other targeted species. The unit of pesticide use is active ingredient in pounds per soybean acre. Viewed alongside aphid pressure trends, pesticide use has a broadly comparable pattern. Consumption rose markedly in the mid-2000s during the initial outbreak, but thereafter exhibited only limited variation, reflecting overall stability over time. Soybean aphids have been less active in recent years, and the pesticide use to control aphids has declined concomitantly.

Change in chemical use

While the pesticide use overall aligns with pest pressure and is stable, farmers’ pesticide choices have changed dramatically. Two notable shifts stand out. First, aphid-targeted spray has tilted more heavily toward pyrethroids. Figure 2 shows the changes in the composition of active ingredients in pesticides targeting aphids in northern and central Iowa. In figure 2, pesticide application is expressed as a percent of total active ingredients per acre per year, partitioned into pyrethroids, organophosphates, and other active ingredients. The gray line in the figure indicates the absolute volume of pyrethroid use, showing that while the overall amount of insecticides applied has remained relatively stable, the composition has shifted significantly over time. Historically divided between organophosphates and pyrethroids, pesticide use now has swung toward pyrethroids. 

While the hazards of organophosphates are already well established, the potential risks of pyrethroids have received comparatively less attention. Although pyrethroids replaced organophosphates, they also carry significant risks. To compare hazards across insecticides consistently, we apply the Environmental Impact Quotient (EIQ), which aggregates risks to workers, consumers, and ecosystems (Eshenaur et al. 2025). For instance, chlorpyrifos (an organophosphate) has an EIQ of 87, while cyhalothrin (a pyrethroid) scores 75—both considerably higher than that of common herbicides used in soybean, such as glyphosate (41) or 2,4-D (30). Thus, the shift from organophosphates to pyrethroids has not substantively reduced overall risk, leaving soybean aphid management heavily reliant on environmentally costly chemicals. This dependence has already led to the emergence of pyrethroid resistance in soybean aphid populations (Hanson et al. 2017) and increases the risk of disrupting beneficial species.

Second, neonicotinoid-treated seeds (NSTs), commercially available since 2006, have diffused rapidly across the agricultural sector. Figure 3 shows NST use in northern and central Iowa soybean production. The figure shows the reported share of acres planted with NST, rising sharply between 2006 and 2013. By 2014, NST adoption had become nearly universal, with approximately 80% coverage. After 2014, data collectors ceased explicitly asking about NSTs because it had become the default practice for almost all farmers, where many farmers were failing to recognize seed treatment as an applied pesticide. 

Increased adoption of NST brings two broad problems. First, research highlights its considerable environmental and health risks. Reflecting these concerns, the US EPA initiated a registration review of neonicotinoids, with completion expected in 2025. Second, these compounds are not deployed in response to pest pressure at critical moments but are instead applied preemptively at planting. This is further problematic regarding soybean aphids because there is a discrepancy between the NST effect window and the aphid outbreak window. Neonicotinoids in soybean fields remain bioactive in foliage for only approximately the first 3–4 weeks after planting, after which their insecticidal protection rapidly declines.

In contrast, soybean aphid populations typically begin to escalate during mid- to late-summer, often around June and more intensively in late July and August, when plants are in the late vegetative to reproductive stages. Consequently, although NST is a multiple-purpose treatment, the temporal overlap between the short-lived protective window offered by seed treatments and the peak of soybean aphid outbreaks is minimal. This mismatch implies that NSTs for soybean aphids likely offer little yield benefit while still imposing ecological and environmental costs (Mourtzinis et al. 2019). Taken together, the evolution of insecticide choice and prophylactic seed treatments highlights how pest control practices have shifted without necessarily lowering overall environmental or agronomic risk.

Change in non-chemical practices

Another agronomic shift is earlier planting dates. Figure 4 shows the planting progress of soybeans for northern and central Iowa. Soybeans now often reach 50% planted by early May rather than late May. This change was driven mainly by weed control, technology, climate, insurance coverage, and workload management with corn, not aphid management. Its implications for soybean aphids are less clear: earlier planting may shorten crop-aphid overlap, but it may also increase exposure to spring migrants and allow more time for colonies to establish and surpass thresholds. We therefore treat this trend as an important feature of the production system while recognizing that its direct influence on aphid pressure has yet to be clearly established.

Conclusion

Severe aphid outbreaks are now rare, yet prophylactic and broad-spectrum insecticide use remains common, and agronomic practice shifts. These practices persist not simply because of pest biology, but because of structural and behavioral factors: time constraints; scouting limitations; and, the appeal of inexpensive, easy-to-apply solutions. The central challenge, therefore, is not only biological but institutional: understanding why integrated pest management adoption remains limited and what incentives or supports might shift behavior toward more targeted, sustainable practices.

Footnotes

1. For more information, see https://soybeanresearch.ppem.iastate.edu/extension (accessed September 24, 2025).
2. These states are Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Michigan, Minnesota, Missouri, Nebraska, South Dakota, and Wisconsin. Those in Louisiana and South Dakota are no longer active. For more information, see Lagos-Kutz et al. (2020) and https://suctiontrapnetwork.org/ (accessed September 24, 2025).
3. Six crop reporting districts (CRD) are covered: Northwest (19010), North Central (19020), Northeast (19030), West Central (19040), Central (19050), and East Central (19060).
4. CAD is calculated by averaging the number of aphids observed at two consecutive time points, multiplying by the time between those observations, and then summing these values across all observation intervals.
 

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Suggested citation

Lee, H., E.W. Hodgson, D.M. Lagos-Kutz, and D.A. Hennessy. 2025. "Soybean Aphid and Changing Pesticide Usage in Iowa." Agricultural Policy Review Fall 2025. Center for Agricultural and Rural Development, Iowa State University. https://agpolicyreview.card.iastate.edu/fall-2025/soybean-aphid-and-changing-pesticide-use-iowa