The surprising connection between neonicotinoids, soybean aphid, and invasive ladybeetles

This post explores a new paper by KBS LTER scientists Drs. Christie Bahlai and Doug Landis, Shifts in dynamic regime of an invasive lady beetle are linked to the invasion and insecticidal management of its prey. The paper is now available as a preprint at Ecological Applications. A version of this post, written by Dr. Bahai, originally appeared on her blog, Practical Data Management for Bug Counters.

Neonicotinoids, a commonly used class of broad-spectrum insecticides, have been the subject of much controversy in recent years, particularly their impacts on beesbirds, and their general efficacy at controlling target pests. This issue is complex, and is at risk of oversimplification if all facets are not examined.

Soybean aphid infested soybean plants

Soybean aphid infested soybean plants- photo by Christina Difonzo

Our study uses three legacy datasets, including the KBS LTER ladybeetle database, to examine another aspect of this issue. In it, we link the population dynamics of an invasive lady beetle, to agronomic practices in the US Midwest, particularly neonicotinoid seed treatments.

Soybean is a major field crop in central North America. In 2000, a new invasive species, the soybean aphid, arrived in North America and swiftly became a major pest in a crop that previously had very few troubles with insects. The Midwest had several major outbreaks immediately following their invasion. Soybean aphid is eaten by a broad suite of predators, including another notable invasive species, the Multicoloured Asian ladybeetleHarmonia axyridis.

Since 1989, ladybeetle populations have been monitored at the KBS LTER in southwestern Michigan. This monitoring captured the 1993 invasion of Harmonia to the region, and the subsequent population dynamics of this species.

In a 2012 paper, my co-author and study PI Doug Landis remarked on an apparent pattern in the dynamics of Harmonia: 1) generally moderate populations of Harmonia prior to soybean aphid arrival (in 2000), 2) a distinct 2-year cycle in peaks of Harmonia during the acute phase of soybean aphid invasion (2001-6) and a return to moderate populations of Harmonia afterwards. In the new Ecological Applications paper, my colleagues and I use quantitative techniques to determine if the patterns we observe fit certain theoretical patterns, and if so, if we could detect changes in the pattern, and figure out when they were.

Line Graph showing Mean abundance of Harmonia axyridis

Figure 7 from Knapp, A. K., et al. 2012. Past, present, and future roles of long-term experiments in the LTER Network. Bioscience 62:377-389

What our model tells us is that after 2000, corresponding with the invasion of soybean aphid, Harmonia’s intrinsic rate of increase (a measure of how fast a population can grow) went up by 20%. Simultaneously, the carrying capacity for Harmonia (a measure of how large a population an environment can sustain) went up by 40%. This is not surprising: a new, highly abundant, and highly suitable aphid food source suddenly became available. This means good times for ladybugs. What is surprising, though, is what happens in 2006: our model tells us that at this time, Harmonia’s dynamics return to values identical to those before soybean aphid’s arrival—essentially, from a quantitative standpoint and looking at it from the Harmonia perspective—it’s like soybean aphid stopped existing.

Now, soybean aphid did not disappear from North America. But around this time, the aphid did become a lot harder to find than it had been previously. Outbreak populations still happened, but they became rarer, less severe, and patchy. It seemed likely that, because Harmonia’s first shift was clearly related to the invasion of soybean aphid, its second shift was related to this apparent decline in soybean aphid. However, we still didn’t have a clear explanation for what actually happened to the aphids.

We turned to scouting records and Extension publications to get regional estimates of soybean aphid infestation. Then we compared these records, region by region, year by year, to estimates for pesticides used in soybean. We found an unexpected result: that soybean aphid outbreaks have a negative association with neonicotinoid use. That is, regions with more neonicotinoid seed treatment use have fewer or less severe aphid outbreaks.

Why was this result unexpected? Well, there is a growing body of literature that shows neonicotinoid seed treatments are largely ineffective at preventing aphid outbreaks in soybean, when you compare treated and untreated fields side-by-side. Neonicotinoids usually only last about 40 days after planting, and soybean aphid outbreaks usually occur mid-to-late summer, so there is no insecticidal activity at the time when aphids cause economic damage. However, for reasons we detail in the paper, it appears that these insecticides can have dramatic effects on early season establishment of these aphids, and, as I’ve argued before, early season survivorship is extremely important in season-long aphid dynamics. This, in turn, affects Harmonia—fewer aphids to eat—both early and late in the season.

Although this result is correlative, the evidence suggests that widespread adoption of prophylactic seed treatments is having indirect negative effects on Harmonia (which could be a good or bad thing depending on your view of Harmonia, as a valuable biocontrol organism or as a disruptive invasive species).

Further, our study suggests that if seed treatments were to be limited (EPA is reviewing their use in America right now, and adjacent Ontario has already introduced legislation to dramatically reduce their use) there could be a return to increased soybean aphid and more explosive Harmonia population dynamics. To mitigate this outcome in the event of changes to neonicotinoid use, we suggest alternate management strategies, including host plant resistance and biocontrol, which could potentially offer similar, landscape-level suppression of these two invasive species, but without the controversy that follows the use of this class of insecticides.