Dual regime shifts in dynamics of an invasive predatory ladybug are linked to the invasion and pesticidal control of its aphid prey

The spread and impact of invasive species may vary over time in relation to changes in the species itself, the biological community of which it is part, changes in climate, or external controls on the system. Here we investigate whether there have been regime changes in the dynamics of two invasive species in the Midwestern US, the Multicoloured Asian Ladybeetle Harmonia axyridis and the soybean aphid, Aphis glycines, and explore the underlying drivers of these changes. We show by model selection that after its first invasion into the American Midwest, the year-to-year changes of H. axyridis were governed by a logistic rule supporting gradual rise to a stable carrying capacity. After invasion of the soybean aphid in 2000, food resources at the landscape level became abundant, supporting higher year­to-year growth rate, and higher but unstable carrying capacity, with a 2-year predator cycle of aphids and ladybeetles as a consequence. During 2005-2007, farmers in the Midwest progressively adopted pesticidal control strategies, combining prophylactic seed treatment with curative spraying based on thresholds. This human intervention largely eliminated the soybean aphid as a major food resource for H. axyridis at landscape level, and model selection showed a reverse regime shift towards the original rule for year-to-year dynamics. Thus, we document a short episode of major predator-prey fluctuations in an important agricultural system resulting from two biological invasions,  that were subsequently nullified by widespread pesticidal controls. Development of plant resistance in soybeans may mitigate the need for pesticides and achieve the same stabilization of pest and predator populations at lower cost and environmental burden.