Choosing Organic Pesticides over Synthetic Pesticides May Not Effectively Mitigate Environmental Risk in Soybeans

Background: Selection of pesticides with small ecological footprints is a key factor in developing sustainable agricultural systems. Policy guiding the selection of pesticides often emphasizes natural products and organic-certified pesticides to increase sustainability, because of the prevailing public opinion that natural products are uniformly safer, and thus more environmentally friendly, than synthetic chemicals. Methodology/Principal Findings: We report the results of a study examining the environmental impact of several new synthetic and certified organic insecticides under consideration as reduced-risk insecticides for soybean aphid (Aphis glycines) control, using established and novel methodologies to directly quantify pesticide impact in terms of biocontrol services. We found that in addition to reduced efficacy against aphids compared to novel synthetic insecticides, organic approved insecticides had a similar or even greater negative impact on several natural enemy species in lab studies, were more detrimental to biological control organisms in field experiments, and had higher Environmental Impact Quotients at field use rates. Conclusions/Significance: These data bring into caution the widely held assumption that organic pesticides are more environmentally benign than synthetic ones. All pesticides must be evaluated using an empirically-based risk assessment

Ladybeetle captures at Kellogg Biological Station, 1989-2012

Counts of 13 ladybeetle species collected from yellow sticky cards at sampling stations in the Main Cropping System Experiment at Kellogg biological station in southwestern Michiga

Data from: The role of exotic ladybeetles in the decline of native ladybeetle populations: evidence from long-term monitoring

Ladybeetles (Coleoptera: Coccinellidae) are ubiquitous predators which play an important role in suppressing pest insects. In North America, the coccinellid community is increasingly dominated by exotic species, and the abundance of some native species has declined dramatically since the 1980s. Several hypotheses have been proposed to describe the mechanism of invasion coupled with native species declines, e.g. vacant niche exploitation, intraguild predation, competitive exploitation and habitat compression. We analyze a 24-year dataset of coccinellid community structure in southwestern Michigan to elucidate the most likely mechanism(s) of native coccinellid decline and implications for their conservation. Correlation analyses indicated that impact of exotic species on native coccinellids varies with their degree of interaction. Although several native species were observed to be in numerical decline, only Adalia bipunctata and Coleomegilla maculata had declines that were statistically significant. The magnitude of decline in these two species varied with the degree of dietary overlap with invaders, thus their decline is most likely driven by competitive exploitation. Habitat use patterns by some native species (A. bipunctata and Coccinella trifasciata) changed during years when the exotic Harmonia axyridis reached high numbers, lending support to the habitat compression hypothesis, where native species survive in ancestral (i.e. natural or semi-natural) habitats when invaders dominate cultivated habitats. Coccinellid communities occurring in semi-natural forested habitats were unique in both composition and variability from those occurring in cultivated habitats. Such semi-natural habitats can act as refuges for native coccinellids and may play a role in maintaining the functional resilience of coccinellid communities

Long-term functional dynamics of an aphidophagous coccinellid community remain unchanged despite repeated invasions.

Aphidophagous coccinellids (ladybeetles) are important providers of herbivore suppression ecosystem services. In the last 30 years, the invasion of exotic coccinellid species, coupled with observed declines in native species, has led to considerable interest in the community dynamics and ecosystem function of this guild. Here we examined a 24-year dataset of coccinellid communities in nine habitats in southwestern Michigan for changes in community function in response to invasion. Specifically we analyzed their temporal population dynamics and species diversity, and we modeled the community's potential to suppress pests. Abundance of coccinellids varied widely between 1989 and 2012 and became increasingly exotic-dominated. More than 71% of 57,813 adult coccinellids captured over the 24-year study were exotic species. Shannon diversity increased slightly over time, but herbivore suppression potential of the community remained roughly constant over the course of the study. However, both Shannon diversity and herbivore suppression potential due to native species declined over time in all habitats. The relationship between Shannon diversity and herbivore suppression potential varied with habitat type: a positive relationship in forest and perennial habitats, but was uncorrelated in annual habitats. This trend may have been because annual habitats were dominated by a few, highly voracious exotic species. Our results indicated that although the composition of the coccinellid community in southwestern Michigan has changed dramatically in the past several decades, its function has remained relatively unchanged in both agricultural and natural habitats. While this is encouraging from the perspective of pest management, it should be noted that losses of one of the dominant exotic coccinellids could result in a rapid decline in pest suppression services if the remaining community is unable to respond

Photo illustrating natural enemy community exclusion treatments used in Study 1.

<p>A) closed plot caged to exclude all natural enemies, B) open plot allowing access to all natural enemies and C) sham plots, which are caged but, with holes on the ground and canopy levels to allow access by natural enemies.</p

Photo illustrating the natural enemy guild exclusion treatments used in Study 2.

<p>Each plot was assigned a cage treatment to exclude different groups of natural enemies from aphid populations. Treatments included exclusion of <b>A</b>) foliar-foraging predators and parasitoids (-F), <b>B</b>) ground-dwelling predators (-G), <b>C</b>) and all natural enemies (-F-G), and <b>D</b>) open plot (O), which allowed access to all natural enemies and served as a control.</p

Agronomic records at study sites.

<p>*DuPont Thifensulfuron-methyl–25%, Tribenuron- methyl- 25%, other ingredients–50%.</p><p>**Stratego YLD -BAYER Prothioconazole–10.8%, Trifloxystrobin, –32.3%, other ingredients–56.9%.</p><p>Michigan State University campus farm fields used for cereal aphid and natural enemy studies, East Lansing, Michigan, 2012–2013. All plots also received a single application of Affinity Broadspec* at 28.0 g/ha.</p><p>Agronomic records at study sites.</p

Aphid density, Study 1.

<p>Mean aphid numbers (± SEM) per tiller of; <b>A</b>) <i>Rhopalosiphum padi</i> in 2012, <b>C</b>) <i>R. padi</i> 2013, <b>B</b>) <i>Sitobion avenae</i> in 2012, <b>D</b>) <i>S. avenae</i> 2013, for closed (exclusion of all natural enemies), open (exposed to all natural enemies), and sham (to control for cage effect) plots in Study 1. ANOVA was used to test statistical differences. Different letters next to the treatments indicate statistically significant differences among aphids per tillers at α = 0.05.</p