Responses of soil faunal food webs to pesticide seed treatments

Responses of soil faunal food webs to pesticide seed treatments

Summary

Pesticide seed treatments (PST) are widely used in commodity cropping systems, but little is known about how this management practice affects the soil faunal community. We conducted a two-year field experiment to examine the effects of insecticidal-fungicidal seed treatments on soil food web composition and function. Corn (2013) and soybean (2014) with and without pesticide seed treatments were planted as a two-year rotation in a completely randomized design with five replications. We measured soil faunal communities, litter decomposition, nutrient cycling, and crop response. Our results suggest this management practice does not cause community-level shifts in soil faunal composition, but can cause reductions in the detritivore trophic group. Our results also suggest pesticide seed treatments can benefit Collembola Isotomidae (fungivores) and a non-targeted pest population, Symphylan, while adversely affecting two predatory taxa, Carabidae Bembidion spp. and Diplura Japygida. These effects on soil faunal composition, however, did not result in measurable differences in the decomposition of surface litter, plant available nitrogen, or grain yields between our treatments. In 2016, we will complete the isotopic analysis of the soil fauna specimen collected in our field experiment to determine if there are shifts in the trophic position of the most common faunal taxa due to the pesticide seed treatment. These results will be presented at agricultural conferences and prepared into manuscripts for peer-reviewed journals.

Objectives/Performance Targets

2. Determine whether pesticide treated seeds alter the trophic position of commonly occurring soil faunal species. Due to the small quantities (15N and ¹³C, a sample must contain a minimum amount of carbon and nitrogen; specifically, 20-150 µg N and 200-2000 µg C. Because we are still interested in how pesticide seed treatments affect consumer-resource links we narrowed the taxa of interest to include only the most commonly occurring taxa. We identified seven taxa in our samples that will meet the minimum sample mass for isotopic analyses and frequently occur among plots, time points, and years. These taxa include:

1. 1. Acari: Oribatida

1. 1. Acari: Mesostigmata

1. 1. Collembola: Onychiuridae

1. 1. Collembola: Entomobryidae

1. 1. Diplura: Japygidae

1. 1. Diplura: Campodeidae

1. 1. Chilopoda: Lithobiomorpha.

At this time, all seven taxa are prepared for isotopic analysis. This involved isolating each taxa, drying the specimen, and transferring them to sample tins. The next step is to submit these samples to the UNH Stable Isotope Laboratory for analysis. We anticipate receiving these results in early March. All further statistical analyses will be conducted by July 2016.

3. Determine the effects pesticide seed treatments have on agriculturally important ecosystem services including decomposition and nitrogen cycling. All litter bags and cation/anion resin strips deployed in 2013 and 2014 were processed and analyzed in 2015. Further analyses on crop growth, including plant heights and leaf chlorophyll content, and yields were also completed.

Accomplishments/Milestones

1. Determine to what extent pesticide seed treatments alter soil food web composition. Soil fauna samples collected in 2014 were processed in the lab in 2015. This involved extracting, counting, and identifying all fauna collected from the litter bag study and the soils. The data from 2013 and 2014 were pooled and analyzed using appropriate univariate and multivariate analyses. The major findings from these data are presented below.

There was little evidence of community-level differences in soil faunal composition between the control and PST in both 2013 and 2014 (PerMANOVA: 2013 (p = 0.6646), 2014 (p = 0.9212)). To further probe the community dataset, we organized the taxa into trophic groups and tested for differences in faunal biomass between pesticide treatment and control (Fig 1.). The amount of detritivore biomass found in the surface residues (litter bags) when both years are combined was significantly reduced with pesticide seed treatments (PST: F_1,8 = 14.26, p = 0.00541). Interestingly, the PST did not significantly affect herbivore biomass (PST: F_1,8 = 2.286, p = 0.169), which includes pest populations, the intended targets of this management practice. These data suggest PST do not affect soil faunal composition at the community-level, but non-targeted trophic levels can be affected by this management practice.

We also used Indicator Species Analysis to determine if specific taxa within the soil faunal community strongly associated with either PST or control than would be expected by chance (Fig. 2). Two taxa, Collembola Isotomidae (fungivore) and Symphylan (non-targeted pest), were identified as indicator species for PST. We also identified three taxa more strongly associated with the control: Carabidae Bembidion spp. (predator), Coleoptera Cucujoididae larvae (variable trophic groups), and Diplura Japygida (predator).  These data suggest differential responses of non-targeted taxa to the PST.

2. Determine whether pesticide treated seeds alter the trophic position of commonly occurring soil faunal species. All remaining fauna samples collected in 2013 and 2014 were processed and prepped for isotopic analysis in 2015. Unfortunately, we were unable to meet our previous deadline due to the very time-intensive process of counting and preparing thousands of soil fauna for isotopic analysis. After numerous methodological improvements and hard work, all samples are prepared for the isotope lab. Unforeseeably, the timeline for this objective was further delayed due to an unexpected renovation of the UNH Stable Isotope Laboratory. After the renovation is complete and the new equipment is up and running, we will submit the samples to the Stable Isotope Laboratory (February 2016). Statistical analyses of these data will commence once we receive the results. 

3. Determine the effects pesticide seed treatments have on agriculturally important ecosystem services including decomposition and nitrogen cycling. All remaining litter bags and resin strips collected in 2013 and 2014 were processed in 2015. Aboveground litter bags were used to measure surface residue decomposition and resin strips were used to measure nitrogen cycling rates. These results were analyzed and the major findings are presented below.

Our data suggests both functions we measured, surface residue decomposition and plant available N-NO₃^–, are not affected by pesticide seed treatments. There was no difference in aboveground decomposition rates between PST and control both years (2013:F_1,8 = 0.367, p = 0.561; 2014: F_1,8 = 0.124, p = 0.734). We also did not detect any differences between plant available N-NO₃^– between PST and the control both years (2013: PST F_1,8 = 1.543, p = 0.249, PST*Time F_2,16 = 0.359, p = 0.704; 2014: PST F_1,7 = 0.668, p = 0.441, PST*Time F_2,15 = 0.895, p = 0.429). These data are not surprising considering we did not observe any community-level changes in the soil food web community (Obj. 1 results). Interestingly, we also did not detect a yield benefit when either corn or soybean was pretreated with pesticides (Corn: F_1,8 = 4.0901, p = 0.07777; Soybean: F_1,8 = 0.3282, p = 0.5825).

• Figure 1
• Figure 2

Impacts and Contributions/Outcomes

Our two years of field data suggest that pesticide seed treatments can differentially affect key soil faunal populations, but have little effect on overall community composition or the soil food web’s capacity to decompose aboveground litter and transform N to plant available nitrate. Our data continue to raise questions about the efficacy and utility of this common agricultural practice in that 1) there is evidence a non-targeted pest, Symphylan, flourishes in systems with pesticide seed treatments, 2) two predatory taxa are more likely to associate with untreated systems, and 3) there were no grain yield benefits for both corn and soybean in our system. Together these data provide evidence that pesticide seed treatments can have unanticipated effects on soil organisms and agroecosystem services.

These data were presented at two separate scientific conferences in 2015. The first was at the 100^th Ecological Society of America Conference held in Baltimore, MD in August. L. Atwood was also invited to present these data at the Entomological Society of America Conference which was jointly held with the Agronomy, Crops, and Soils Societies of America in Minneapolis, MN in November. Both presentations were well attended and provided excellent opportunities to network. The first manuscript with the data funded by this award is in preparation. 

Collaborators: