Despite their undeniable contribution to ecosystem function and stability, macro-decomposers are seriously under-represented in research on agroecosystems (Wurst, 2013). Our limited knowledge of the macro-decomposer community in crop fields hinders our understanding of the value of these decomposers for agroecosystem function, primarily for plant residue incorporation and nutrient cycling. Moreover, our weak grasp of these communities hampers our ability to assess the influence of agricultural practices on abundance and diversity of the animal species that compose the community.
I have been working to address these gaps regarding our knowledge of macro-decomposer communities. Through my first objective, I have been defining the macro-decomposer communities in no-till and cover-cropped maize and soybean fields in the Northeastern United States, and I have been working to understand the role of macro-decomposers in residue breakdown and nutrient cycling. I will pair my field tests and laboratory-based toxicity assays to investigate the indirect and direct influence of prophylactic insecticide use on macro-decomposer community composition and decomposition. I have focused on three prophylactic insecticides—the pyrethroid lambda-cyhalothrin, and the neonicotinoid seed treatments clothiainidin and imidacloprid—which I have chosen due to their ubiquity in maize and soy production across the Northeast.
This purpose of this project is to identify and understand the role of macro-decomposers in maize and soybean fields in the Northeastern United States. Additionally, this project was designed to assess the effect of two pest-management tactics – neonicotinoid seed treatments and post-planting Warrior applications – on these decomposers and their services. This research is important for sustainable agriculture in the Northeast, as sustainable pest management methods should minimize detrimental effects on other managed aspects of farming (pollination, nutrient cycling, and soil health).
I had four main objectives:
Objective 1: Define the decomposer communities in conventional soy and maize fields of the Northeastern U.S., and Pennsylvania in particular.
Objective 2: Determine the influence of macro-decomposers on decomposition characteristics, including residue breakdown rate, nutrient mineralization, and production of soil organic matter.
Objective 3: Determine the acute sensitivity of representative macro-decomposers to the pyrethroid lambda- cyhalothrin and the neonicotinoids clothianidin and thiamethoxam.
Objective 4: Assess how prophylactic insecticide use (neonicotinoid seed treatments and post-planting pyrethroid applications) affects macro-decomposer community composition and decomposition.
Field Management (objectives 1, 2, & 4)
In June 2016 at Penn State’s Entomology Research Farm, I established research plots in two no-till fields – one larger field planted with soybeans, one smaller field planted with maize. I assigned three pesticide treatments to the plots:
- control plots (untreated seeds; no insecticides)
- neonicotinoid seed treatments (maize coated with clothianidin, soybeans coated with imidacloprid)
- pyrethroid plots (lambda-cyhalothrin (Warrior®) applied four weeks after planting)
The maize field comprised 12 plots (40 x 70 ft2) and the soybean field comprised 18 plots (60 x 60 ft2). After harvest, both fields were both planted with a rye cover crop which was burned down the following spring.
For the 2017 season, the crops were rotated (soy in the smaller field, maize in the larger field) but the three pesticide treatments were kept consistent. Again, both fields were planted with a rye cover crop in late fall.
Sampling and Identification of Arthropods (objectives 1 & 4)
I used pitfall traps to sample the macroarthropod community in my plots in both 2016 (3 times) and 2017 (2 times). Two pitfall traps per plot were open for 72 hours during each sampling event. Additionally, I used Berlese funnels to heat-extract decomposers from the litterbags used for objectives 2 and 4. Initially I had proposed to extract decomposers from soil and litter samples, however after further reading and experience, I found extractions from litterbags simplified my collection process and meant I could work with more samples of standardized volume and area.
All specimens collected from the pitfall traps and litterbags were kept in alcohol for counting and identification. Since macro-decomposer diversity was low in my samples (mostly a single species of invasive millipede, Oxidus gracilis), I have been making an effort to count and identify mites and collembolans as well.
Litter bag Experiments (objectives 2 & 4)
I have been using litter bags to determine the influence of neonicotinoid seed treatments and post-planting Warrior applications on decomposition rate. As proposed, I placed pairs of litterbags in each plot; each pair of bags has included a macro-decomposers-exclusion litter bag constructed from 1 mm polyester mesh and a macro-invertebrate-permitting bag with a 5 mm polyester mesh top. The litterbags are 10 cm by 20 cm rectangles 3.90- 4.10 grams of rye straw (the first two batches of bags) or 4.18- 4.30 grams of wheat straw. While I originally had planned to use maize and soy litter in my bags, I decided to use straw, a standardized litter, instead. Since I am primarily interested in the decomposer community, I decided to eliminate the complications that would arise from using different litter types/qualities. As I am using a rye cover crop, the decomposer community should be ‘familiar’ with straw-like residue.
I decided to place batches of litterbags in both spring and fall, to correspond to the times during the year where residues/litter deposition is highest (spring = cover crop termination, fall = crop residue post-harvest). I have placed four batches of litterbags; two in June right after planting, and two in November after harvest:
- In June 2016, I placed 300 pairs of fine and coarse-mesh litterbags (10 pairs per plot); subsets of the bags were collected in June, August, and November of 2016 as well as March, May, and July of 2017.
- In November 2016, I placed 270 pairs of bags; subsets were collected in November 2017, March, May, July, August, September, and November 2017.
- In June 2017, I placed 180 pairs of bags; subsets were collected in July, August, September, and November 2017 (remaining bags will be collected in March and May 2018).
- In November 2017, I placed 150 pairs of bags; subsets will be collected in March, May, July, August/September, and November 2018.
Litter bags remained in place during most management activities (herbicide and nutrient applications, corn harvest), but during planting and soy harvest they have been be temporarily removed, held in individual plastic bags, then replaced to their original field locations.
To quantify decomposition rate across the growing season, I have collected litter bags at various time points over the past two growing seasons, and will collect the remaining bags over the upcoming season. I weighed the contents of all litter bags before placement and again after arthropod extraction, rinsing, and air-drying. Due to high soil contamination, I am determining the ash-free dry weight of the remaining litter to get an accurate loss of mass due to decomposition.
Initially I had proposed analyzing the soil directly beneath each litterbag to assess decomposition dynamics. Due to logistical challenges and consulting the literature and my PhD committee, I decided to instead analyze the litter chemistry over time; I have analyzed protein and lignin content under my different treatments / mesh-sizes at different time points. However, I am still taking soil samples to see if there is a detectable acute effect of pesticide treatments on overall soil chemistry.
Toxicity Assays (objective 3)
I have run into challenges while trying to maintain lab colonies of macro-decomposers. I had planned to keep colonies into the winter to avoid running assays during the field season, however this hasn’t been possible due to the biology (an annual life cycle) and sensitivity of the most common millipede species (Oxidus gracilis) and the low density of isopods. Since I will not be deploying litterbags at the start of the 2018 season, I will be able to collect specimens and quickly run toxicity assays during the field season. I still plan to run standardized, acute dose-response assays on Oxidus gracilis. I will collect individuals from untreated fields and expose them to one of four treatments; 1. lambda-cyhalothrin treated soil, 2. lambda-cyhalothrin treated crop residue, 3. clothianidin treated soil, and 4. thiamethoxam treated soil. Per standard procedures, I will run a preliminary range-finding assay followed by more precise dose-response assays to generate LC50 values. In short, sets of individuals will be exposed to known pesticide concentration across a specific range, and mortality at each concentration will be assessed every week for five weeks. Mortality will be determined as no movement stimulated by being flipped over with forceps.
Objective 1 Results: In Pennsylvania, I have found the macro-decomposer community in my fields to be dominated by a single millipede species, Oxidus gracilis. This is a non-native species that reaches very high numbers in our research plots (over 150 individuals caught in some of the pitfall samples). Additionally, I have found lower densities of julid millipedes and isopods. Qualitatively I have seen a lot of evidence of earthworm activity (castings, tunnels) – I hope to quantitatively measure earthworm activity/density in the following season.
Although not initially proposed for this experiment, I have also been able to identify mesofaunal members of the decomposer community – orbatid mites, other mites, and a handful of springtail species reach high densities throughout the season.
Objective 2 Results: My preliminary results show macroinvertebrate exclusion slows decomposition rate; so macro-decomposers (or other macroinvertebrates) accelerate the early stages of decomposition. My preliminary litter analysis suggests macroinvertebrates increase lignin breakdown.
Objective 4 Results: So far I have observed a negative effect of neonicotinoid seed treatments on springtail densities and a moderate negative effect of the early season pyrethroid application on mite populations. Unexpectedly, the activity-density of millipedes increased in response to the pyrethroid treatment; it is possible this is an increase in population density, but I hypothesize it is an increase in activity as the millipedes attempt to avoid the pesticide. My dose-response assays will allow me to test this hypothesis.
Decomposition is accelerated at certain times of the year under seed treatments.
Education & Outreach Activities and Participation Summary
Although I only have preliminary results, I have been able to share my research with farmers and members of the community, and my peers in entomology. I gave a talk about my research to farmers and extension agents at the Penn State Annual Agronomic Weed Tour. I have shared my research with people from the community – from gardeners at the Insect Identification and Soil Health Workshop (Penn State Student Farm) to children at school visits and community events such as Wings in the Park at the Snetsinger Butterfly Garden and the Great Insect Fair at Penn State. For the Great Insect Fair, I collaborated with my colleague to create an interactive display to teach people about soil invertebrates and their contribution to agroecosystems. We hope to expand this display and to use it at future events. Over the past two years, I have also helped mentor 7 undergraduate students hosted by my lab. Finally, I have presented my research at two Annual Entomology Society of America meetings.