Macroarthropod decomposers in field crops: Influence on residue breakdown and response to prophylactic insecticides

Macroarthropod decomposers in field crops: Influence on residue breakdown and response to prophylactic insecticides

Summary

This project has been undertaken to define the macro-decomposer communities in maize and soybean fields, to understand the role of macro-decomposers in nutrient cycling, and to assess the effect of two pest-management tactics – neonicotinoid seed treatments and post-planting Warrior applications – on these decomposers. To do this, I am sampling invertebrates from maize and soy fields using pitfall traps and litterbags. The litterbags are also a tool to measure decomposition rate, with small and large mesh sizes to specifically measure the role of macroinvertebrates. In conjunction with this field experiment, I am running toxicity assays to determine the sensitivity of specific macro-decomposers to neonicotinoids and lambda- cyhalothrin, the active ingredient in Warrior.

Although my project began just this fall, I was able to conduct some preliminary work this summer; this past field season helped me optimize my pitfall trapping and litterbag experiment for the coming seasons. For within-year comparisons, I expanded my project to include both maize and soy plots for each season and I believe my preliminary litterbag experiment, from this past season, is robust enough to include in my final project. Results from my preliminary work do suggest interactions between the decomposer community/activity and pest management tactics.

Objectives/Performance Targets

I had four main objectives in my original plan and I am on target with my timetable.

Objective 1: Define the decomposer communities in conventional soy and maize fields of the Northeastern U.S., and Pennsylvania in particular.

• Preliminary field season (2016) of pitfall traps and litterbags to sample the decomposer community in soy and maize under the same treatments to be used in 2017 and 2018

• Cover-cropped these plots with rye in preparation to be planted with maize and soy for the 2017 season; soy will be planted in the 2016 maize plot, maize in the 2016 soy plot

Objective 2: Determine the influence of macro-decomposers on decomposition characteristics, including residue breakdown rate, nutrient mineralization, and production of soil organic matter.

• In addition to the 600 litterbags placed in June 2016 for preliminary work, in November I placed 540 litterbags in the plots for next season
• took soil samples for analysis of initial soil quality

Objective 3: Determine the acute sensitivity of representative macro-decomposers to the pyrethroid lambda- cyhalothrin and the neonicotinoids clothianidin and thiamethoxam.

• Collected the most common millipede in my plots, Oxidus gracilis, to start a lab colony for toxicity assays this winter

Objective 4: Assess how prophylactic insecticide use (neonicotinoid seed treatments and post-planting pyrethroid applications) affects macro-decomposer community composition and decomposition.

• Same progress as Objective 1

Accomplishments/Milestones

As this was my first year, my activities for 2016 were primarily in preparation for the 2017 field season. This past June I established both maize and soy plots with the same treatments I will be using for these experiments. I was lucky to have to opportunity to expand my project to include 30 sub-plots (instead of my originally proposed 12), two different fields, with maize and soy plots for both years, not just maize in 2017 and soy in 2018.

I removed pitfall traps in September so they would not be damaged from harvest/planting, but next fall I plan to take pitfall samples later into the season.

The plots I used this past summer were harvested in October (soy) and November (maize), and on November 10^th the plots were planted with a rye cover crop. My fall batch of litterbags was place in the plots at the end of November. As I am working in 30 plots instead of just 12, I used 540 litterbags instead of just 384. I collected 60 bags right after placing to get baseline values, as some of the litter may be lost just from moving the bags. 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 not to collect litter this November, as I would like these samples to reflect plots that have gone a full season after cover cropping. Also, I had great success extracting invertebrates from my litterbags collected in August and October (bags originally placed in June 2016), so I expect I will get a good representation of the litter-dwelling invertebrates from these samples alone.

For the toxicity assays I will run this winter, I have established a lab colony of juvenile millipedes (the dominant species I found in my plots, the invasive Oxidus gracilis). I also have invertebrate samples to sort/ID from this past summer, which will provide an additional season of data and will help me identify the invertebrates I am likely to find in later samples.

Impacts and Contributions/Outcomes

As I have just started, I have not participated in any specific outreach or produced any publications on this research. I look forward to doing so in the coming years!

Collaborators: