Evaluations of economic benefits and long-term sustainability of neonicotinoid seed treatment use in the mid-Atlantic
Seed treatments are a convenient and economical way to protect a wide variety of crops from insect pests. Thiamethoxam and imidacloprid are neonicotinoid insecticides that are registered for use as seed treatments on wheat, corn, and soybeans. When applied as seed treatments, germinating plants will absorb a portion of the active ingredient, but most of the chemical remains in the soil. Neonicotinoid seed treatments have been shown to have non-target effects on both below and above ground communities. They may affect earthworms, soil microbial communities and beneficial arthropods, including predators, parasitoids, and pollinators. In this study, we are investigating the repeated use of thiamethoxam and imidacloprid seed treatments in a commonly practiced mid-Atlantic three-year grain crop rotation; specifically, soybean, followed by fall-planted wheat, double-cropped soybean and corn.
Our specific objectives in this study are to determine the impacts of neonicotinoid seed treatments upon both beneficial and pestiferous foliar and soil organisms, including soil microorganisms and measure how seed treatments impact grain growth and yield. Thus far, we have found that neonicotinoid seed treatments do reduce abundance of some pests early in the growing season, but may also negatively impact beneficial arthropods. However, these treatments have not led to a significant impact on yield in soybean or wheat. Results of the study will help producers make informed long-term management decisions regarding seed and seedling protection.
The primary goal of this study is to examine the effects of repeated use of neonicotinoid seed treatments in a three year crop rotation of full season soybean, winter planted wheat, double cropped soybean and corn. Specifically, we are studying the impacts of thiamethoxam and imidacloprid treated seed on:
- Arthropod pests and beneficial arthropods – data from visual counts has been analyzed for full season soybean, wheat and double cropped soybean, while samples collected from sticky cards, pitfall traps, and litter are being processed. All of these counts will be repeated for corn in 2017.
- Crop growth parameters and yield – neonicotinoid treated seeds did not have a significant impact on growth parameters such as stand count or yield in full season soybean, wheat or double cropped soybean. Growth parameters and yield will also be measured for corn in 2017.
- Soil health, including physical soil parameters and microbial abundance, which will be measured through quantitative PCR – soil parameters such as compaction, aggregate stability and percent carbon, oxygen and nitrogen were measured at the start of the study in 2015. These measurements will be repeated after corn is harvested in 2017. Soil samples have been collected from soybean and wheat and stored for quantitative PCR, which will be conducted in 2017 and 2018, and will also be repeated for corn.
- We are also testing whether commercially available Solvita soil respiration test kits are an accurate indicator of soil microbial abundance relative to qPCR. When soil was collected for qPCR, Solvita tests were carried out on soil from the same samples immediately after collection.
Funding from this grant, which began in July 2017, will be used specifically for objectives 3 and 4. In 2017 and 2018, qPCR will be conducted on stored soil samples from wheat and soybean and samples that will be collected from corn in 2017.
Full season soybean was grown in 2015 and wheat was planted immediately afterwards in fall 2015. Double crop soybean was planted after winter wheat was harvested in the summer of 2016.
- Pest and beneficial arthropods:
Wheat – Visual counts for insect pests, primarily aphids and cereal leaf beetle (Oulema melanopus) were conducted twice in the fall (Feekes stages 2 and 3), and three times in the spring (Feekes stages 6, 10 and 11). Additional arthropod sampling was conducted at Feekes stages 6, 10 and 11. Pitfall traps were set up to sample the epigeal community, and foliar arthropods were trapped using sticky cards. Soil and litter dwelling arthropods were collected by passing litter samples through Berlese funnels.
Preliminary results: In the winter, seed treatments significantly impacted aphid abundance at Queenstown (F3,59 = 33.04, p <0.0001) and Beltsville (F3,56 = 4.17, p = 0.0098); both neonicotinoid seed treatments significantly reducing the aphid population on both sampling dates (12/4 and 12/16) at Queenstown and the first sampling date (12/4) at Beltsville. At Beltsville, the sampling date had a significant impact on the model during the winter sampling period, so data from the two dates is analyzed and presented separately. In all other instances (spring at Beltsville, winter and spring at Queenstown), date did not have an impact on aphid abundance and so the data was pooled across dates. There was no significant reduction in aphid numbers due to seed treatments in the spring at either site. Cereal leaf beetle was present in low numbers at both sites in the spring, but there were no significant differences in its abundance between seed treatments at either site.
Double cropped soybean – Visual insect counts were conducted at V2 and R1 stages, and foliar insects were also captured by sweepnets at the R1 stage. Sticky cards and pitfall traps were deployed at the V2, R1 and R3 stages, and litter samples were also collected at this time.
Preliminary results: The most abundant groups of pests were plant thrips and planthoppers. The seed treatments did not have a significant impact on abundance of planthoppers at either date or on the abundance of plant thrips on the first sampling date (P>0.05). On the second sampling date, there were significantly lower levels of plant thrips in the Cruiser plots, but not in the Gaucho plots (F3,24 = 5.96, P=0.0035).
The most abundant beneficials were minute pirate bugs and predatory thrips. There was no significant treatments effect on minute pirate bugs at either date (P>0.05). On the first sampling date, Cruiser led to a significant decrease in predatory thrips, but Gaucho did not have a significant impact (F3,21 = 13.43, P<0.0001). On the second date, there was no significant difference between any treatments (P>0.05).
- Plant growth and yield:
Wheat – Stand density was measured at emergence and one week post-emergence, and plant height was measured six weeks post planting. Normalised Difference Vegetation Index (NDVI) was measured three times in the fall and once in the spring (Feekes stages 1, 2, 3 and 4) using a Crop Circle optical sensor as another measure of plant growth. Plants were dug up to count tillers in the fall and again in the late spring after wheat had completed tillering (Feekes stages 3 and 6). Yield, moisture and test weight were recorded at the time of harvest.
Preliminary results: Five sets of samples were taken for Normalised Difference Vegetation Index, which measures the presence of green vegetation. There was no significant difference between NDVI in the different treatments at any of the sampling dates (P>0.05). At Beltsville, there was no significant difference in yield between any of the treatments (F3,6 = 2.9219, P=0.1222). At Queenstown, the control plots had a significantly lower yield than the other three treatments, likely due to the absence of fungicide seed treatment in the bare seed control, (F3,6 = 8.2244, P=0.0151); the insecticide seed treatments did not significantly affect yield.
Double cropped soybean – At emergence (VC Stage), stand count was measured and growth stage was recorded for forty plants per plot. Stand counts and growth stage measurements were repeated the following week (V2 stage); plant height was measured at the R1 stage. We intended to measure NDVI for soybean as well; however this was deemed impractical due to abundance of weeds early in the season, which would interfere with the measurement of green plant matter.
Preliminary results: There was no significant difference in stand count or plant height between any of the treatments at either site (P>0.05). There were no significant differences in yield between any of the treatments at either Beltsville or Queenstown (P>0.05).
- Soil health and microbial abundance:
Wheat – At Feekes stages 4, 6, 10 and 11, soil cores were collected from each plot and used to conduct Solvita respiration tests (Woods End Laboratories); a portion of each soil sample was stored at -80°C for later microbe identification using qPCR.
Soybean – Soil samples were collected at V2, R1 and R3 stages for Solvita respiration tests, and a portion of each sample was also stored for later qPCR.
Future work: Arthropods collected from litter samples, pitfall traps, sticky cards and sweep nets in wheat and soybean are being identified in the lab, which will continue through 2017 and 2018. Data from Solvita tests wills be analysed, and qPCR will be conducted on stored soil samples starting early in 2017. Corn will be planted in 2017 and arthropod, soil and plant growth sampling will be conducted similar to wheat and soybean. Soil measurements taken before planting in 2015 will be repeated after corn is harvested in 2017.
Impacts and Contributions/Outcomes
Results from this study will be used to make recommendations to Maryland grain producers about using neonicotinoid treated seeds in a sustainable and beneficial manner. This project is partially funded by grants from the Maryland Soybean Board and the Maryland Grain Producers Board, and reports on the progress and results of the project are also submitted to these stakeholder organizations.
Results from this study were presented to ~400 farmers at several extension events in 2017, including the Mid Atlantic Crop School, Northern Maryland Field Day, Maryland Commodity Classic, Eddie Mercer Field Day, and Small Grain Twilight Tour. Surveys of Mid Atlantic Crop School attendees (45 responders) found that 80% will use the information presented about this project, 35% planned to change how they use seed treatments, and 87% would share the information with others. An extension article on this work was published in the research edition of the University of Maryland Extension Agronomy News in October 2016. A talk about this study was presented to a scientific audience as part of the Student Competitions at the 2016 International Congress of Entomology. Findings from this study will continue to be communicated to both growers and researchers at extension events and scientific conferences in coming years.
Our results so far suggest that the use of neonicotinoid seed treatments may not always be beneficial in Maryland soybean and wheat. Although seed treatments did provide some pest protection early in the growing season in both wheat and soybean, pest pressure was always low, and did not reach treatment threshold for any pests in either crop. Seed treatments did not lead to an increase in yield in full season soybean, wheat or double-cropped soybean. Our results are in keeping with an EPA report that found that the use of neonicotinoid seed treatments in soybean does not provide economic benefits in most cases, and the northeast was identified as a region where this is especially true. We have also found that seed treatments do have a negative impact on some beneficial arthropods, and we may also see an impact on soil health, as well as cumulative effects as a result of repeated use over four crops in three years. By sharing our findings with growers, we will help them make management decisions about how to best use neonicotinoids in a way that is both sustainable and economically beneficial.
University of Maryland College Park
4112 Plant Sciences Building
College Park, Maryland 20742