Evaluating the Impact of Insecticides on Arthropods in Cover Crop to Corn Transitions.

Final report for GNC18-258

Project Type: Graduate Student
Funds awarded in 2018: $11,716.00
Projected End Date: 05/31/2020
Grant Recipient: University of Nebraska - Lincoln
Region: North Central
State: Nebraska
Faculty Advisor:
Dr. Justin McMechan, Ph.D., D.P.H.
University of Nebraska Eastern Nebraska Research and Extension Center
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Project Information


Cover crops have continued to increase in acreage across the US as a sustainable means of improving soil health, reducing soil runoff from fields, and as an alternative strategy to manage weeds. To maximize these benefits producers are encouraged to allow cover crops to grow as long as possible in the spring before planting a cash crop. Conversely, such practices can be counterproductive if a cover crop harbors arthropods that cause damage to the subsequent cash crop. In such cases, farmers may attempt to mitigate these threats by tank mixing an insecticide at the time of termination of the cover crop. Tank mixed insecticides are relatively cheap but are unlikely to control insects that secluded within the stems of the cover crop that emigrates to the cash crop several days after a herbicide application. In addition, the use of insecticides will likely kill beneficial insects in cover crop systems. In 2017, we reported a newly emerging pest, the wheat stem maggot (WSM) (Meromyza americana Fitch) in rye and wheat cover crop to corn systems. Yield losses from this pest have spurred an increase in insecticide use when terminating a cover crop. Farmers also face potential losses from other insects associated with cover crops such as black cutworm, true armyworm, common stalk borer, and stink bugs. Insects such as common stalk borer and wheat stem maggot are only likely to be controlled when migrating between the cover crop and cash crop. Research is needed to address the best management practices for arthropod pests and beneficials in a cover crop to corn transition systems. The proposed study will be conducted with two farmers to address the efficacy of delayed insecticide applications relative to cover crop termination on insect pests and the impact of this practice on beneficial arthropods. Results from this study will enhance the education of farmers and consultants by providing them with the skills necessary to scout cover crops for pest and beneficial arthropods, protect the environment by reducing unnecessary insecticide applications, and increasing the profitability of growers by providing them with information on when to apply. Findings will be disseminated through field days, conferences, CropWatch articles, as well as extension and research publications.

Project Objectives:

This study will increase our participant knowledge of arthropods and their pest or beneficial impact in a cover crop to corn transition systems. Knowledge gains will occur through hands-on participation in data collection during field visits, demonstration plots at the Eastern Nebraska Research and Extension Center, extension publications, and presentations at field days and conferences. These experiences will increase participant awareness on the activity and abundance of pest and beneficial arthropods when insecticides are used in cover crop systems. Participation in data collection, demonstration plots and information from presentations will provide participants, with the skills to properly scout for and group arthropods in these systems as beneficial or pest. Results from the study will also provide farmers with information on the efficacy of insecticides application timing to control key pests and skills to properly time applications.
Several action outcomes are anticipated from this project. We expect an increase in scouting efforts of cover crop fields prior to termination as a result of the training and hands-on experience as well as the ability to weigh the presence of beneficial insects in cover crop systems. Such skills should result in a reduction in tank-mixed insecticides at cover crop termination. When pest pressure is high we anticipate that producers will adjust timings of insecticides to achieve greater efficacy resulting in a reduction in situations with significant economic losses and a greater return on investment.


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Materials and methods:

A total of three fields were used for the insecticide management study, two on-farms and one at the UNL Eastern Research and Extention Center. Field visits started in April 2019. All on-farm site visits were scheduled with growers. The first visit was to choose the field location. During this visit, we also talked about field activities, logistics, expectations, and possible outcomes. We also talked with growers about their management practices and past problems regarding cover crop management. The following visits to collect samples and apply treatments were scheduled by phone or text messages. We had a very good communication, always informing them about our plan/questions/or if anything changed.

Pitfall samples started 15 days before applying the first treatment (at the cover crop termination time). The second pitfall trap was done at the first treatment application (herbicide only, tank mix) at the cover crop termination date. And the last pitfall trap was at the late insecticide application, around 25 days after the cover crop termination. Cover crop biomass and extended leaf height were collected at the cover crop termination time. Cover crop biomass was collected to measure how much rye biomass was present in the field at the cover crop termination time (maximum cover crop biomass). We believe that cover crop biomass might create a microhabitat for some arthropods, so quantifying it is important. Two cover crop biomass samples were done per experimental unit (0.017m2 each sub-sample). Calculation of biomass/acre was further done. Cover crop extended leaf height was also done at the cover crop termination time. This is a parameter used by growers and recommended by USDA/NRCS Cover Crop Termination Guideline, what set cover crop height and days prior/after planting corn as a threshold for the cover crop termination. At V3, V10 and R2 corn stages, insect damage assessment was done. All corn plants in 15 feet in the two mid-rows were inspected for insect damage. The damage percentage was recorded. All insect pest was counted, and unknown pests were collected for identification. In early stages, if corn plants were a stage behind the plot average, it was dug, and roots were inspected for any insect damage. 15 feet mid-rows were hand-harvested. Moisture and weight were recorded to adjust the final corn yield. 

Communication with farmers was done before, during, and after studies. Farmers are very busy during the season, so they did not participate in field samples collection as we wish, with an expectation of one time. However, they were being informed of our findings regularly. Visitor scholars and our research project coordinator were directly involved in this project. They helped with logistics, data collection, and data processing. Results were shared with other growers at the Scal Field Day, ENREC Corn Management Clinic, and also with scientists and students, at the 2019 Entomological Society Annual Meeting, at St. Louis.  

Research results and discussion:

Cover crop biomass was measured at the cover crop termination time. ENREC location had the lower cover crop biomass (485 lbs/acre), followed by on-farm location 1 (942 lbs/acre), and on-farm location 2 had the highest cover crop biomass (1496 lbs/acre) (Figure 1).

Cover crop extended leaf height (ELH) was also measured just prior to cover crop termination. ELH in all locations met the minimum threshold set by NRCS guidelines. On-farm location 1 had the lowest ELH, followed by ENREC location, and on-farm location 2, respectively. 

A total of 16,695 arthropods were collected in three pitfalls samples across three locations. On-farm location 1 did not have any no-cover crop treatment. This was the result of historic flooding in Nebraska forcing us to acquire a new on-farm location (very low cover crop establishment).

In total, 36 arthropod families were collected across the three locations for this study. However, an analysis was only performed with arthropod families composing >5% of the total arthropods collected. Table 2 indicates arthropod families per arthropod group analyzed in this study.

Arthropod group response to the tank-mixed insecticide and herbicide treatment varied between locations. For the on-farm location 1, there was no difference between no insecticide and tank mix insecticide-herbicide regarding arthropods groups (Figure 3). However, for the ENREC site, prey was higher in the no-insecticide treatment. Macroarthropods were unexpectedly higher in the tank mix insecticide-herbicide treatment (Figure 4). This unanticipated result might be related to the small plot design, facilitating the more mobile insects to move between treatment. Unfortunately, the first pitfall traps set were lost at the on-farm location 2 due to heavy amounts of rain shortly after setting up the traps.

Late insecticide application resulted in a reduction of predators, prey, and total arthropods collected compared to the no insecticide treatment (Figure 5). At the ENREC site, the impact of the late insecticide application resulted in a reduction in immatures, prey, microarthropods, and total arthropods compared to the no insecticide treatment (Figure 6). For the on-farm location 2, the late insecticide application reduced immatures and total arthropods collected when compared to the no insecticide treatment (Figure 7). In contrast, predators were greater in treatments when insecticide was applied at the on-farm location 2. Since these predators are highly mobile, this could be a result of movement between plots due to the small plot sizes.

            Only at the ENREC and on-farm location 2, there was “no cover crop plots”, illustrated as “check” in red in Figures 6 and 7. When this treatment is compared to the cover crop no insecticide treatment, the check had lower immatures populations in both locations.

Corn yield varied from 7513 kg/ha at ENREC (only herbicide treatment) to 10388 kg/ha at on-farm location 2 (tank-mix treatment). However, there was no corn yield a significant difference in any location (Figure 8).

In conclusion, insecticide applications as a tank mix at the cover crop time and late insecticide application are not warranted as a preventive practice to minimize pest transition from a cover crop to the following corn. Experiments on larger scales are needed to better evaluate arthropod abundance and diversity, with the objective to minimize arthropod movement between plots.

SARE final report - Results Figures


Participation Summary
2 Farmers participating in research

Educational & Outreach Activities

2 Curricula, factsheets or educational tools
3 On-farm demonstrations
2 Webinars / talks / presentations
1 Workshop field days
4 Other educational activities: Data collection with visiting scholars. Data collection (pitfall traps and other) demonstration at the SCAL field day

Participation Summary:

10 Farmers participated
15 Ag professionals participated
Education/outreach description:

We recently got the Partnership Grant as a follow up of the Graduate Student Grant. The Graduate Student Grant one-year data will be included in the final manuscript along with the Partnership Grant results (totalizing 3-years field study and 7 locations). 

Project Outcomes

1 Grant received that built upon this project
2 New working collaborations
Project outcomes:

This project aimed to study insecticide use and management in the cover crop – corn transition system. Due to the 2017 Wheat Stem Maggot transition from rye and wheat to the following corn, several growers in NE were using insecticide tank mix at the cover crop termination to decrease pest transition. However, this practice may impact several other arthropods in the system. In general, our results showed that an insecticide applied as a tank mix with a herbicide reduced prey populations. And surprisingly, it increased the macro arthropods numbers and did not have any impact on predators, decomposition, immatures, micro and total number of arthropods collected. Insecticide applied 25 days after termination resulted in decreased on immatures, prays, microarthropods, total, and reduced and/or increased of predators. No significant pest pressure was observed for any of the treatments across all three sites which reinforces the fact that insecticides not warranted if there are no pests in the system. Insecticide use decreased prey (food) for predators and negatively impacted other arthropod groups. This negative response to insecticide use demonstrates how important scouting is in cover crop systems. Utilizing insecticides as a “insurance” policy against unexpected or unanticipated pest pressure has a significant negative effect and can potentially set growers up for secondary pests to occur in the systems. To maximize returns and minimize impacts on the environment, clientele will have to adopt a strong routine of scouting cover crop fields. This will be especially important in the transition between cover crop and cash crop.

Knowledge Gained:

We started this project with the objective of educating growers about insecticide management in a cover crop-corn system. This project was part of our continuous effort to demonstrate to growers how scouting is crucial in order to make an informed decision pest management. Our ultimate goal was to maximize beneficial arthropods, minimize pests, and identify the most profitable and sustainable management strategy in this context. We want to keep doing that, working closely with growers to reduce their reliance on pesticides and move towards a more integrated pest management strategy. The partnership grant will be an excellent follow-up to this study. We will have 4 locations in 2020 and much larger plots. This will help us to account for the field variability and will minimize arthropod movement between plots/treatments. 


The Graduate Student Grant was a great learning opportunity for me. This learning opportunity went well beyond the project itself. Having the opportunity to organize and lead an on-farm study was a huge responsibility. I was responsible for the whole project and all steps (from contacting growers to analyze the data). My advisor was there in case I had questions, but he let me lead everything. Conducting this type of study was a real challenge for me. Building a trustful connection with growers is not easy. It requires time and a lot of competence. In the beginning, I was insecure, mainly because I am an international student and a woman. But at the end of the project, I looked back and could see how much I had grown as a professional. The proof is that the two growers of the Graduate Student Grant agreed to be part of the Partnership Grant project. 

This opportunity actually changed my graduate program. I started in 2017 as an M.S. student, and after getting this grant and conducting it, we decided to change my studies to a Ph.D. program. This project alongside the Partnership results will be included as one of my dissertation chapters. After conducting this grant, I feel much more capable, secure, and a much better communicator with farmers. Farmers are “today’s maker decisions” and working alongside them is crucial to build a more sustainable production system. I really believe that university-extension-farmers should work together to build a better world. I am very happy and proud to contribute (a little bit) to SARE values and objectives. 

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Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.