Insecticide use in cover crops: Understanding impacts at a field scale

As this work has already been published, the Material and Methods, Results and Discussion follow the publication "Carmona et al. 2022. Impact of the Timing and Use of an Insecticide on Arthropods in Cover-Crop-Corn Systems. 2022 Mar 31;13(4):348. doi: 10.3390/insects13040348."

"Results

Arthropod Activity

The pitfall traps collected 33,316 total arthropods across all sites and years of the study. During the 2019 growing season (sites one, two, and three), a total of 13,292 individual arthropods were collected representing 42 different taxa. For the 2020 growing season (sites four, five, and six), a total of 14,028 individual arthropods were collected, representing 38 different taxa. In 2021 (sites seven and eight), a total of 6,026 individual arthropods were collected, representing 33 different taxa.

2019 Growing Season

For 2019, the most dominant taxa were Collembola (39.9%), Acari (35.6%), Coleoptera: Zopheridae (4.8%), Hemiptera: Aphididae (4.5%), Coleoptera: Nitidulidae (2.0%), Coleoptera: Staphylinidae (1.9%), Diptera: Anthomyiidae (1.8%), Coleoptera: larvae (1.7%), Araneae (1.6%), and Coleoptera: Carabidae (1.5%), representing 95.3% of total arthropods collected (Table 3).

Table 3. Arthropod taxa activity, the percentage from total activity, and the classification of arthropod taxa that composes more than 1% of the total arthropod activity per site and year.

Total arthropod and Aphididae activity from site three were the only taxa impacted by the study treatments. For total arthropod activity, a significant interaction occurred between treatment and sampling period (Table 4; Figure 1A). A multiple-treatment test within the sample dates indicated that the interaction was due to a lack of differences between the treatment means during sample two (F = 0.22; df = ; = 0.8056), while the herbicide only (387) had greater total arthropod activity compared to the tank mix (159; t = 3.87; df = 9; p = 0.0096) and late insecticide (136; t = 4.02; df = 9; p = 0.0077) treatments in sample period three. The treatment effect was approaching significance for Aphididae as a result of greater arthropod activity in the herbicide only (15), followed by the late insecticide (4), and then the tank mix (0.1) treatments (Table 4; Figure 1B).

Fig 1 a,b,c,d,e,f - Arthropod Activity

Figure 1. Natural log of total arthropod activity per sampling period and treatment from site 3 (A), Aphididae activity per treatment from site 3 (B), Aphididae activity per sampling period and treatment from site 5 (C), Carabidae activity per sampling period and treatment from site 5 (D), of total arthropod activity per sampling period and treatment from site 8 (E), and of Collembola activity in log per sampling period and treatment from site 8. Sampling period two was performed at the tank-mix application and three at the late insecticide application time (F). Error bars indicate the standard error of the natural log of the means. * Represents statistical difference at p < 0.05 between treatments at a given sampling period. Same letters represent no statistically significant difference at p < 0.05. Grey dash lines indicate the natural logmean equivalent.

Table 4. Analysis of covariance with significant interactions or main effects per site and year for each arthropod taxa composing more than 1% of the total arthropod.

n/a represents taxa activity lower than 1% of the total arthropod activity; therefore, analysis was not performed. represents the sampling periods. Trt represents treatments. ¹ sample period one was lost due to the rain; therefore, analysis of variance was performed for this site. ² Numerator and denominator degrees of freedom, respectively. Significant p-values (<0.05) are shown in bold. * represent the interaction between sample (SMP) and treatment.

2020 Growing Season

For 2020, the most abundant taxa were Collembola (27.5%), Coleoptera: Zopheridae (21.3%), Coleoptera: Nitidulidae (16.7%), Hemiptera: Aphididae (15.2%), Acari (3.9%), Araneae (3.3%), Diptera: Anthomyiidae (3.0%), Coleoptera: larvae (2.7%), and Coleoptera: Carabidae (1.7%), representing 95.1% of the total arthropod collected (Table 3).

In 2020, Aphididae and Carabidae activity in site five were the only taxa impacted by the study treatments. A significant interaction between the treatment and sampling period was identified for Aphididae (Table 4; Figure 1C) and Carabidae (Table 4; Figure 1D). For Aphididae, multiple treatment tests within sample dates indicated that the interaction was a result of a lower Aphididae activity in the tank mix treatment (17) compared to the herbicide only (72; t = 5.56; df = 9; p = 0.0004) and late insecticide (68; t = 5.40; df = 9; p = 0.0011) treatments during sample two. In contrast, no differences between treatment means were observed during sample three (F = 0.64; df p = 0.5477) (Table 4; Figure 1C). The interaction between the treatments and the sampling period for Carabidae activity was the result of greater Carabidae activity in the tank mix treatment (7) compared to the herbicide only (0.08; t = −3.14; df = 9; p = 0.0291) and late insecticide (0.05; t = −3.29; df = 9; p = 0.0230) in sample two, while no differences between treatments occurred in sample three (F = 0.40; df = 2, 9; p = 0.6834) (Table 4; Figure 1D).

 2021 Growing Season

For the 2021 growing season, the most abundant taxa were Collembola (54.3%), Acari (17.1%), Coleoptera: Zopheridae (6.0%), Diptera: Sciaridae (5.6%), Coleoptera: Nitidulidae (5.2%), Araneae (3.4%), Chilopoda (1.7%), and Coleoptera: Carabidae (1.5%), representing 95.1% of the total arthropods collected (Table 3).

The total arthropod and Collembola activity in site eight were the only taxa influenced by the study treatments. The interaction between treatment and sampling period was significant for the total arthropod (Table 4; Figure 1E) and Collembola activity (Table 4; Figure 1F). For total arthropod activity, a multiple treatment test within sample dates found a similar response in treatments for sample two (F = 2.91; ; p = 0.1306). In contrast, the total arthropod activity was greater for the herbicide-only (234) treatment compared to the tank mix (121; t = 3.59; df = 6; p = 0.0267) and late insecticide (91; t = 2.96; df = 6; p = 0.0254) for sample three (Table 4; Figure 1E), resulting in an interaction between treatments for the two sample dates. For Collembola, a multiple treatment test within sample dates indicated that the interaction was a result of greater activity in the tank mix treatment (126) compared to the herbicide only (59; t = −3.65; df = 6; p = 0.0107) and late insecticide (43; t = −2.18; df = 6; p = 0.0425) for sample two. In sample three, the herbicide-only treatment (31) had greater Collembola activity compared to the tank mix (14; t = 2.41; df = 6; p = 0.0325) and to the late insecticide (11; t = 2.53; df = 6; p = 0.0419) treatments (Table 4; Figure 1F). Although Collembola was the most abundant arthropod overall, Acari was the most abundant during the late insecticide application sample period driving the interaction at that sample period. The results from the other arthropod taxa that composed more than 1% of the total arthropod activity were only affected by the sampling period Table 4.

Agronomic Parameters

 Cover-Crop Extended Leaf Height

All sites in this study met the cover-crop extended leaf height NRCS threshold (152 mm) for cover-crop termination at the termination time (The extended leaf height varied between sites (F = 156.91, df , p = <.0001) with sites seven and eight having the greatest extended leaf-height mean, followed by site six, then site two. Sites three, four, and five did not differ from each other but had lower extended leaf heights compared to sites two, six, seven, and eight. Site one had a significantly lower extended leaf-height mean compared to all other sites (Figure 2A).

Fig 2 (2)

Figure 2. Cover crop extended leaf height (ELH) (A) and cover-crop biomass (B) per site taken at cover-crop termination. Error bars indicate the standard error of the means. The same letters represent no statistically significant difference at p < 0.05.

Cover-Crop Biomass

The cover-crop biomass varied from 629 (site three) to 4,806 kg.ha⁻¹ (site eight) (Figure 2B). The cover-crop biomass varied between sites (F = 70.30, df = p <.0001). Site eight had the greatest cover-crop biomass (4806 kg.ha⁻¹), followed by sites seven (3,946 kg.ha⁻¹) and six (3,744 kg.ha⁻¹), and then site one (2013 kg.ha⁻¹). Site one was not different from sites two (1,897 kg.ha⁻¹) and four (1,927 kg.ha⁻¹), but it was greater than sites five (1,019 kg.ha⁻¹), and three (629 kg.ha⁻¹). Finally, sites two and four had greater cover-crop biomass compared to site three but did not differ from site five (Figure 2B).

Corn Grain Yield

The corn yield varied from 7,831 (site one) to 13,208 kg ha⁻¹ (site seven). Significant differences in corn-grain yield occurred between sites (F = 13.32; df p <.0001), while treatment effect was not significant (F = 0.29; df = 2, 89; p = 0.7496). Site seven had the greatest corn grain yield mean (13,208 kg ha⁻¹), followed by site five (12,003 kg ha⁻¹), eight (10,540 kg ha⁻¹), four (10,011 kg ha⁻¹), two (9,998 kg ha⁻¹), three (9,866 kg ha⁻¹), six (9,856 kg ha⁻¹), and the lowest yield was recorded in site one (7,831 kg ha⁻¹) (Figure 3).

Fig 3

Figure 3. Corn grain yield in kg.ha⁻¹ per site. Error bars indicate the standard error of the means. The same letters represent no statistically significant difference at p < 0.05.3.3. Environmental Conditions During the Pitfall Trap Sample Periods

The cumulative average temperature and precipitation varied between sites and sampling periods (Table A1). During 2019, the cumulative average temperature varied from 127.5 °C (sample one, site one) to 210.6 °C (sample two; site one). Cumulative precipitation varied from 1.3 (samples one and two; site two) to 43.9 mm (sample two; site one). For 2020, the cumulative average temperature varied from 125.3 °C (sample one; sites five and six) to 200.0 °C (sample three; site four). The cumulative precipitation varied from 0.0 (sample two; site four, samples one and two for sites five and six) to 10.9 mm (sample three; site six). Finally, during 2021, the cumulative average temperature varied from 79.4 °C (samples one and two for sites seven and eight) to 194.4 °C (sample three; site eight). The cumulative precipitation varied from 0.0 (sample one; sites seven and eight, sample three; site eight) to 7.9 mm (sample three; site seven).

Discussion

This is the first multi-site-year on-farm study, to our knowledge, to evaluate insecticide management in a cover-crop-corn rotation system. Cover-crop use is rapidly increasing, and growers need scientific information to guide sustainable and profitable management decisions [2]. Our study showed that insecticide application as the tank mix could reduce Aphididae activity, an insect group present in the upper canopy of the cover crop, and an increase in Carabidae activity were observed in the same treatment at the same site. Based on previous similar findings, we believe that the Carabidae activity increase was led by a prey–predator relationship with dead Aphididae in the same plotHowever, most of the arthropods evaluated were not affected by any insecticide application. Those findings led to the hypothesis that the cover-crop biomass created a physical shelter–barrier that may have protected beneficial and potential insect pests at the lower cover-crop canopy from the contact insecticide application; however, further research to test this hypothesis is needed. The reduction in total arthropod activity only with late insecticide application provides insights supporting this hypothesis, as the cover crop was decomposing, and the cover crop’s physical shelter–barrier was reduced when the late insecticide application was made.

Increases in ground beetles (Coleoptera: Carabidae) captured by pitfall traps after insecticide applications appear to be surprisingly common [18–23]. Indirect and direct effects have been cited in the literature as possible causes for this increase. An increase in collected carabids after 17 days of pyrethroid application was reported in the literature [19]. The authors suggested that the reason for it was carabid movement into the insecticide-treated plots to feed on dead and impaired insects. Indirect mechanisms such as changes in mobility, abundance, and distribution of prey have also been reported as the reason for an increase in Carabidae activity-density after insecticide application [19,21]. In addition, low doses of insecticide might trigger a hormesis effect on arthropods. Hormesis is characterized by a low-dose response that is opposite in effect to that seen at high doses Moreover, it has been reported that hormesis responses can accelerate insect population growth [22–24]. Our results indicated increases in Carabidae activity after tank-mix application. The results of our study also showed that Aphididae was reduced in the same treatment and that the same site that Carabidae activity increased with the tank-mix treatment. We hypothesize that the increase in Carabidae activity was led by a prey–predator relationship with dead Aphididae at the same treatments or by a hormesis effect.

Early pest pressure in the cover-crop-corn system has been reported, such as true armyworm, black cutworm, and wheat-stem maggot [12,13]. However, no significant pest pressure was observed in any site-year of this study. The lack of pest pressure might be explained by growers using Bt hybrids, which control most of the early-season corn pests, or due to natural low pest pressure in the area. However, Bt hybrids are not effective against wheat-stem maggot. A recent experiment in a cover-crop-corn system conducted in eastern and central Nebraska, using either Bt or non-Bt corn hybrids, reported less than 1% of pest pressure in their studies [25].

Pest transition from the cover crop to corn has been recently reported [12,13]. Field surveys of wheat-stem maggot injury to corn from cover crops suggested that terminating the cover crop before planting corn or having the cover crop completely dead before the cash-crop planting is a strategy to minimize pest transitions [13]. However, maximizing cover-crop biomass is often a primary goal for growers to increase cover-crop benefits, such as weed suppression, erosion control, and water quality improvement [1,4,8]. As a result, growers are hesitant to terminate the cover crop too early. The occurrence of pest pressure in this system can lead some growers to add pyrethroid insecticides, often applied with other chemicals as a tank mix at the cover-crop termination time to reduce any possible pest transition to the following cash crop. In our study, we hypothesized that if a pest such as wheat-stem maggot were present, pest pressure could be reduced if a late insecticide application coincided with larval movement between the cover crop to corn. Nonetheless, no pest pressure was present in any site-year, so the efficacy of insecticide management against pests in this system could not be evaluated. The lack of pest pressure in this study highlights the fact that pests are infrequent in a cereal rye to corn system, and the use of an insecticide is often not warranted.

Insecticide applications as a preventive strategy are frequently used by growers, especially when pest populations in the area have been a problem [26]. Unnecessary insecticide application might decrease total arthropod activity in the system. This reduction might impact the arthropods’ prey–predato dynamics, potentially making beneficial insects less effective in controlling pest populations or potentially resulting in secondary pest outbreaks due to an imbalanced system [27–33]. We hypothesized that any preventive insecticide application in a cover-crop system would reduce the total arthropod activity. The results of our study partially supported our hypothesis as insecticide applications reduced the total number of arthropods in two site years. Even though carabids, a beneficial arthropod, increased with the tank-mix application, it might be a disadvantage for the sustainability of the system. We believe that the increased activity of Carabidae was the result of abundant dead or immobilized prey; therefore, Carabids could potentially be exposed to pesticides as well. Additional studies will be needed to evaluate any potential sublethal effects from the consumption of prey exposed to a pyrethroid in a cover-crop system. If sublethal effects occur, it could make Carabids less effective in controlling pests. The observed reduction in total arthropods and the potential disadvantage of Carabid exposure to a pyrethroid reinforces the need to avoid unnecessary insecticide applications in cover crops when pest pressure is low.

Despite the decrease or increase in activity of some arthropod taxa in our study, most of the taxa evaluated were not impacted by any of the insecticide applications. We hypothesize that the lack of insecticide impact on arthropods in our on-farm studies could result from cover-crop biomass . However, this study was not designed to address the role of biomass protection for ground-dwelling arthropods. A pyrethroid is a broad-spectrum, contact insecticide. Therefore, barriers that protect the ground-level arthropods from contact insecticides, such as cover-crop biomass accumulation, might limit the insecticide from reaching arthropods below the cover-crop canopy. Moreover, this could pose an issue for insecticide efficacy against insect pests present in the cover-crop lower canopy. In addition, the reduction in Aphididae activity was expected as Aphididae is often on the cover-crop canopy, and, as a result, they are vulnerable to insecticide contact applications. In addition, the Aphididae species found in this study do not pose any significant threat to vegetative-stage corn.

Conclusions

This research assists growers in making informed, profitable, and sustainable decisions that could result in a decrease in unneeded insecticide use. Insecticide applications did not impact corn-grain yields as was hypothesized. With no pest pressure at any site in the study, insecticide applications were unnecessary, resulting in additional expense, labor, and time for growers with no return on investment. Finally, the infrequent presence of pests in a cover-crop-to-corn system highlights the importance of scouting for pests prior to any management decision. Future research will be needed to address the role of cover-crop biomass production and insecticide management to test the hypothesis that cover-crop biomass production creates a physical shelter–barrier, protecting ground-dwelling arthropods. More information regarding the side effects on Carabidae when feeding on pesticide-exposed prey will also be important to evaluate. Future research should consider artificial pest infestations to further evaluate the impact of insecticide use in cover-crop systems and corn-grain yield trade-offs. In addition, predator gut analyses should be performed in future studies to better understand the predator–prey relationships in a cover-crop-corn cropping system.

Data Availability Statement: The data presented in this study are openly available in Zenodo at: http://doi.org/10.5281/zenodo.6091653.

Acknowledgments: We would like to thank the University of Nebraska Lincoln Insect Field Ecology Lab members for helping with the data collection and support throughout the seasons (Elliot Knoell, Earl Agpawa, Dania Vieira Branco Ozorio, Tauana Ferreira de Almeida, Juan Bettencourt Cardona, Osler Ortez, and Joana Schroeder de Souza).

Appendix A

A1 Figure

Figure A1. Cover crop status on a late cover-crop termination application at site 5 (A), site 4 (B), and site 7 (C).

Appendix B

Table A1. Cumulative average temperature (°C) and cumulative precipitation (mm) before any insecticide application (sampling period 1), at the tank mix application (sampling period 2), and at late insecticide application sampling period 3) per site and year.

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