Seed Treatment Combined with a Spot Application of Clothianidin

Article pubs.acs.org/JAFC

Seed Treatment Combined with a Spot Application of Clothianidin Granules Prolongs the Efficacy of Controlling Piercing−Sucking Insect Pests in Cotton Fields Zhengqun Zhang,‡,§ Yunhe Zhao,†,§ Yao Wang,† Beixing Li,† Jin Lin,† Xuefeng Zhang,† and Wei Mu* †

College of Plant Protection, Shandong Agricultural University, 61 Daizong Street, Tai’an 271018, Shandong, China College of Horticulture Science and Engineering, Shandong Agricultural University, 61 Daizong Street, Tai’an 271018, China

‡

S Supporting Information *

ABSTRACT: Seed treatments can directly protect cotton from early season piercing−sucking insect Aphis gossypii Glover but hardly provide long-term protection against Apolygus lucorum (Meyer-Dür). Therefore, the efficacy of clothianidin seed treatments combined with spot applications of clothianidin granules at the bud stage of cotton was evaluated to control piercing− sucking pests during the entire cotton growing season. Clothianidin seed treatments (at the rate of 4 g ai/kg seed) combined with a clothianidin granular treatment (even at low rate of 0.9 kg ai/ha) at the bud stage can effectively suppress A. gossypii and A. lucorum infestations throughout the seedling and blooming stages after planting and can improve cotton yield. The spot application of clothianidin granules also reduced the population densities of Bemisia tabaci (Gennadius). The dynamic changes of clothianidin residues demonstrated that the control efficacy of clothianidin against A. gossypii and A. lucorum might be related to the residues of this neonicotinoid in cotton leaves. This pest management practice provided long-term protection against cotton piercing−sucking pests for the entire growing season of cotton plants and could supplement the short-term control efficiency of clothianidin used as a seed treatment. KEYWORDS: Aphis gossypii, Apolygus lucorum, seed treatment, granular treatment, clothianidin

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which increase pesticide use and labor output.2,9 Frequently applied insecticides can negatively affect natural enemies, biodiversity, and the environment in the broader agroecosystem and potentially lead to the evolution of insecticide resistance in the targeted insect pests.10 Hence, some alternatives for reducing pesticide use while controlling A. gossypii, A. lucorum, and other piercing−sucking insects in cotton production are urgently needed. Among various pest management technical measures, seed treatments can directly protect crops from early season foliar pests by allowing more precise targeting of the active insecticide ingredient to the pest organism, which results in decreased applicator exposure and the amount of active ingredient used.11,12 Neonicotinoid insecticides are agonists of nicotinic acetylcholine receptors and have excellent biological activity against a wide range of sucking insect pests via contact or by ingestion.13 Moreover, neonicotinoid insecticides have excellent plant systemicity and are suitable for use as a seed treatment to manage sucking insect pests and certain chewing species affecting seedling stage crops.13−15 Previous studies showed that neonicotinoid seed treatments provide early season seedling protection against a range of sucking pests such as A. gossypii, Bemisia tabaci (Gennadius), and Amrasca devastans (Distant) in cotton fields.16−18 However, seed treatments can effectively control pests only during the plant’s early growth

INTRODUCTION Since the 1990s, the commercial planting and area-wide application of transgenic Bt cotton in China has effectively controlled Lepidoptera larvae, but it has no significant protective effect against piercing−sucking cotton plant herbivores such as aphids, mirids, whiteflies, leafhoppers, and thrips.1−3 At present, the cotton aphid Aphis gossypii Glover (Hemiptera: Aphididae) and the mirid bug Apolygus lucorum (Meyer-Dü r) (Hemiptera: Miridae) are the commonly occurring and economically important cotton pests that cause serious damage in Bt cotton fields. Aphis gossypii infests cotton plants by sucking phloem sap, causing damaged leaves to roll up, thus resulting in retarded growth or even growth cessation and death.4 Moreover, A. gossypii may cause indirect damage because it can transmit several debilitating plant viruses.5 For A. lucorum, feeding by this piercing−sucking pest on the cotton plants results in bud blast, flower abortion, and missing or shrunken squares and bolls.6 The infestation of these two cotton pests can result in extremely large cotton yield losses if not effectively controlled. Chemical control is currently the main control measure for A. gossypii, A. lucorum, and other piercing−sucking pests in cotton fields in China.7,8 At present, chemical foliar sprays of insecticides such as organophosphates, pyrethroids, neonicotinoids, and antibiotic insecticides have been widely used to control A. gossypii and A. lucorum due to their rapid action and high efficacy in cotton fields in China. However, due to the short residual effects of insecticides by spraying and the resistance development of commonly used insecticides, cotton farmers occasionally use up to 10−15 applications per season, © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

July 6, 2017 August 16, 2017 August 24, 2017 August 24, 2017 DOI: 10.1021/acs.jafc.7b03120 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry

ai/kg seed), (5) a foliar spray treatment performed in accordance with the pest management program of the cotton breeding center (details of the spraying treatment are listed in Supporting Information, Table S1), and (6) an untreated treatment. Treatments were arranged in a randomized complete block design with four replications. The cotton seeds were sowed on 27 April 2015 and 26 April 2016. Seed furrows (80 cm apart and at a 5 cm depth) were made by a mechanical furrow opener. The seeds were sowed via manual dibbling, three seeds per hole, and at a 25 cm distance. Approximately 22 kg seeds per ha were used, and planting densities were approximately 45000 per ha. Each plot consisted of 10 rows that were 8 m long, containing approximately 288 cotton plants. Plots were separated by 1.6 m of bare cultivated ground. Pendimethalin was applied at the rate of 800 g ai/ha after sowing, and every row was covered with translucent plastic film. On 27 June 2015 and 25 June 2016 (60 days after sowing), clothianidin granules were buried 10 cm deep in soil on both sides of the cotton plants by a shovel and 15 cm from the cotton plant. The amount of 2% clothianidin granules used in each plot in the three granular treatments (at the rate of 0.9, 1.8, and 3.6 kg ai/ha) were 288, 576, and 1152 g, respectively. The number of A. gossypii and A. lucorum on 100 randomly selected plants were counted in each plot from 13 May to 29 August in 2015 and from 13 May to 28 August in 2016. When aphids and mirids were counted, natural enemies (including coccinellids, lacewings, syrphids, spiders, and predatory bugs) were also monitored. Ladybeetles (mainly including Harmonia axyridis, Propylaea japonica, and Coccinella septempunctata), lacewing (mainly including Chrysopa phyllochroma and Chrysopa sinica), syrphid (mainly including Epistrophe balteata, Syrphus nitens, and Syrphys corollae), spiders (Lycosidae, Gnaphosidae, and Titanoecidae), and predatory bugs (mainly including Orius sauteri, Orius minutes, Geocoris pallidipennis) may all be found on cotton plants or in the surrounding areas. Other piercing−sucking pests, including B. tabaci adults and nymphs, Frankliniella intonsa (Trybom), and Empoasca biguttula (Shiraki), mainly occur in the bud and boll stages of cotton. Therefore, these pest species were counted in each plot from 26 June to 29 August in 2015 and from 26 June to 28 August in 2016. Adults of B. tabaci (Q biotype) and E. biguttula were counted on 100 randomly selected plants, and B. tabaci nymphs and F. intonsa were counted on 30 randomly selected plants. The number of cotton bolls per plant and the boll weight were assessed on 30 randomly selected plants in each plot in midSeptember. The samples for yield were collected at three different times beginning mid-September to late-October (on 16 September, 5 October, and 29 October in 2015, and 17 September, 5 October, and 29 October in 2016), and tons per ha were determined. Residual Determination of Clothianidin in Cotton Leaves and Soil. During the period of the seed treatments, cotton leaves and soils were sampled every 5 days from 13 May through 26 June of 2015. After applying clothianidin granules on 27 June 2015, cotton leaves and soils were sampled on 27 June, 28 June, 30 June, 4 July, and every 7 days after 4 July. Ten cotton plants were randomly selected, and approximately 20 g of young leaves were collected from each plot. The soil around 10 randomly selected plants was sampled using a soil auger (8 cm inner diameter, 15 cm in depth). The soil samples from each plot were pooled and divided, and approximately 500 g of soil was stored. All samples were stored at −20 °C until analysis. The extraction and cleanup of samples followed the method of Zhang et al.21 The leaf samples were finely chopped using a blender, and 10 g of the sample was transferred into a 250 mL triangular flask and 50 mL of acetonitrile were added. The mixture was shaken for 90 min at 200 rpm using a mechanical shaker and was then transferred into a 100 mL centrifuge tube, and 5 g of NaCl and 8 g of anhydrous MgSO4 were added. The mixture was manual shaken for 1 min vigorously and was centrifuged at 4000 rpm for 5 min. Then, 25 mL of the supernatant was evaporated to near dryness on a rotary evaporator and water bath at 40 °C and blown dry using an aurilave. The residue was reconstituted with 2 mL of acetonitrile. The cleanup procedure was used by a dual-layer GCB/PSA SPE cartridge (500 mg of graphitized carbon black (GCB) in the upper layer and 500 mg of primary−secondary amine (PSA) in the lower layer; Anpel, Shanghai,

stages and does not provide long-term protection against the targeted pests through the entire growing season of the crops owing to a very low concentration of insecticides in the leaves of the treated plants.19 Neonicotinoid insecticidal granular treatments at sowing are also a control method that can provide long control efficacy against piercing−sucking insects on cotton plants. For example, a clothianidin granular treatment at sowing can prevent yield loss and A. gossypii and A. lucorum infestations from the seedling to blooming stages of Bt cotton.3 Moreover, spot applications of neonicotinoid granules had a longer residual effect on the targeted insect pest than did seed dressings. 16 However, the dosage of active insecticide ingredient in granule treatments is higher than that of seed treatments, which could pose a greater threat to agroecosystems. Clothianidin, a systemic plant neonicotinoid, is widely used for seed treatments in different crops and has a spectrum of activity against a broad range of pests from different orders.20 Currently, clothianidin has not been registered for use on cotton in China to control cotton pests, and little information has been published about the efficacy of clothianidin used as a seed treatment on the potential pest complex in cotton fields. The objective of this study was to evaluate the efficacy of a clothianidin seed treatment combined with spot applications of clothianidin granules at the bud stage of cotton in controlling A. gossypii, A. lucorum, and other piercing−sucking pests during the entire cotton growing season in order to supplement the short-term control effect of seed treatments and reduce the frequency of insecticide sprays. The impacts of these treatments on natural enemy populations were also evaluated. Additionally, the concentrations of clothianidin in cotton leaves and soil were determined to investigate the relationship between control effects and clothianidin residue.

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MATERIALS AND METHODS

Cotton Seeds and Insecticides. The transgenic Bt cotton seeds, variety Xinqiu-1 (expressing the Bacillus thuringiensis δ-endotoxin Cry1A(c)), were supplied by Shandong Xinqiu Agricultural Science and Technology Co., Ltd. The seeds were delinted and selected before the insecticide seed treatment. Clothianidin (Poncho 600 g/L FS, Bayer CropScience LP, Monheim, Germany) was diluted to a uniform slurry with water before the seed treatment. Additionally, 98.5% technical-grade clothianidin (Veyong Bio-Chemical Co., Ltd., Hebei, China) was used for generating 2% granules (G). The granules were prepared by dissolving the active compound in N,N-dimethylformamid (DMF), adding methylene chloride, mixing that with coal gangue, and allowing the mixture to dry in a fume hood. Field Experiments. In the 2015 and 2016 cotton growing seasons, the field experiments for evaluating the efficacy of the clothianidin seed treatment combined with spot applications of clothianidin granules for managing A. gossypii and A. lucorum were conducted at a cotton breeding base of Shandong Xinqiu Agricultural Science and Technology Co., Ltd. in Xiajin, Shandong, China (site: 36.93° N, 115.95° E). The soil type was a silt loam (clay 12.15%, silt 61.88%, sandy 25.97%), with a pH of 7.53 and an organic content of 1.41%. Farmers usually sow cotton seeds in late April, covering them with a translucent plastic film to keep warm and harvesting in midSeptember, producing only one crop a year. Six treatments were as follows: (1) a clothianidin seed treatment at the rate of 4 g active ingredient (ai)/kg seed combined with a spot application of clothianidin granules at a rate of 0.9 kg ai/ha at the bud stage, (2) a clothianidin seed treatment (at the rate of 4 g ai/kg seed) combined with the clothianidin granular treatment (1.8 kg ai/ha) at the bud stage, (3) a clothianidin seed treatment (at the rate of 4 g ai/ kg seed) combined with the clothianidin granular treatment (3.6 kg ai/ ha) at the bud stage, (4) clothianidin seed treatment (at the rate of 4 g B

DOI: 10.1021/acs.jafc.7b03120 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Figure 1. Population densities (mean ± SE per 100 cotton plants) of A. gossypii (A,B) and A. lucorum (C,D) in various insecticide treated field plots in 2015 and 2016. Plots with no insecticide treatment served as the control. Different letters and the asterisks on a given date indicate significant differences between the different treatments (Tukey’s HSD test, P < 0.05). China). The concentrated extract was transferred to the SPE cartridge preconditioned with 5 mL of acetonitrile and rinsed three times with 6 mL of acetonitrile. The eluents were evaporated to dryness at 40 °C under a nitrogen stream. The residue was then dissolved in 2 mL of acetonitrile and passed through the filtration membrane (0.22 μm) and made ready for analysis. The soil samples were air-dried and strained through a 10-mesh sieve prior to extraction. Then 20 g of soil was placed into a 250 mL triangular flask and 50 mL of acetonitrile were added. This mixture was shaken for 1 h and then ultrasonically extracted for 30 min, and 3 g NaCl were added, followed by vigorous manual shaking for 1 min. After a 30 min settling period, 25 mL of the supernatant was transferred to a 100 mL flat-bottomed flask and then evaporated to near dryness on a rotary evaporator and water bath at 40 °C. Finally, the remainder was blown dry by an aurilave. The residue was reconstituted with 2 mL of acetonitrile and transferred into a 2 mL microcentrifuge tube, and 50 mg of PSA and 150 mg of anhydrous magnesium sulfate were added. Themixture was vortex-mixed for 1 min and then centrifuged at 12000 rpm for 3 min. The supernatant was passed through the filtration membrane (0.22 μm) and made ready for analysis. The extracts were analyzed by an Agilent 1290 Infinity UHPLC system coupled to an Agilent 6460 MS/MS system. Chromatographic separation was performed using an Agilent SB-C18 column (100 mm × 2.1 mm i.d., 1.8 μm particle size) at 30 °C. The mobile phase consisted of solvent A (acetonitrile) and solvent B (0.1% formic acid in water), and the flow rate was 0.2 mL/min. The mobile phase gradient

programs were as follows: 0−2.5 min, 60% A; 2.5−4 min, 100% A; and 4−5 min, 60% A. The retention time of clothianidin in the respective mobile phase was 1.40 min. The injection volume was 2 μL. Tandem mass spectrometry was performed in positive electrospray ionization (ESI+) mode, and the source parameters were as follows: gas temperature, 325 °C; gas flow, 8 L/min; nebulizer gas, 40 psi; and capillary voltage, 4000 V. Nitrogen gas served as the nebulizer, and argon gas was used as the collision gas. The multiple reaction monitoring (MRM) parameters of clothianidin were as follows: precursor ion 250.1 m/z; production 169/132.1 m/z; fragmentation voltage 80 V; and collision energies 8/12 V. The analytical method was validated. The linear standard curves were obtained from the matrix-matched working standard solutions at concentrations ranging from 0.002 to 2.0 mg/L. The percentage recovery of clothianidin in the examined samples was determined to establish the efficiency of the extraction method. The recoveries were determined by adding known amounts of clothianidin standards (0.01, 0.1, and 1 mg/kg) into known weight of a pesticide-free cotton and soil samples before extraction. Further, these samples were subjected to the extraction and cleanup procedures as described above, and all the recovery assays were replicated three times. The matrix-matched calibration curves had good linearity for the correlation coefficients (R2) > 0.99 for clothianidin in cotton leaves and soils. Mean recovery values for clothianidin were 87.25%, 95.71%, and 96.59% in cotton leaves samples and were 93.03%, 95.75%, and 94.40% in soil samples, respectively, which were in the acceptable range (80−110%) specified by the SANCO guidelines.22 C


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Figure 2. Population densities of whitefly adults (A,B) and nymphs (C,D), flower thrips (E,F), and leafhoppers (G,H) in various insecticide treated field plots after spot applications of clothianidin granules in 2015 and 2016. Plots with no insecticide treatment served as the control. Different letters and the asterisks on a given date indicate significant differences between the different treatments (Tukey’s HSD test, P < 0.05). D


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Figure 3. Population densities (mean ± SE per 100 cotton plants) of coccinellid adults (A) and larvae (B), lacewings (C), syrphids (D), spiders (E), and predatory bugs (F) in various insecticide treated field plots in 2015. Plots with no insecticide treatment served as the control. Different letters and the asterisks on a given date indicate significant differences between the different treatments (Tukey’s HSD test, P < 0.05). Data Analysis. All statistical analyses were carried out using SPSS statistical software (version 18.0, SPSS Inc., Chicago, IL, USA). Statistically significant mean values were compared using one-way ANOVA tests, followed by Tukey’s HSD method (P < 0.05).

significantly lower than that in the control treatment and was below the economic threshold 22−43 DAS in 2015 (P < 0.05) and 27−46 DAS in 2016 (P < 0.001). After spot applications of clothianidin granules, the densities of A. gossypii were significantly lower in the clothianidin seed treatments combined with spot applications of clothianidin granules at the rates of 3.6 kg ai/ha compared with the untreated control plots at 89 DAS in 2015 (F5,23 = 10.23, P < 0.001) and 89−103 DAS in 2016 (89 DAS, F5,23 = 4.12, P = 0.011; 41 DAS, F5,23 = 9.091, P < 0.001; 46 DAS, F5,23 = 5.96, P = 0.002) (Figure

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RESULTS Population Dynamics of A. gossypii and A. lucorum. The highest population density of A. gossypii occurred in the untreated control plots in early June, 2015 and 2016. At the seedling stage, the number of A. gossypii in plots treated with the clothianidin seed treatment and spray treatment were E


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Figure 4. Population densities (mean ± SE per 100 cotton plants) of coccinellid adults (A) and larvae (B), lacewings (C), syrphids (D), spiders (E), and predatory bugs (F) in various insecticide treated field plots in 2016. Plots with no insecticide treatment served as the control. Different letters and the asterisks on a given date indicate significant differences between the different treatments (Tukey’s HSD test, P < 0.05).

lucorum) 75−124 DAS in 2015 (P < 0.001) and 68−124 DAS in 2016 (P < 0.001) (Figure 1C,D). Population Dynamics of Other Piercing−Sucking Insects. At 111 DAS, the density of B. tabaci adults in the clothianidin seed treatment combined with a spot application of clothianidin granules at a rate of 3.6 kg ai/ha and spray treatments were significantly lower than the clothianidin seed treatment and the untreated control. The density of B. tabaci adults in clothianidin seed treatments combined with spot applications of clothianidin granules at the rates of 1.8 and 3.6 kg ai/ha and were significantly lower than the clothianidin seed

1A,B). At the seedling stage, the number of A. lucorum in plots treated with clothianidin seed treatments were not significantly different compared with spray treatments and control treatments 16−54 DAS in 2015 and 17−46 DAS in 2016. After spot applications of clothianidin granules, the number of A. lucorum in plots treated with clothianidin seed treatment + clothianidin granular treatment at all three concentrations were not significantly different compared with spray treatments but were significantly lower than the control treatment and was below 20 per 100 plants (the economic threshold of A. F


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Figure 5. Weight of cotton boll (A,D), number of cotton bolls per plant (B,E), and yield of cotton per plot (C,F) from the different treatments in 2015 and 2016. Different letters indicate significant differences between the different treatments (Tukey’s HSD test, P < 0.05).

6.194, P = 0.02). The density of coccinellid adults in the clothianidin seed treatments combined with spot applications of clothianidin granules at the rates of 0.9, 1.8, and 3.6 kg ai/ha and spray treatments were significantly lower or low compared to the clothianidin seed treatments and the untreated control 75−124 DAS in 2015 (P < 0.05) (Figure 3A) and 68−124 DAS in 2016 (P < 0.05) (Figure 4A). The density of coccinellid larvae in the clothianidin seed treatments and spray treatments were significantly lower than the untreated control plots from 22 DAS though 41 DAS in 2015 and 2016 (P < 0.05). The density of coccinellid larvae in plots treated with clothianidin combined with spot applications of clothianidin granules at all three rates and spray treatments were significantly lower than the clothianidin-treated plots and the untreated control plots at 103 DAS and 110 DAS in 2015 (103 DAS, F5,23 = 10.975, P < 0.001; 110 DAS, F5,23 = 8.829, P < 0.001) and at 110 DAS in 2016 (110 DAS, F5,23 = 11.821, P < 0.001) (Figures 3B, 4B). For syrphids, the density of this predator in clothianidin seed treatments and spray treatments was significantly lower than the untreated control treatments 27−41 DAS in 2015 (27 DAS, F2,11 = 18.75, P < 0.001; 32 DAS, F2,11 = 8.272, P = 0.009; 37 DAS, F2,11 = 22.792, P < 0.001; 41 DAS, F2,11 = 12.483, P = 0.003) and 27−32 DAS in 2016 (27 DAS, F2,11 = 13.364, P = 0.002; 32 DAS, F2,11 = 44.074, P < 0.001) (Figures 3D, 4D). Overall, there were no significant differences in the number of lacewings, spiders, and predatory bugs among the five different treatments (Figures 3, 4). Cotton Yield. The boll weights in the spray treatment were higher than that in the clothianidin seed treatments and untreated control treatments but were not significantly different compared to the three granular treatments in 2015 (F5,23 = 4.974, P = 0.005) (Figure 5A). However, in 2016, The boll weights in untreated plots were lower than that in other treatments, but the effects were not significant (F5,23 = 3.167, P

treatments and the untreated control plots but were not significantly different than the spray treatments at 118 and 125 DAS in 2015 (118 DAS, F5,23 = 9.526, P < 0.001; 125 DAS, F5,23 = 14.788, P < 0.001) and 118 and 125 DAS in 2016 (118 DAS, F5,23 = 6.161, P = 0.002; 125 DAS, F5,23 = 10.613, P < 0.001) (Figure 2A,B). The population densities of B. tabaci nymphs in three different granular treatments were significantly lower than the densities in the plots treated only with the seed treatment and the untreated plots at 104, 111, 118, and 125 DAS in 2015 (P < 0.001) and at 97, 104, and 125 DAS in 2016 (P < 0.001) (Figure 2C,D). There was no significant difference in the number of F. intonsa in all treatments at all sampling dates in 2015 (Figure 2E). At 90, 97, and 104 DAS in 2016, the density of F. intonsa adults in the clothianidin seed treatment combined with a spot application of clothianidin granules at a rate of 3.6 kg ai/ha was significantly lower than the untreated control (90 DAS, F5,23 = 4.996, P = 0.005; 97 DAS, F5,23 = 5.422, P = 0.003; 104 DAS, F5,23 = 3.908, P = 0.014) (Figure 2F). For E. biguttula, the density of this pest in the clothianidin seed treatment combined with a spot application of clothianidin granules at a rate of 3.6 kg ai/ha was significantly lower than the clothianidin seed treatments and the untreated control but was not significantly different than the other two granular treatments and spray treatments at 125 DAS in 2015 (F5,23 = 4.477, P = 0.0079) and at 97, 104, 111, and 125 DAS in 2016 (97 DAS, F5,23 = 7.26, P = 0.001; 104 DAS, F5,23 = 16.655, P < 0.001; 111 DAS, F5,23 = 9.642, P < 0.001; 125 DAS, F5,23 = 5.574, P = 0.003) (Figure 2G,H). Population Dynamics of Natural Enemies. At the seedling stage, the population densities of coccinellid adults in the clothianidin seed treatments and the spray treatments were significantly lower than the densities in the untreated plots at 27 and 32 DAS in 2015 (27 DAS, F2,11 = 9.000, P = 0.007; 32 DAS, F2,11 = 8.471, P = 0.009) and 41DAS in 2016 (F2,11 = G


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Journal of Agricultural and Food Chemistry = 0.032) (Figure 5D). The number of bolls per plant and cotton yields were significantly higher in the clothianidin seed treatments combined with spot applications of clothianidin granules at 0.9, 1.8, and 3.6 kg ai/ha and spray treatments compared to the untreated control treatments in 2015 (bolls/ plant, F5,23 = 12.529, P < 0.001; yield, F5,23 = 22.005, P < 0.001) and 2016 (bolls/plant, F5,23 = 5.123, P = 0.004; yield, F5,23 = 7.056, P = 0.001) (Figure 5). Dynamic Changes of Clothianidin Residues in Cotton Leaves and Soil. The residues of clothianidin in cotton leaves declined gradually in the clothianidin treatments throughout the sampling periods. Their highest values were 0.940 mg/kg, which occurred at the first sampling date (16 DAS). At 54 DAS, clothianidin was not detected in cotton leaves in the seed treatments. After spot applications of clothianidin granules, the residues of clothianidin in cotton leaves increased rapidly until 82 DAS. At 82 DAS, the residues of clothianidin in cotton leaves were 0.855, 1.013, and 1.555 mg/kg in the granular treatments at the rates of 0.9, 1.8, and 3.6 kg ai/ha, respectively. The residues of clothianidin in cotton leaves decreased rapidly again after 82 DAS. At 124 DAS, the residues of clothianidin in cotton leaves were 0.005, 0.006, and 0.008 mg/kg in the granular treatments at the rates of 0.9 and 1.8 and 3.6 kg ai/ha, respectively (Figure 6A). Clothianidin in soils degraded from 0.953 mg/kg (at 16 DAS) to 0.243 mg/kg (at 54 DAS) in the seed treatments before being treated with granules. After being treated with clothianidin granules, the residue levels of clothianidin in the soil were 5.606, 12.666, and 25.233 mg/kg in the granular

treatments at the rates of 0.9, 1.8, and 3.6 kg ai/ha at 61 DAS, respectively. At 185 DAS, the residue levels of clothianidin in the soil were very low and were only 0.005, 0.010, and 0.027 mg/kg in the three granular treatments, respectively (Figure 6B).

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DISCUSSION Aphis gossypii at the seedling stage and A. lucorum at the blooming stage are economically important cotton pests that cause serious damage, and their infestations increase gradually in Bt cotton fields in the Yellow River Region of China.2 Cotton aphid outbreaks occurred from mid-May until mid-June at the experiment site. The clothianidin seed treatments maintained lower population densities of A. gossypii compared to the untreated control plots throughout the seedling stage. The control effect of the clothianidin seed treatments was equal to that of the spray treatments. There were very low densities of A. gossypii at the blooming stage of cotton plants in July and August. After applying clothianidin granules, to some extent, the population densities of A. gossypii were also inhibited. The population densities of A. lucorum maintained a very low presence at the seedling stage and increased rapidly to a high level at the blooming stage. Clothianidin seed treatments at the rate of 4 g ai/kg seed had no significant control efficiency of A. lucorum at the blooming stage of cotton. However, the spot applications of clothianidin granules, even at the low concentration of 0.9 kg ai/ha, provided long-term protection against A. lucorum through this stage and made up for lack of control efficiency from clothianidin seed treatment. Therefore, the clothianidin seed treatment combined with spot applications of clothianidin granules is a method that can effectively suppress the infestations of A. gossypii and A. lucorum throughout the cotton growing season. Zhang et al.3 also showed that clothianidin granules applied at a rate of 3.6 kg ai/ ha at sowing can provided long-term protection against A. gossypii and A. lucorum from the seedling to blooming stages. However, the minimum amount of active ingredient used in the control method in this study was approximately 1 kg/ha, which was less than the amount of active ingredient used in the granular treatments at sowing in Zhang et al.3 In addition, the spot applications of clothianidin granules at the blooming stage also exhibited control efficiency of B. tabaci, which is a commonly occurring, highly destructive, and invasive sucking pest at the late growing stage of cotton plants. Among various neonicotinoids, clothianidin exhibited high toxicity against A. gossypii23,24 and A. lucorum.25 Recently, imidacloprid is the most commonly used neonicotinoid insecticide for the control of sucking pests on cotton when used as seed treatments and foliar sprays. Many previous studies have shown that A. gossypii populations have developed high resistance to imidacloprid with potential cross-resistance to other neonicotinoids.24,26 However, the imidaclopridresistant A. gossypii has shown little or no cross-resistance to clothianidin.26,27 Chen et al.28 demonstrated that clothianidin could significantly inhibit the activities of carboxylesterase, glutathione S-transferase, and acetylcholinesterase in imidacloprid-resistant cotton aphids and be used to manage imidacloprid-resistant cotton aphids. Additionally, clothianidin showed significant adverse effects on the biological characteristics of A. gossypii, including body weight, honeydew excretion, longevity, and fecundity.26 The control efficacy of clothianidin against A. gossypii and A. lucorum might be related to the residues of this neonicotinoid

Figure 6. Dynamic changes of clothianidin concentrations (mg/kg) in cotton leaves (A) and soil (B). H


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necessary for the successful control of insect pests on cotton plants, along with the scientific application of these insecticides. Second, insecticides with different mode of actions should be alternately used for managing cotton pests. Neonicotinoids with moderate persistence and water solubility as seed treatments have raised concerns about environmental contamination.34 After being applied as a seed treatment or granular treatment, the bulk of clothianidin entered the soil and typically declined rapidly by plant uptake, degradation leaching, and absorption.35 However, neonicotinoids may persist under some conditions, and successive applications of neonicotinoids may result in residue accumulation in the soil.36 If clothianidin is widely applied as a seed treatment and a granular treatment for the control of cotton pests in the future, further research should investigate the persistence of this neonicotinoid in soils in a typical field crop ecosystem dominated by cotton production.

in cotton leaves. After seed emergence, the majority of clothianidin used as seed treatments was transferred from the seed coat to the soil, resulting in high residue levels of clothianidin in the soil. Then clothianidin was translocated to cotton leaves through the xylem tissues.29 The clothianidin residues in cotton leaves at seedling stages transferred from the seed coat and provided excellent control efficiency against A. gossypii. The uptake of clothianidin by the plants was influenced by the residue levels of clothianidin in the soil. The degradation of clothianidin in the soil might be the main factor influencing the concentration reduction of this insecticide in cotton leaves. The spot applications of clothianidin granules increased the residue level of clothianidin in the soil again, resulting in the rapid increase in the concentrations of clothianidin in the leaves. During this period, clothianidin had high residue levels in cotton leaves, which resulted in relatively obvious control efficacy against A. lucorum at the blooming stage. At the end of the growing season of cotton plants, the densities of A. lucorum in plots treated with clothianidin granules also remained at low levels even clothianidin with a very low concentration in cotton leaves. This phenomenon can be explained by the fact that clothianidin effectively suppressed the population of A. lucorum to a very low level during the earlier study period, and consequently, the insects had insufficient time to develop again. The clothianidin seed treatment at the seedling stage reduced the abundance of coccinellid adults and larvae and syrphids, which are the major predators in cotton fields. Zhang et al.3 also demonstrated that granular treatments of clothianidin at sowing decreased the population densities of H. axyridis (Pallas) and Propylea japonica (Thunberg) in cotton fields. There are some factors resulting in the reduction of the population densities of natural enemies. First, the abundance of prey is the main factor that determines the distribution of these predators.30 Natural enemies tend to emigrate to, remain, and oviposit in areas with sufficient food resources.31 Higher population densities of A. gossypii at the seedling stage in the untreated plots attracted more predators of cotton aphids to feed or oviposit. At this period, significantly lower densities of natural enemies found in the clothianidin-treated plots were very likely to result from low prey densities. Second, cotton floral and extrafloral nectar act as a food source for some natural enemies.32 The nectar containing insecticides are ingested directly by predators as a supplemental diet and might be a factor in natural enemy reduction. After the spot applications of clothianidin granules at the blooming stage, high residue levels of clothianidin were found in the leaves of cotton plants. Therefore, we speculated that nectar from cotton plants in the plots treated with clothianidin granules also contained high concentrations of clothianidin, which had lethal and sublethal effects on these predators and reduced their population densities. Overall, the clothianidin seed treatments combined with spot applications of clothianidin granules might weaken the biological control services provided by these natural enemies. In conclusion, clothianidin seed treatments (at the rate of 4 g ai/kg seed) combined with clothianidin granular treatments (even at a low rate of 0.9 kg ai/ha) at bud stage can reduce A. gossypii and A. lucorum infestations during the seedling to blooming stages of Bt cotton within 124 days after planting and can improve cotton yield. These above operations could reduce the frequency of insecticide sprays and lower the labor costs. Currently, clothianidin resistance has been reported in A. gossypii populations from some geographic regions.33 Therefore, the monitoring of neonicotinoid insecticide resistance is

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.7b03120. The application of insecticides in spray treatment during experiment period in 2015 and 2016 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-0538-8242611. E-mail: [email protected]. ORCID

Zhengqun Zhang: 0000-0003-1726-2472 Wei Mu: 0000-0002-9836-478X Author Contributions §

Z.Z. and Y.Z. contributed equally to this work.

Funding

This work was supported by a grant from the National Key Research and Development Program of China (2017YFD0201900) and Funds of Shandong “Double Tops” Program. Notes

The authors declare no competing financial interest.

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REFERENCES

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Seed Treatment Combined with a Spot Application of Clothianidin

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