Analysis of particle size regulating the insecticidal efficacy of phoxim

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Analysis of particle size regulating the insecticidal efficacy of phoxim polyurethane microcapsules on leaves Jian Luo, Xue-ping Huang, Tong-fang Jing, Da-xia Zhang, Beixing Li, and Feng Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04567 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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ACS Sustainable Chemistry & Engineering

Analysis of particle size regulating the insecticidal efficacy of phoxim polyurethane microcapsules on leaves

Jian Luo, †,‡ Xue-ping Huang, †,‡ Tong-fang Jing, †,‡ Da-xia Zhang, †,# Beixing Li, †,‡ Feng Liu †,‡,* †

Key Laboratory of Pesticide Toxicology & Application Technique, Shandong

Agricultural University, Tai’an, Shandong 271018, P.R. China ‡

College of Plant Protection, Shandong Agricultural University, Tai’an, Shandong

271018, P. R. China #

Research Center of Pesticide Environmental Toxicology, Shandong Agricultural

University, Tai’an, Shandong 271018, P.R. China *

To whom correspondence should be addressed.

E-mail address: [email protected] (F. Liu)

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The size of microcapsules (MCs) is an important and easily adjustable parameter; however, the function of this parameter in the movement behavior of pesticide MCs had not yet been studied. Phoxim-loaded polyurethane MCs with three various size distributions (average diameters of 1.39 μm, MC-S; 5.78 μm, MC-M; and 23.60 μm, MC-L) were obtained. In the greenhouse experiment, the insecticidal activities of MC-S and MC-M occurred mainly in the first three days and that of MC-L was maintained from 3 to 10 days after application. The direct and secondary distributions of a pesticide were defined and used to investigate the effects of particle size on the insecticidal activity of MCs in the field. The results indicated that the reason why MC-S had an excellent initial activity was that it was more widely distributed on the surface of the organism, was more likely to be adhered to by pests, and had greater resistance to rain washing. MC-L had excellent later-stage insecticidal activity, which was mainly because of its outstanding light stability. Then, retained phoxim was released through a crack caused by a light shining onto the shell. The increase in the size of the MCs improved the amount of pesticide swallowed by the insect and the movement distance of the pesticide within the digestive system of the insect. Thus, increasing the size of MCs helps increase the utilization rate of pesticides if a chemical group responding to alkaline conditions can be added into the capsule shell. The transfer and release behavior of pesticide MCs in the field can be regulated by simply adjusting the particle size, which is of great value to the application of pesticide MCs in agriculture and could provide a new approach for the


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efficient utilization of pesticide MC formulation. KEYWORDS: Microcapsule; Size distribution; Polyurethane; Phoxim; Direct and secondary distribution; Insecticidal predominant period



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INTRODUCTION With the widespread application of pesticides in agriculture, the yield and quality of crops have been improved obviously, 1 but with the heavy use of pesticides comes serious environmental pressure.2-6 Under this premise, controlled release formulations (CRFs), as an effective method for reducing the amount of pesticide waste and side effects, have been substantially developed in recent years.7-10 Polymer microcapsules (MCs), as an important part of CRFs, have been widely used in pest control

11,12

due to their simple

preparation process, easy availability of raw materials and wide application in fields. Polymer MCs are widely used as carriers for storing the active ingredients of pesticides, which can be released gradually at the target site; the retained active ingredients inside the MCs can be protected by the polymer shell. 13 However, during the actual application, especially for unstable active ingredients, the products of polymer MCs frequently have shown unstable efficacy; in some cases, the initial and long-term efficacies have been unsatisfactory.

14

This would reduce the effective use of

encapsulated pesticide and cause a negative impact on the environment. This phenomenon was linked to a lack of understanding of the effect of various MC parameters on the mechanism of the bioactivity of pesticide MCs under complicated and changeable field conditions. For various MC parameters, the size parameters were significant during the industrial production of MC formulation and were easily adjusted


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from a technical perspective. In recent years, research showing that the properties and insecticidal efficacy of MCs are affected by the size parameters of MCs gradually has become valued by researchers. However, research on the size parameters of MCs has been limited mainly to nanosized MCs; moreover, the raw materials and preparation process have been reported to be complicated

2,12;

little research has focused on the

micron-sized polymer MCs that have been widely industrialized.

Figure 1. The processes by which pesticide microcapsules function in the insecticidal efficacy on the surface of a plant It is a complicated process by which pesticides function in insecticidal activity on plant foliage in the field environment. Normally, there are two main processes by which pesticide particles function in the insecticidal activity of a foliar spray application (Fig. 1). One process occurs when pesticide particles are deposited and distributed directly onto the organism (pest, crop, etc.) surface by spraying, which was defined as “direct distribution” in this study and the broader direct distribution of pesticide particles can



increase the exposure of harmful organisms to pesticides. Another process occurs when larvae come into contact with and ingest pesticide particles through crawling or feeding behavior on the leaves on which pesticides were deposited, which was defined as “secondary distribution” in this study. Moreover, “secondary distribution” is more effective for controlling pests due to the pests escape behavior during spraying and in various field conditions. Meanwhile, pesticide on the foliage are degraded by various environmental factors (rain, ultraviolet light and so on). Although this process is well known in pesticide application, the behavior of the active ingredients that are encapsulated in the MC shell has not been researched. In particular, the effect of the size distribution on the transfer behavior and insecticidal activity of pesticide MCs in the field has not been studied clearly.. Previous research mainly paid attention to the properties of MC particles, yet few studies studied the distribution and stability properties of MC particles on the surface of the organism in the application environment. In particular, the distribution of MC particles on the surface of target crops and pests and the effect of the size parameter on the mechanism of the MC bioactivity had not been studied in detail. There is not enough data to support such an important parameter which makes the adjustment to the particle size parameter blind during the production process of the microcapsules and it is very adverse for the development and application of controlled-release technology. Previous studies by the author of this paper have found that the size distribution of MC formulation could lead to differences in the release time of the cargo 14. The main reason


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is that there are obvious differences in the thickness and specific surface area of MC shells of different sizes. Although the cargo release and bioactivity would be affected by the stability of wall materials in the environment and by the “direct distribution” and “secondary distribution”' of MC particles on target organisms, the effect of the MC size parameter on the mechanism of these factors had not been studied. As an important organophosphorus insecticide, phoxim presents effective control over a variety of agricultural pests.

15,16

Nevertheless, the photodegradation character of it

should be alleviated via microencapsulation technology. In this study, phoxim-loaded polyurethane MCs of different size distributions were prepared and the agricultural pest Agrotis ipsilon was selected as the targeted insect. Moreover, the initial efficacy was researched by the direct distribution of MC particles on the surface of larvae and plants; the later-stage efficacy was studied by the secondary distribution and the UV (ultraviolet) stability of MC particles after they were applied to organisms. The content of this study is conducive to revealing the mechanism by which the size parameter of MC formulation affects the field efficacy of core-shell pesticide MCs; it also provides an important reference for rationally designing the size distribution of MC formulation.



EXPERIMENTAL SECTION Chemicals The technical grade material (purity = 95%) of phoxim was provided by Hubei Xianlong Chemical Industry Co. (Hubei, China). Other chemicals used to fabricate the MCs are listed in the Supporting Information. Preparation of the emulsifiable concentrate (EC) and MCs MCs were obtained using the interfacial polymerization method according to previously reported procedures, 17 and the detailed fabrication process is described in the Supporting Information. To show the distribution of MC particles on the organism, the MCs were prepared by using the same method but adding 0.5% solvent blue 35 into the oil phase. The ingredients for all MC formulations in this study were the same; only the size distributions of the MC formulations were different. The emulsifiable concentrate formulation was prepared by mixing 36.5 g of phoxim technical material, 4 g of emulsifier 500#, and 6 g of emulsifier 600#. The final volume of the solution was brought 100 mL with xylene. Microcapsule characterization The size distributions and specific surface areas of the MC samples were evaluated by a laser particle size analyzer (LS-POP 6, Zhuhai OMEC Instrument Co., Ltd., Guangdong,


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China). The surface morphology of the MCs was observed using scanning electron microscopy (SEM) (JSM-6610LV, JEOL, Tokyo, Japan). The standard procedures of the release properties and encapsulation efficiency of the MCs are described in the Supporting Information. Greenhouse experiments The greenhouse experiments were conducted to verify the insecticidal effect of MCs with different size distributions. The MC and emulsifiable concentrate samples of phoxim were sprayed at a concentration of 200 mg/L. To promote foliage spreadability, 0.05% Silwet 408 (silicone additive, Momentive Performance Materials, USA) was added to the diluted solutions. The average temperature of the greenhouse where the experiments were carried out was maintained at 15-25 °C. The treated cabbage leaves were picked periodically and made into round samples with a diameter of 1 cm using a circular punch. Two roundels and one larva were placed into each hole of 24-hole cell culture plates. Each experiment was replicated three times. The temperature and relative humidity were maintained at 25 ± 1 °C and 70 ± 5%, respectively. Larval mortality was evaluated after 24 h. 16 Wettability and distribution of MCs on the leaf surface To assess the wettability of the MC diluent, the surface tension and contact angle on the hydrophobic (cabbage) and hydrophilic (cucumber) leaves were measured by using a surface tension meter (QBZY-2, Fangrui Co., Ltd., Shanghai, China) and a contact angle machine (JC2000C2, Zhongchen Digital Technic Apparatus Co., Ltd., Shanghai, China)



at 25 °C. The assessment of the retention on the leaf was referenced to the impregnation method.

18

The diluent of surfactants in the MC formulation was used as a control, and

each of the experiment was replicated three times. The adhesion property of the MC particles on the leaf was investigated according to a reported method but with some modifications.

19

The 0.5 mL MC diluent at a concentration of 200 mg/L was sprayed

onto cabbage and cucumber leaves in a petri dish. After drying naturally, the dish was put at an angle of 30 degrees from the flat table. To simulate rainwater washing, 3 mL of deionized water was dropped on the resulting leaf. Then, the distribution (before and after washing) of the MC particles on the leaf surface was observed through a super deep scene microscope (KEYENCE VHX-900, Guangdong, China). Direct and secondary distribution on the larval surface The 10 mL MC diluent at a concentration of 200 mg/L was sprayed onto 30 larvae in a petri dish. After drying naturally, 6 larvae were selected and treated with narcosis. The direct distribution of MC particles on the larval surfaces was observed through a super deep scene microscope. Then, the remaining larvae were placed into 24-hole cell culture plates. The temperature and relative humidity were maintained at 25 ± 1 °C and 70 ± 5%, respectively. The bioactivity experiment was supplemented with pesticide concentrations of 50 mg/L and 100 mg/L, and the adopted emulsifiable concentrate sample was used as the control. Each of the bioactivity tests was repeated three times, and the mortality was assessed after 24 h. The 10 mL MC diluent at a concentration of 200 mg/L was sprayed onto cabbage


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leaves in a petri dish. After drying naturally, 30 larvae were placed on the treated cabbage leaves and separated by plastic sheets for 10 minutes. Then, 6 larvae were selected and treated with narcosis. The secondary distribution of the MC particles on the larval surface was observed by using the above method. The bioactivity assessment of the remaining larvae was supplemented with pesticide concentrations of 50 mg/L and 100 mg/L, and the adopted emulsifiable concentrate sample was used as the control. Each of the bioactivity tests was repeated three times, and the mortality was assessed after 24 h. Distribution inside the larvae The distribution of MCs inside the larvae in vivo was performed by using the leaf sandwich method

20.

The 2.5 μL sample diluent at a concentration of 200 mg/L was

dropped on a cabbage leaf with a diameter of 0.5 cm. Subsequently, another leaf was used to fabricate the pesticide-loaded sandwiched leaves through the tackiness of gelatinized corn starch. The leaves were utilized to feed larvae that had been starved for 24 h. After four hours, the 40 larvae that ate all the leaves were selected and stored at -18 °C for 1 hour. To specify the cut position, after anatomic observation, the locations of the larval foregut, midgut and hindgut were located in the head (the first and second somite of the larvae), body (the third to seventh somite of the larvae) and tail (the eighth to tenth somite of the larvae), respectively. Then, each larva was excised into three parts according to the location of the larval alimentary canal

21;

next, the larval parts were

smashed and collected into a container. Finally, 5 mL methanol was added to the containers, which were then ultrasonically disrupted, and, after centrifugation, the



supernatant was taken. The phoxim content was measured by HPLC, and the conditions used for HPLC detection are detailed in the Supporting Information. UV Stability The emulsifiable concentrate and MC samples with different size distributions were diluted at a concentration of 200 mg/L. Then, 10 μL of the diluent was applied to a silicon wafer (0.5 cm × 0.5 cm). After evaporation of the water, the silicon wafer was exposed to ultraviolet rays with an average irradiation intensity of 25 μW/cm2. Then, the silicon wafer was removed at regular intervals, and the surface morphology of the MCs was observed by using SEM. To assess the insecticidal effect of the sample after UV irradiation, the silicon wafers were replaced with leaf discs that had a diameter of 1 cm. Each leaf disc that was dropped onto 30 μL diluent of samples was exposed to ultraviolet rays with an average irradiation intensity of 25 μW/cm2 and was removed at regular intervals for bioactivity test. The method of the bioactivity test and the counting mortality was referenced to the greenhouse experiment, and each of the bioactivity tests was repeated in triplicate. The UV degradation curve was assessed according to a reported method. 16,22


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RESULTS AND DISCUSSION Microcapsule characterization

Figure 2. Characterization of the microcapsules containing phoxim: (A) the particle size distribution, (B) the encapsulation efficiencies and specific surface area and (C) the release profiles of MCs with different size ranges. The error bars represent the standard errors of the means of three replicates. Table 1. Release kinetics equations for microcapsules with different size distributions Sample

Fitting equation

R2

MC-S

Q = 99.47 - 99.47 / (1 + (t / 0.38) ^ 3.17)

0.99

MC-M

Q = 95.86 - 95.86 / (1 + (t / 0.79) ^ 1.23)

0.98

MC-L

Q = 94.21 - 94.21 / (1 + (t / 9.6) ^ 2.82)

0.99

Note: Q represents the amount of cumulative release, and t represents the release time.



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As depicted in Figure 2A, the size distributions of the MC samples were unimodal, and the average diameters of the MC samples were 1.39 μm (MC-S), 5.78 μm (MC-M) and 23.60 μm (MC-L). The result demonstrated that the size distributions of MC samples showed an obvious difference in the micron level, although previous research had been less focused on the accuracy of this size adjustment. The encapsulation efficiencies were 96.61%, 98.17% and 98.76%, respectively (Fig. 2B). The similar encapsulation efficiencies of the three MC samples indicated that almost all of the active ingredient would release from the interior of the MCs under the same environmental conditions. The specific surface areas of the MC samples were calculated as 4.44 m2/cm3, 1.16 m2/cm3 and 0.39 m2/cm3 (Fig. 2B). It was clear that the MC sample with a smaller size distribution possessed a larger contact area in the release medium, which might increase the release rate. As depicted in Figure 2C, the MC-S sample had the fastest release in the initial stage, and its cumulative release proportion reached 94.9% at 1 h. The release rate of MC-M, which was slower than that of MC-S, reached 52.3% at 1 h and took 8 h to reach 95.9%. The release rate of MC-L was the slowest, and the cumulative release proportion was only 1.9% at 1 hour and took 24 hours to reach 92.2%. These results indicated that the release rate of the MCs would increase as the size distribution decreased, and the release curves of the three MCs were fitted with the different release equations (Table 1). Previous studies had reported that increasing the size distribution of the MCs would affect the release rate of the encapsulated active ingredients


23.

However, the size

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distribution of MC reported was narrow (3.2-9.0 μm); moreover, accurately adjusting the size in such a narrow range was difficult in industrial production. Obviously, the size range used for this study was larger, and the polyurethane shell was widely used in industrial production. However, the release property of polyurethane MCs with a larger difference in size distribution has not been studied in the agricultural field. Therefore, these results would be valuable for actual applications. Size distribution was an important factor that provided a controllable release of MCs

24,

and an adjustment to the size

distribution has the potential to become a more workable way to adjust the MC release property in industrial production. In the meantime, further studies on application efficacy are still necessary. Greenhouse experiment

Figure 3. The insecticidal efficacies of emulsifiable concentrate, MC-S, MC-M and



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MC-L in the greenhouse. Data with different lowercase letters are significantly different at the p < 0.05 level by Tukey's multiple range test, and the data were arcsine square root transformed prior to statistical analyses. In all of the above data, the error bars represent the standard errors of the means of three replicates. A good pesticide control release system should have favorable actual efficacy.

25

To

verify the insecticidal activity of the MC samples that possessed various release rates, simulation experiments (Fig. 3) were conducted in a greenhouse. The mortality of A. ipsilon was investigated at 1st, 3rd, 5th, 7th and 9th day after treatment. On the first day, the mortality of the emulsifiable concentrate group was the highest at over 93%. Meanwhile, as the size distribution of MCs increased, the mortality on the first day gradually decreased. The mortality of the MC-S (90.2%) was slightly lower than that of the emulsifiable concentrate group, but the mortality of the MC-M group (59.7%) was significantly lower than that of the emulsifiable concentrate group. The mortality of MC-L group (22.2%) was the lowest of all other samples. On the 3rd day after treatment, the mortality of the emulsifiable concentrate group decreased to only 5.6%, which indicated the loss of insecticidal activity. Although the mortality of the MC-S group was significantly higher than that of emulsifiable concentrate group at this time, the mortality of 34.7% was unacceptable to the grower. The mortality of the MC-M group was 66.7% on the 3rd day but rapidly decreased to 18.1% on the 5th day after treatment. In contrast, the MC-L sample began to show outstanding bioactivity; its mortality was 69.4% on the 3rd day after treatment and reached a peak of 88.9% on the 5th day. On the 7th day after


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treatment, the mortality of the MC-L group decreased to 48.6% but outlasted the other groups, which had lost insecticidal activity. The greenhouse experiment demonstrated that the initial bioactivity of MC samples was lower than that of the emulsifiable concentrate sample, but the persistence periods were prolonged. The initial bioactivity of MC increased as the size distribution of the MC sample declined; moreover, the peak of the bioactivity appeared later as size distribution of the MC sample increased. However, after the pesticide MCs were applied into the environment, the release behavior and bioactivity of the active ingredients were affected not only by the structure and thickness of the MC shell and the solubility of the cargo in the shell but they also by the combined action of environmental factors (sunlight, rainfall, etc.). Moreover, the distribution of the pesticide on the organism surface was also a key factor in affecting the efficacy. Therefore, how the size parameter of MCs interacted with the environmental factors to affect the insecticidal efficacy is worth studying, and the distribution behaviors on the organism and the UV stability of the MC particles were studied in-depth in this study. Wettability and distribution of MCs on the leaves



Figure 4. The surface tension (A), contact angle (B) and retention (C) of surfactant control, MC-S, MC-M and MC-L. The distribution of MC particles before and after washing on the cabbage (D) and cucumber (E). Data with different lowercase letters are significantly different at the p < 0.05 level by Tukey's multiple range test. In all of the above data, the error bars represent the standard errors of the means of three replicates. The excellent wettability and adhesion of pesticide diluent on the target organism would be beneficial to improving the deposition and distribution of pesticides, which are very important for providing better bioactivity. 26,27 The surface properties would change after the pesticide was encapsulated by the polymer material, which may affect the property of wetting and distribution of particles on target organism surfaces. Furthermore, the effect of the MC size parameter on the property of wetting and


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distribution of the MC diluent has not been previously studied. The results of the study indicated that the surface tensions of the MC diluent with the different size distributions did not show significant differences at the same concentration and were not observably different from those of the control group. (Fig. 4A) However, surface tensions decreased as the concentration increased. This result demonstrated that the change in the MC size parameter had no significant effect on the surface tension of the MC diluent. As shown in Figure 4B, the contact angle of all MC sample diluents on the hydrophobic cabbage leaf was slightly larger than that on the hydrophilic cucumber leaf, and the contact angle decreased as the concentration increased. Particle size distributions had no effect on the wettability. Meanwhile, the control group showed the same rule. In addition, Figure 4C shows that the retention of the same diluent on the cucumber leaf was much larger than that on the cabbage leaf, and the retention of the MC diluent in the higher concentration was slightly higher than that of the lower concentration. This might be due to the higher concentration of surfactant providing a better retention property. This shows that the surface tension, contact angle and retention of the MC diluent on the leaves were related mainly to the concentration of surfactant in the MC diluent and the leaf surface construction. Therefore, the different bioactivities of the MC samples with different size distributions was uncorrelated with the wettability of the MC diluent. As shown in Figures 4D and 4E, the distribution of MC particles of various size distributions on the cabbage and cucumber leaves showed the same rule. The MC sample



with the smaller size was more evenly distributed and possessed larger area distribution on the leaves. This result indicated that MCs with a smaller size could provide a better direct distribution property of MC particles on the leaf. Pests have more possibilities for contact with pesticide. Meanwhile, the MC particles on the leaf surface might be washed away by rain and cause a secondary distribution of MC particles. The change in the number of MC particles on the leaf surface before and after rain washing could indicate washing resistance. The results showed that the MC particles of a smaller size were harder to wash away by rain compared with the MC particles of a larger size, possibly because the smaller MC particles could more easily deposit into the gaps of the leaf surface and provide better adhesion. MC-L could provide an excellent insecticidal effect in the later stage, and rain wash should not be ignored in the field. Therefore, improving the retention property of pesticide on the plant surface is necessary. One method of adding a construction containing the pyrocatechol functional groups (such as polydopamine) into the MC polymer shell could increase the adhesion of MC particles on the plant surface.

22,28

Another approach was to add absorbability substances into the

diluent. Cai et al. had developed an approach for effectively reducing chlorpyrifos loss by adding ash-based biochar and biosilica into liquid. 29 The distribution and adhesion of MC particles was an important basis for playing bioactivity. The broader direct distribution and faster release characteristic of MC-S could provide excellent initial insecticidal activity. The adhesion property of MC-L on plants should be enhanced.


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Direct and secondary distribution on larval surface

Figure 5. The direct distribution and secondary distribution of three MC samples on larval surface (A1 and A2) and the bioactivity (B1 and B2). Data with different lowercase letters are significantly different at the p < 0.05 level by Tukey's multiple range test. In all of the above data, the error bars represent the standard errors of the means of three replicates. The direct distribution and secondary distribution of the pesticide on the larvae have a great effects on insecticidal activity. Therefore, the two distribution processes of MC particles on the larval surface (Figs. 5A1 and 5A2) were simulated, and the corresponding insecticidal activities were evaluated (Figs. 5B1 and 5B2). As shown in Figure 5A1, there were few MC-L particles on all three sites of larvae, and distribution was sporadic. Compared with MC-L particles, the MC-M particles had a greater quantity



and broader distribution on the larval surface. The MC-S particles showed a uniform layer of the blue mark on the three sites of larvae, which indicated that the MC-S particles had the broadest and most uniform distribution among the three MC samples, and the number of MC-S particles was highest. The results of "direct distribution" showed that the MCs of smaller size possessed more uniformity and a larger coverage area on the surface of larvae, which increased the probability of larvae coming into contact with pesticide. This phenomenon was beneficial to providing an excellent initial insecticidal activity in the MC-S group. As shown in Figure 5B1, the mortality of the MC-S group was much higher than that of the MC-M and MC-L groups at the concentration 50 mg/L. This result might be due to the lower coverage of MC-L particle decreasing the probability that pesticide attached to the sensitive sites of the larvae, and the release rate of MC-S was faster than those of MC-M and MC-L. However, the mortalities of the MC-S and MC-M groups were closer at higher concentrations (100 mg/L and 200 mg/L). Because the number of MC particles in the diluent at higher concentrations was much greater than that at the low concentration, the probability of the MC-M particles attached to the larval body improved. Therefore, the mortality of the MC-M group was slightly lower than that of the MC-S group because of the slower release rate of MC-M than that of MC-S; however, MC-L almost did not release in the initial stage, so the mortality of the MC-L group was much less than that of the MC-S and MC-M groups at these three concentrations. The study on "direct distribution" verified that the MC system with the smaller size


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distribution could provide an excellent initial bioactivity by improving the coverage of pesticide on the larval surface. In consideration of the activity behaviors of the larvae in field conditions, the "secondary distribution" of MC particles might play a more important role in the process of controlling pests. As shown in Figure 5A2, the MC coverage rule of "secondary distribution" was the same as that of "direct distribution"; the MCs with the smaller size distribution possessed broader coverage, and the mortality of the MC-S group was much higher than that of the MC-M and MC-L groups (Figure 5B2). However, compared with "direct distribution", the mortality of "secondary distribution" for all treatments was slightly higher. This result might be due to the cumulative amount of MC particles on the larval surface after crawling being higher than that of "direct distribution", and the MC particles were densely distributed at the feet of larvae or in the gap around the foot. More pesticides might be permeated to larvae because of the stronger adhesion of MC particles in these gaps. Furthermore, the mortality of the MC-L group was lowest for "direct distribution" and "secondary distribution". This result demonstrated that the active ingredient inside MC-L had difficulty being exposed to larvae because of the slow initial release rate, and the stable polyurethane shell could not be punctured during larval crawling. The above studies have indicated that in both “direct distribution” and in “secondary distribution” processes, the particles of the MC-S group had a more uniform and broader distribution on the larval surface. Combined with the fast release rate of MC-S, larvae were more likely to come into contact with active ingredients. Thus, the MC-S group's



excellent character of “direct distribution” and “secondary distribution” could provide a better insecticidal activity in the initial stage. In contrast, the insecticidal activity of the MC-M group was lower than that of the MC-S group, and the insecticidal activity of the MC-L group was very low in the initial stage. Distribution of phoxim inside the larvae

Figure 6. The total concentration and proportion distribution of phoxim inside larvae In addition to the distribution of pesticide on the integument of insects, the "secondary distribution" of pesticide inside larvae during feeding also affects MC bioactivity. As shown in Figure 6, the total concentration of the active ingredient (phoxim) inside the larvae was vastly different among all the samples. The total content of phoxim in polypide treated with emulsifiable concentrate was the lowest at only 2.2 μg/g; those of the three MC treatments were in the range of 3.95 -5.72 μg/g and declined as the MC size


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decreased. This result might be due to that part of the phoxim that was spit out when larvae consumed the food containing pesticides. On the other hand, phoxim have led to an obvious decline in the food consumption of insects. Therefore, the amount of pesticide ingested into larvae would reduce as the phoxim outside of the MCs increased. That is, the amount of pesticide ingested into the larval body was increased after microencapsulation, but the insecticidal efficacy was not increased. Moreover, the pesticide was mostly (70-80%) distributed in the body site where the larval midgut, which mainly plays a role in digestive function, is located. For the MC treatment, the pesticide content proportion (12-21%) of tail (hindgut) was significantly more than the pesticide content proportion (4%) of tail (hindgut) of the emulsifiable concentrate treatment. The pesticide of the emulsifiable concentrate treatment was mainly distributed at the site of the head (foregut) and body (midgut), but the pesticide of the MC treatment was mainly distributed in the site of the body and tail. Since the digestive juice in the midgut of Lepidoptera larval was alkaline, the functional response to the alkaline conditions of the MC shell was expected.

26,30

The utilization rate of pesticide would be

observably improved during pest control.

UV Stability



Figure 7. Influence of UV irradiation on the stability of the MC appearance (A), the retained proportion (B) and bioactivity (C). In all of the above data, the error bars represent the standard errors of the means of three replicates. Greenhouse experiments indicated that the insecticidal activity of the MC-M and MC-L groups was significantly better than that of the MC-S group on the 3rd day. The insecticidal activity of the MC-L group was still satisfactory on the 5th day when other treatments almost lost bioactivity. Therefore, the MC-L group showed the lowest initial


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bioactivity and the longest persistence among the emulsifiable concentrate and three MC samples. In the field environment, phoxim was easily degraded by ultraviolet light; moreover, the polyurethane material was easily aged under the light condition.31

After

the initial release behavior of the MC particles on the surface of leaves, the UV-stability of the MC shell would become the main factor affecting core material release. Therefore, the appearance and the retained and insecticidal activity of the phoxim MCs under UV irradiation were evaluated to research how the size distribution affected the insecticide activity period of the MC samples. As shown in Figure 7A, after exposure to UV light for 15 minutes, the shell of MC-S was damaged seriously and that of MC-M was damaged slightly; nevertheless, the shell of MC-L was intact. The shell of MC-M had been damaged seriously also after 1 h, but that of MC-L was only damaged partially after 3 h. This result was due to the thicker shell of MC particles of larger sizes 14. As shown in Figure 7B, the retained proportion of phoxim in the emulsifiable concentrate group was reduced to 12.2% after exposure to UV light for 0.25 h, while that of the MC-S group was slightly higher, with a value of 21.5%. Experiencing the same exposure time, the retained proportions of phoxim in the MC-M and MC-L groups were 43.4% and 79.2%, respectively. The active ingredients of the emulsifiable concentrate and MC-S groups were almost degraded after 1 h and 3 h, respectively, while that of the MC-M and MC-L groups were 9.0% and 37.5%, respectively, after 3 h. Our previous studies [14] have shown that the wall thickness of polymer microcapsules increases with size increasing. Therefore, MC-L possessed better



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UV-shielding properties because of the thicker MC wall, and it can effectively block the degradation of the pesticide under ultraviolet irradiation. The fitting equation of pesticide degradation is described in Table 2. Moreover, the half-times (T1/2) also indicated that the anti-ultraviolet property of MC-L was the best in the all samples.

Table 2. The kinetic equation of UV degradation for microcapsules of different size distributions. Sample

Fitting equation

R2

T1/2 (h)

EC

Q = 100 / (1 +(t / 0.08) ^ 1.76)

0.99

0.06

MC-S

Q = 100 / (1 + (t / 0.05) ^ 1.37)

0.99

0.08

MC-M

Q = 100 / (1 + (t / 0.19) ^ 0.98)

0.99

0.19

MC-L

Q = 100 / (1 + (t / 1.21) ^ 1.05)

0.96

1.21

Note: Q represents the remaining amount, and t represents the UV irradiation time. Meanwhile, the bioactivity of each of the emulsifiable concentrate and MC samples under UV was evaluated, and the results are shown in Figure 7C. On the first day after treatment, the bioactivity of the emulsifiable concentrate, MC-S and MC-M groups each decreased as the UV exposure time increased, and only the bioactivity of the MC-L group gradually increased after exposure to UV light. Then, the mortality of each treatment was assessed for 3 days. The mortality of the MC-L group increased gradually during the 3 days. The MC-M and the MC-L groups that had been exposed to UV light for 0.25 h showed the same result. This result demonstrates that damage to the MC shell was


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beneficial to the release of the phoxim encapsulated in MC-M and MC-L. These results confirmed the change to the MC shell after exposure to UV light. Hereto, the reason that the MC-L group showed a peak of insecticidal efficacy in the later stage was revealed: The phoxim was protected by the shell and its release amount was less in the initial stage; then the retained phoxim of the MC-L group was released gradually to the outside in the later stage because of the damage to the MC shell caused by the UV light. In conclusion, the release performance of MC not only depended on the parameters of the MC particles but was also affected by the stability of the shell in the environment. This study proves that the bioactivity peak period of polyurethane MCs could be adjusted purposefully by controlling the size parameter of MC. It has been reported that the MC polymer shells could combine their light-response functional groups by complex chemical reactions to obtain control of the release performance.

32,33

In contrast, the

method of adjusting the MCs size made it easier to achieve similar goals. Our previous study had achieved excellent initial and later efficacy by combining the polyurea MC formulation of the various size distributions

14.

Based on the mechanism study which

showed that insecticidal activity was affected by the MC size parameter, the mixtures of phoxim polyurethane MCs of different size distributions were obtained and used for field trials. This result was consistent with what we reported earlier and that the MC(S+L) groups showed excellent initial and later efficacy. The detailed results are shown in the Supporting Information. Meanwhile, the results also demonstrated that the size of the combined system of MC formulations has the potential to be used in other MC shell



materials. CONCLUSION Under the condition of spraying the pesticide on leaves, the effect of the size parameter on the mechanism of the insecticidal efficacy of MCs was researched in this study, and the research scope was the polymer MCs prepared by interfacial polymerization, which has been widely industrialized. It was found that the size parameter of MC formulation had no effect on the surface tension and wettability of MC diluents, but it has a great effect on the direct distribution and secondary distribution of MC particles on the surface of organisms. The decrease of MC size distribution was beneficial to the fast release of MCs, which could provide excellent insecticidal efficacy in the initial period. However, the persistent efficacy of MCs was closely related with the UV resistance of MC shell material. The mechanism of this study would have great potential to improve the efficient utilization of MC controlled release system and possesses universality in the controlled release field.


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ACKNOWLEDGMENTS This work was supported by grants from the National Key Research Development Program of China (2017YFD0200307) and the National Natural Science Foundation of China (31772203). ASSOCIATED CONTENT Supporting Information Chemicals used to fabricate MCs; the detailed preparation of MCs; the detection method the encapsulation efficiency and release property of MCs; field experiment of combined MC formulation. AUTHOR INFORMATION Corresponding Author *(F.L.) Phone: +86 0538-8242611. E-mail: [email protected]. Notes The authors declare no competing financial interest. ORCID Feng Liu: 0000-0002-0271-3632 REFERENCES (1) Enserink, M.; Hines, P. J.; Vignieri, S. N.; Wigginton, N. S.; Yeston, J. S. The Pesticide Paradox. Science 2013, 341 (7), 1763-5,DOI:10.1126/science.341.6147.728.



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DOI: 10.1039/C2PY20735D.

Graphical abstract

SYNOPSIS The reasonable control of microcapsule size distribution can effective improve the comprehensive application property of microcapsule formulation, reducing pesticide loss while possessing the excellent initial and long-term efficacy.




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Analysis of particle size regulating the insecticidal efficacy of phoxim

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