| The low utilization rate of pesticide in the foliar application is a common problem in agricultural chemicals,which is caused by many factors such as the complexity of the pesticide application environment.Microencapsulation technology has become one of the popular areas of pesticide preparation research and development because of improvement of the utilization rate and safety of pesticides in recent years.However,there is still a problem in that they cannot balance the initial and long-term efficacy in practical applications.Due to the limitation of preparation process,the particle size of conventional microcapsules（MCs）generally showed a unimodal distribution.Studies have shown that the size of MCs affected its release,thereby affecting the control efficacy in recent years.Therefore,it is clear whether it is feasible to use the particle size parameters of MCs to balance the initial and long-term efficacy and improve the utilization rate of pesticides.In this study,lambda-cyhalothrin（LC）loaded polyurea MCs of three different sizes were successfully prepared by interfacial polymerization method.The effects of MC size parameters on the performance and insecticidal activity of LC loaded polyurea MCs was clarified by characterization of their morphology,particle size,release properties and photolysis resistance and evaluation of biological activity,exposure evaluation of MCs under natural conditions and rain-washing resistance before and after photolysis.LC loaded polyurea MCs of different sizes were mixed in pairs at ratios of 2:1,1:1,and 1:2.Subsequently,the particle size,release properties and photolysis resistance were measured to clarify their performance changes.The effect of MC size combination on the control efficacy was verified by greenhouse and field experiments.The main results were as follows:1.The prepared LC loaded polyurea MCs with three different particle sizes（average particle size of 1.38 μm,MC-S;5.13 μm,MC-M;and 10.05 μm,MC-L）were spherical and showed different degrees of collapse.In addition,MCs with smaller sizes had obvious collapse,thinner shell,larger specific surface area,faster release rate and higher insecticidal activity,but poor photolytic resistance.2.The exposure level of MCs on peanut leaf surface was evaluated under natural conditions.The results showed that the MCs with smaller size had a faster release rate under natural light.Compared with the release in the release solution,the difference was that MC-S was suddenly released after the evaporation of the water（79.62 % was released after the evaporation of the water）.Degradation was accelerated with the decrease of MC size.The amount of LC on the surface of emulsifiable concentrate（EC）and MC-S showed a downward trend,while MC-M and MC-L first increased and then decreased.Therefore,combined MCs of different sizes are expected to achieve sustained release,thereby maintaining high insecticidal bioactivity throughout the control period.There was no release behavior or degradation behavior of MCs under dark conditions.This indicates that light is an important factor affecting the release of LC loaded polyurea MCs.3.The experiment of rain-washing resistance of LC loaded polyurea MCs on peanut leaves showed that the retention of MC-S on peanut leaves was highest in three size MCs after simulated rainfall no matter whether in lucifugal or UV irradiation conditions.And the rainwashing resistance was improved after the degradation of polyurea MCs under UV irradiation for 1 h.4.The release and degradation of LC loaded polyurea MCs with different sizes that mixed in pairs at different ratios were controlled in two stages.The release rate and degradation rate of combined MCs with same proportion and different particle sizes were MC（S:M）> MC（S:L）>MC（M:L）.The release rate and degradation rate of combined MCs with same proportion and different particle sizes were MC（2:1）> MC（1:1）> MC（1:2）.The results showed that MCs with smaller sizes and greater proportions in the size combination samples had faster release and degradation properties.5.LC loaded polyurea MCs with different sizes that mixed in pairs at different ratios showed the same trend in the greenhouse experiment of Agrotis ipsilon（They were sprayed at a concentration of 10 mg/L）and the greenhouse experiment and field experiment of Hyphantria cunea（The dilution multiple of 2.5% EC and 10% MCs were 3000 times and 12000 times,respectively）.In the 21-day control period,the insecticidal activity of MC（S:L）was relatively stable.Compared with EC,MC（S:M）prolonged the duration of insecticidal activity,but the insecticidal activity decreased in the later stage,which could not meet the requirement of longterm efficacy.MC（M:L）had higher insecticidal activity in the later stage,but the insecticidal activity was low in the initial stage.MC（S:L）had higher insecticidal activity in 1:2 of the three ratios.Therefore,MC（S:L=1:2）was the most balanced combination among nine MC size combinations.In summary,particle size could affect the performance and insecticidal activity of LC loaded polyurea MCs,and light had an important impact on its release and control efficacy.After particle size control and refined combination of MCs,it can achieve the balance of initial and long-term efficacy and improve the utilization rate of pesticides.Therefore,it has certain development and application potential. |
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