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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 3 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 efficient utilization of pesticide MC formulation.
Luo et al. (Tue,) studied this question.