A pesticide delivery system made of biocompatible components and having sustained release properties is highly desirable for agricultural applications. In this study, we report a new biocompatible oil-core silica-shell nanocapsule for sustained release of fipronil insecticide. Silica nanocapsules were prepared by a recently reported emulsion and biomimetic dual-templating approach under benign conditions and without using any toxic chemicals. The loading of fipronil was achieved by direct dissolution in the oil core prior to biomimetic growth of a layer of silica shell surrounding the core, with encapsulation efficiency as high as 73%. Sustained release of fipronil in vitro was tunable through control of the silica-shell thickness (i.e., 8–44 nm). In vivo laboratory tests showed that the insecticidal effect of the fipronil-encapsulated silica nanocapsules against economically important subterranean termites could be controlled by tuning the shell thickness. These studies demonstrated the effectiveness and tunability of an environmentally friendly sustained release system for insecticide, which has great potential for broader agricultural applications with minimal environmental risks.