Lepidopteran species fly freely in the environment and their larvae feed on the leaves of host plants which may be exposed to nanomaterials. As an ecological model of nanoparticle exposure, 5th instar larvae of the sweet potato hornworm (Agrius convolvuli) were subcutaneously injected with suspensions of 10 μL (100 μg/mL) titanium dioxide nanoparticles (TiO₂-NPs), 10 μL (100 μg/mL) zinc oxide nanoparticles (ZnO-NPs) or saline (control) and the effects on spermatogenesis were examined in the pupal testis. During the larval wandering stage, larval tissues (except the testis) underwent extremely rapid histolytic changes. Pupation and emergence were not affected by the injection. On pupal day 4, there was a significant decrease in testis weight and the number of sperm bundles in the ZnO-NPs group. Electron microscopic observation revealed that cyst cells surrounding the spermatogenic cells took on small agglomerates of TiO₂-NPs or ZnO-NPs by phagocytosis. As spermatogenesis advanced in the nanoparticle-injected groups, vacuoles of various sizes were found in the nuclei of spermatocytes, the nuclear chromatin of spermatids was uncondensed and some vacuoles were found in the nuclei of sperm bundles. A possible mechanism for this is that abnormal vacuoles disturbed the chromatin condensation, resulting in the decrease of sperm bundles. Toxicity of manufactured TiO₂-NPs and ZnO-NPs were demonstrated detriment to insect spermatogenesis.