Invasion of crops by stemborers previously known in natural vegetation only requires close monitoring for early detection of stemborer exchange between the habitats. This in turn demands up-to-date knowledge about the diversity of stemborers in natural habitats. The current list of known stemborers in South Africa includes 49 species associated with 65 wild host plant. The Eastern Cape and KwaZulu-Natal biogeographic regions received little coverage in earlier studies. This study sought to fill this knowledge gap. We conducted surveys from 14 November 2014 to 18 March 2015 in the Eastern Cape and KwaZulu-Natal biogeographic regions. Potential host plants in families Poaceae, Cyperaceae, and Typhaceae were dissected in the field, stemborer life stages collected, reared on artificial diet, and identified to the lowest possible taxonomic level. Thirty-three species belonging to Crambidae, Noctuidae, Phycitidae, Pyralidae and Tortricidae were recovered from 27 host plant species. Twenty-four species were recorded in Eastern Cape and 28 species in KwaZulu-Natal. The Chao 1 asymptotic species richness estimator predicted 3 and 5 more species for the biogeographic regions indicating an equitably complete species inventory of the study area. Fifteen species collected in this study are part of the existing inventory, and 19 are first records on the various host plants. The new records expand the list of stemborer species in South Africa from 49 to 68. The main factors influencing diversity were endemism, the number and disproportionate abundance of common and rare species.