Insect diversity and population dynamics of natural enemies under sorghum–legume intercrops

Abstract

The effect of intercropping sorghum with legumes on insect diversity and population dynamics of natural enemies was evaluated in the 2015/2016 cropping season at Botswana University of Agriculture and Natural Resources in Sebele, Gaborone. Sorghum monocrop, Sorghum bicolar (L) Moench, was intercropped with four commonly grown legumes, namely cowpea, Vigna unguiculata (L) Walp.; groundnut, Arachis hypogea L.; Bambara groundnut, Vigna subterranean (L.) Verdc; and chickpea, Cicer arietinum L. The study showed that intercropping significantly increased insect abundance, from 176.50 individuals on the monocrop to 438 individuals when sorghum was intercropped with cowpea being the highest among the intercrops. Similarly, species diversity significantly increased from 0.24 on monocrop to 0.54 when sorghum was intercropped with cowpea. There were more herbivores than predators, and the predominant insect herbivore found infesting sorghum was Melanaphis sacchari (Zehntner) (n = 2495), followed by Chilo partellus (Swinhoe) (n = 650). There was a significant difference in the level of parasitism between monocrop and intercrop systems. There were significantly more stemborer parasitoids found in the intercrops, ranging from 1 to 5%, compared to sorghum monocrop where no parasitism was observed. Predatory coccinellid density was not significantly influenced by intercropping but was by time of sampling. The results add to the body of knowledge indicating that intercropping significantly increases insect diversity and reduces the pressure of herbivores on crops in agroecosystems. Intercropping sorghum with legumes increased the number of biological control agents and reduced herbivore populations, and therefore can be recommended as a component of integrated pest management.