Diversity and seasonality of Chrysomelidae (Coleoptera) on kenaf, Hibiscus cannabinus Linnaeus (Malvaceae), in South Africa, with special reference to preference indices of Podagrica testacea (Chapuis) (Alticinae)

English: Kenaf (Hibiscus cannabinus L.) is being developed as a new fibre crop in South Africa and its potential in this regard has been under investigation since 2000. Kenaf is a short-day, annual, herbaceous plant. It belongs to the Malvaceae, a plant family notable for both its economic and horticultural importance. The kenaf stalk consists of two types of fibre, an outer ‘bast’ and an inner ‘core’, thereby making it a multi-purpose crop, with a variety of applications, for example, paper, building materials, adsorbents, textiles, livestock feed. Monitoring of all arthropods associated with kenaf was conducted at different trial-site localities throughout South Africa and leaf beetles (Chrysomelidae) were recorded as the most abundant phytophagous insect group. The aim of this study was, to first of all, determine the diversity of Chrysomelidae feeding on kenaf. In this regard, a total of 20 species was sampled during the study period. This data was analysed, and preference indices with regard to season, cultivar or cultivation method were determined for the most abundant species. The flea beetle, Podagrica testacea (Chapuis) (Alticinae), proved to be the most abundant and wide spread species, also causing the most damage by skeletonizing the leaves of the plants, characteristically resulting in a ’shot-hole’ appearance. This species occurred in high densities at four of the six locations (i.e. Winterton (western KwaZulu-Natal), Rustenburg (Northwest Province), Addo (Eastern Cape Province) and Makhathini (northern KwaZulu-Natal)) where kenaf trials were planted. The entomopathogen, Beauveria bassiana Balsamo (Vuillemin), was tested as a biopesticide on P. testacea flea beetles sampled at the Winterton (KwaZulu- Natal) site. Results obtained were inconclusive suggesting that the specific formulation used was not successful in controlling these specific flea beetles. Because of the close relationships that exist between plant diseases and insects, the possibility exist that leaf beetles, P. testacea and Monolepta cruciata Guérin- Méneville occurring on kenaf were acting as possible plant disease vectors, was investigated. Beetles from Winterton were incubated and a total of twelve different fungal species were isolated from these specimens, of which Alternaria spp., Fusarium spp. en Penicillium spp. were the most abundant. World-wide defoliation of kenaf plants by insect herbivores has serious consequences in terms of its successful cultivation. Selective artificial defoliation as a partial insect control mechanism has some potential. It implies removing leaves of the plant, thereby excluding the insects on the basis that their food source is no longer abundantly available. Artificial defoliation trials in this study did not demonstrate this, but rather contributed towards an understanding of the level of impact that simulated beetle herbivory exerts on kenaf yield. Overall the impact of Chrysomelidae in the agricultural environments covered by this study has demonstrated that they are prominent role-players with regard to phytophagy and disease transmission. Therefore, these beetles contribute to the wide variety of factors that should be considered and evaluated to justify the ultimate goal of successfully establishing a viable and sustainable fibre industry for South Africa. Future recommendations for the successful cultivation of kenaf in South Africa would be to invest in a resistant cultivar breeding program, for cultivars particularly suited to South African conditions. Accompanying this would have to be an understanding of the array of anthropocentric activities in the agroenvironment and their influence on the status of pests, crop cultivar and density, and environmental variables, and to what extent, all of these would contribute to the justification and scope of a pest management program.