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)

[摘要] English: Kenaf (Hibiscus cannabinus L.) is being developed as a new fibre crop in SouthAfrica 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, aplant family notable for both its economic and horticultural importance. Thekenaf 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, forexample, paper, building materials, adsorbents, textiles, livestock feed.Monitoring of all arthropods associated with kenaf was conducted at differenttrial-site localities throughout South Africa and leaf beetles (Chrysomelidae) wererecorded as the most abundant phytophagous insect group. The aim of this studywas, to first of all, determine the diversity of Chrysomelidae feeding on kenaf. Inthis regard, a total of 20 species was sampled during the study period. This datawas analysed, and preference indices with regard to season, cultivar orcultivation method were determined for the most abundant species. The fleabeetle, Podagrica testacea (Chapuis) (Alticinae), proved to be the most abundantand wide spread species, also causing the most damage by skeletonizing theleaves of the plants, characteristically resulting in a 'shot-hole' appearance. Thisspecies occurred in high densities at four of the six locations (i.e. Winterton(western KwaZulu-Natal), Rustenburg (Northwest Province), Addo (Eastern CapeProvince) and Makhathini (northern KwaZulu-Natal)) where kenaf trials wereplanted.The entomopathogen, Beauveria bassiana Balsamo (Vuillemin), was tested as abiopesticide on P. testacea flea beetles sampled at the Winterton (KwaZulu-Natal) site. Results obtained were inconclusive suggesting that the specificformulation 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, wasinvestigated. Beetles from Winterton were incubated and a total of twelvedifferent fungal species were isolated from these specimens, of which Alternariaspp., Fusarium spp. en Penicillium spp. were the most abundant.World-wide defoliation of kenaf plants by insect herbivores has seriousconsequences in terms of its successful cultivation. Selective artificial defoliationas a partial insect control mechanism has some potential. It implies removingleaves of the plant, thereby excluding the insects on the basis that their foodsource is no longer abundantly available. Artificial defoliation trials in this studydid not demonstrate this, but rather contributed towards an understanding of thelevel of impact that simulated beetle herbivory exerts on kenaf yield. Overall theimpact of Chrysomelidae in the agricultural environments covered by this studyhas demonstrated that they are prominent role-players with regard to phytophagyand disease transmission. Therefore, these beetles contribute to the wide varietyof factors that should be considered and evaluated to justify the ultimate goal ofsuccessfully establishing a viable and sustainable fibre industry for South Africa.Future recommendations for the successful cultivation of kenaf in South Africawould be to invest in a resistant cultivar breeding program, for cultivarsparticularly suited to South African conditions. Accompanying this would have tobe an understanding of the array of anthropocentric activities in theagroenvironment and their influence on the status of pests, crop cultivar anddensity, and environmental variables, and to what extent, all of these wouldcontribute to the justification and scope of a pest management program.

[发布机构] University of the Free State