Van Deventer, Francina Elizabeth
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.
Afrikaans: Kenaf (Hibiscus cannabinus L.) word sedert 2000 as ‘n potensiële nuwe veselgewas vir Suid-Afrika ontwikkel. Kenaf is ‘n kortdag, eenjarige, kruidagtige plant. Dit behoort aan die plantfamilie Malvaceae, wat veral vir hul ekonomiese en tuinboukundige belang bekend is. Die kenaf stam bestaan uit twee tipes vesel, die buitenste ‘bas’ vesel en die binneste ‘kern’ vesel, hierdie einskap gee aanleiding tot ‘n wye verskeidenheid van produkte waarvoor die plant aangewend kan word. Dit sluit in produkte soos, papier, boumateriaal, tekstiele, absorbeer middels asook voer vir diere. Monitering van alle geleedpotiges wat met kenaf geassosieer is, is by verskillende proefperseel lokaliteite dwarsdeur Suid-Afrika uitgevoer en blaarkewers (Chrysomelidae) was die volopste fitofage insekgroep wat aangeteken is. Die doel van hierdie studie was, eerstens, om die verskeidenheid van Chrysomelidae wat op kenaf voed, te bepaal. In hierdie konteks is ‘n totaal van 20 verskillende spesies vervolgens tydens die studietydperk versamel. Hierdie data is toe geanaliseer, en voorkeur indekse wat seisoen, kultivar of bewerkingspraktyk betref van die volopste spesie is vasgestel. In hierdie verband is die vlooi kewer, Podagrica testacea (Chapuis) (Alticinae), as die volopste en mees wydverspreide spesie uitgewys, en het dit ook die meeste vreetskade aan die plant veroorsaak, naamlik ‘n kenmerkende ‘hael-skade’-tipe voorkoms. Hierdie spesie het by vier van die ses lokaliteite (d.i. Winterton (westelike Kwazulu-Natal), Rustenburg (Noordwes Provinsie), Addo (Oos Kaap Provinsie) en Makhathini (noordelike Kwa-Zulu Natal)) waar kenaf proewe geplant is, voorgekom. Die entomopatogeen, Beauveria bassiana Balsamo (Vuillemin), is as ‘n biologiese beheeragent teen P. testacea (Chapuis) vlooikewer eksemplare, wat by die Winterton (KwaZulu-Natal) perseel versamel is, getoets. Resultate wat verkry is was nie oortuigend nie en dui moontlik daarop dat die formulasie wat hier gebruik is nie suksesvol sal wees in die beheer van hierdie spesifieke vlooikewer spesie nie. Met die noue assosiasie wat tussen plantsiektes en insekte bestaan, is die moontlikheid dat P. testacea en Monolepta cruciata blaarkewers wat op kenaf voorkom, om as vektore van plantsiektes op te tree, ondersoek. Kewers vanaf Winterton is geïnkubeer en ‘n totaal van 12 fungus spesies is van hierdie eksemplare geïsoleer, waarvan Alternaria spp., Fusarium spp. en Penicillium spp., die volopste was. Die ontblaring van kenaf plante wat deur insekherbivore veroorsaak word, het orals in die wêreld ernstige probleme tot gevolg. Daar bestaan ‘n geringe moontlikheid dat selektiewe, kunsmatige ontblaring, as ‘n meganisme wat gedeeltelike insekbeheer kan bewerkstellig, gebruik kan word. Dit berus op die beginsel dat die verwydering van blaarmassa kan lei tot die onbeskikbaarheid van genoegsame voedsel vir die insek. Onder die omstandighede kon die kunsmatige ontblarings proewe in hierdie studie nie hierdie moontlikheid demonstreer nie, maar het dit eerder bygedra tot ‘n begrip van die impaksvlak van gesimuleerde kewer herbivorie op kenaf opbrengs. In die geheel het die impak van Chrysomelidae op die landbou omgewings van hierdie studie gedemonstreer dat hulle vername rolspelers is wat fitofagie en siekteoordrag betref. As sulks dra hierdie kewers by tot die wye verskeidenhed van belangrike faktore wat in ag geneem en ge-evalueer moet word om die suksesvolle ontwikkeling en vestiging van kenaf as ‘n nuwe kommersiële gewas in Suid- Afrika te regverdig. Verdere aanbevelings vir die suksesvolle verbouing van kenaf in Suid-Afrika, sluit ‘n daadwerklike belegging in ‘n weerstandbiedende teelprogram vir kultivars, wat aan die spesifieke Suid - Afrikaanse toestande voldoen, in. Gepaardgaande hiermee sou ‘n begrip van die verskeidenheid van mensgedrewe aksies in die agro-omgewing wees, en daarmee saam ‘n bepaling van hul invloed op plaagstatus, gewaskultivar en digtheid, en omgewingsveranderlikes, en in welke mate dit alles bydra tot die regverdiging en omvang van ‘n plaagbestuur program.