DSpace Repository

Post release evaluation of Stenopelmus Rufinasus Gyllenhal (Coleoptera : Curcilionidae) : a natural enemy released against the red water fern, Azolla Filiculoids Lam. (Azollaceae) in South Africa

Show simple item record

dc.contributor.advisor Byrne MJ en
dc.contributor.advisor Hill M en
dc.contributor.author McConnachie AJ en
dc.date.accessioned 2016-09-22T11:14:08Z
dc.date.available 2016-09-22T11:14:08Z
dc.date.created 1998 en
dc.date.submitted 2004 en
dc.identifier.uri http://hdl.handle.net/20.500.11892/106316
dc.description.abstract Red water fern, <i>Azolla filiculoides</i> Lamarck (Pteridophyta: Azollaceae), is one of the five declared aquatic weeds in South Africa. Native to South America, it was first recorded in South Africa in 1948. Dense mats of the weed (up to 30cm thick) severely degraded aquatic ecosystems and affected all aspects of utilization. The frond-feeding weevil, <i>Stenopelmus rufinasus</i> Gyllenhal (Coleoptera: Curculionidae), was released as a biological control agent in December 1997. This thesis deals with the post-release evaluation of this insect over the first six years after its release. The weevil was found to be tolerant of a wide range of environmental temperatures, and predictive models showed that its establishment and distribution would not be restricted by climate. Field-cage trials confirmed laboratory findings that the weevil had a high rate of increase, long-lived adults, several overlapping generations per year, and a high per capita feeding rate. To date, <i>S. rufinasus</i> has been released at 112 sites throughout South Africa. The weevil has been responsible for clearing 91 of these sites completely, all of which were cleared in less than a year. The remaining 21 were either washed away during flooding, not revisited, or are in the early stages of control. Laboratory studies showed that phosphorous concentration was found to have little effect on the growth of <i>A. filiculoides</i> or the development of <i>S. rufinasus</i>. The weevil was found to disperse actively, migrating distances of over 300 km from the point of release. <i>Azolla filiculoides</i> megaspores were found to be highly viable (73%) under optimal conditions. At 7% of cleared sites, the weed has reappeared, up to two years after the initial disappearance. The weevil has located 90% of these recurrences and controlled the weed. Host-specificity testing further confirmed the oligophagous habit of <i>S. rufinasus</i> within the genus <i>Azolla</i>. The weevil was recorded to feed and develop on the native <i>Azolla pinnata</i> var. <i>africana</i> (Desv.) under both laboratory and field conditions. Phylogenetic analysis revealed <i>A. pinnata</i> var. <i>africana</i> to be more closely related to the natural host of the weevil, <i>Azolla caroliniana</i> Wild, than to its target, <i>A. filiculoides</i>. For the year 2000, the benefit- cost ratio of the biological control programme was calculated at 2.5:1, increasing to 13:1 in 2005 and 15:1 in 2010 as the annual costs of the programme decrease. These indicators reinforce the overall economic viability of biological control. The biological control of red water fern can be ranked alongside other successful programmes on aquatic weeds in the world. The biggest challenge facing aquatic ecosystems in South Africa, however, remains eutrophication, of which invasion by aquatic weeds is only a symptom. It is highly likely that, in many of the systems in which red water fern has been controlled, other, and possibly worse aquatic weeds will take hold unless the levels of eutrophication are reduced. en
dc.language English en
dc.title Post release evaluation of Stenopelmus Rufinasus Gyllenhal (Coleoptera : Curcilionidae) : a natural enemy released against the red water fern, Azolla Filiculoids Lam. (Azollaceae) in South Africa en
dc.type Doctoral degree en
dc.description.degree PhD (Science) en


Files in this item

Files Size Format View

There are no files associated with this item.

This item appears in the following Collection(s)

Show simple item record