Browsing by Author "Roberts, Anthony"

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    Constraints on Neltumius arizonensis (Coleoptera: Bruchidae) as a biocontrol agent of prosopis in South Africa : the role of parasitoids
    (2000) Roberts, Anthony; Hoffmann, John H
    1. Two species of seed feeding bruchid, Algarobius prosopis and A. bottimeri, were introduced to South Africa in 1987 and 1990 respectively, to curb the spread of the invasive weed Prosopis. Failure of A. bottimeri to establish and reduced effectiveness of A. prosopis due to interference by livestock resulted in the release of a third species, Neltumius arizonensis in 1992. 2. Algarobius prosopis populations have remained high but N. arizonensis has not been as successful. 3. Populations ofboth bruchids showed similar emergence patterns through the sampling period hence there is no temporal partitioning of resources. 4. Percentage emergence of N. arizonensis was low at the start of the season but increased as the season progressed with higher proportions of N. arizonensis emerging from 'tree' pods than 'ground' pods. Regardless of the spatial partitioning of resources by the two bruchids, N. arizonensis emergence numbers are still far lower than those of A. prosopis (approximately 1:4 respectively). 5. Oviposition strategies of N. arizonensis results in high levels of egg parasitism by Uscana sp., Trichogrammatidae, that A. prosopis escapes by ovipositing in concealed places avoiding detection by parasitoids. 6. Egg parasitism is extremely high on N. arizonensis eggs (67%) and has resulted in failure by N. arizonensis to establish large populations and hence its effectiveness as a biocontrol agent is negligible.
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    The influence of acacias on below canopy grass communities
    (2000) Roberts, Anthony; Midgley, Jeremy J
    The influence of acacia species through their varying life stages (seedlings, juveniles, adults and dead trees) on sub-canopy grass communities was investigated at various sites in the Hluhluwe-Umfolozi Park (HUP). There were significant differences between classes (at p < 0.05} to each other and to the open sites, but to differing extents at the different rainfall regions within the HUP. Significant differences occurred between young acacias and the open sites more frequently at the drier sites than at the wetter sites. Young acacias do not influence below canopy grass communities in the high rainfall regions. ln the low rainfall sites, the young acacias act as refugia for 'bunch' grass species in the sea of 'lawn'. A switch back to lawns occur below the adult acacias dominated by the 'lawn' grass Dactyloctenium australe which occurs at differing proportions to the 'bunch' grass Panicum maximum below the canopy depending on the extent of animal disturbance, measured as density of droppings. Rainfall was responsible for separating communities out between the various sites in HUP. However, the impact of animals on the grass communities within each of the sites had more influence on the resulting grass communities in each of the classes. The response of a variety of grass species to light levels failed to explain the presence of particular species below adult acacias and others away from the influence of these acacias. Succession is occurring between grass communities as a result of establishment of acacias with the process being more evident in the dry areas of the HUP. The grass communities in the high rainfall areas revert back to early successional communities more rapidly once a tree dies than they do in the low rainfall areas. The influence of the adult acacias on the below canopy soils is greater and affects grass communities for a longer period of time in thc low rainfall areas than it is in the high rainfall areas even after the tree has died.