Browsing by Author "Van Alphen-Stahl, Jonathan"

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    Influence of feeding-location on nutritional quality for cochineal insects (Homoptera : Dactylopiidae) on Opuntia ficus-indica (Cactaceae)
    (2002) Van Alphen-Stahl, Jonathan; Hoffmann, John H
    The cochineal insect Dactylopius opuntiae has been an extremely successful biocontrol agent of the cactus weed Opuntia ficus-indica. Its resounding success has allowed us to take a closer look at how the relationships between biocontrol agents and their host plants are maintained. The prickly pear, O. ficus-indica, is the sole source of food for the cochineal insect and provides it with refuge and shelter. Wind, sun and particularly rain leave the cochineal insects vulnerable to predators as their protective waxy coating is eroded by the elements. The behaviour of the insects to settle on the sheltered side of cactus cladodes is not unexpected when one takes the stresses into account. One curious fact that has arisen from this is that the insects that do settle on the sheltered side of the cladode are larger and more fecund than those that settle on the exposed surface of the cladode. The nutritional quality of the plant was investigated as a possible explanation for the disparity in size. Phloem sap was collected from the severed stylets of cochineal insects and analysed for polyphenols (which inhibit growth) and amino acids (responsible for growth). The indication from the polyphenol analysis was that it was not responsible for inhibiting growth in cochineal insects. Amino acid analysis from two different cladodes gave conflicting results and no definite conclusions could be drawn from these. Increasing the number of samples analysed will give us a better idea of any trends that could possibly exist. It is certain that there is still much to be discovered in the way of insect-plant interactions and future studies in this field could yield some potentially remarkable findings.
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    Molecular systematics and phylogeography of the Helmeted Guineafowl (Numida meleagris)
    (2005) Van Alphen-Stahl, Jonathan; Crowe, Timothy M; Bloomer, P
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    Phylogeograpy of Aphanicercella cassida (Plecoptera : Notonemourida) : cryptic speciation?
    (2002) Van Alphen-Stahl, Jonathan; Hedderson, Terry A
    Stoneflies (Plecoptera) in South Africa are limited to cool pristine mountain streams. This provides them with very limited habitat in which to survive. The species of stonefly studied in this project (Aphanicercella cassida) has a very wide distribution unlike most other species which have a limited range. Of particular interest is a population in Mpumalanga (north of South Africa) which is very disjunct from the other populations of A. cassida found widely in the southwestern Cape. Despite this large geographic distance the insects appear to be morphologically identical to one another. It is believed that there is limited or no gene flow between the northern and southern populations as stoneflies are poor fliers. Molecular analysis of the cytochrome oxidase I gene in the mitochondrial DNA was analysed to see if any genetic differentiation was occurring inspite of the morphological homogeneity. A 557 base pair region was compared across three populations of A. cassida one in Mpumalanga and two in the southwestern Cape. Two congeneric species, A. bullata and A. scutata, were used as outgroups. The findings of this study were quite dramatic. The northern population is extremely different from the southern populations at the molecular level. There is more variation between the two populations of A. cassida than between the two outgroup species and the split between the two disjunct populations displays evidence of an ancient vicariance event. This finding definitely warrants further investigation into whether the population of A. cassida in Mpumalanga is a cryptic species. More sampling has to be done within the studied populations and more populations should be added to the analysis before any definite decision can be made regarding this fascinating emergence of possible cryptic species in South African stoneflies.