Influence of feeding-location on nutritional quality for cochineal insects (Homoptera : Dactylopiidae) on Opuntia ficus-indica (Cactaceae)
Bachelor Thesis
2002
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University of Cape Town
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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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Van Alphen-Stahl, J. 2002. Influence of feeding-location on nutritional quality for cochineal insects (Homoptera : Dactylopiidae) on Opuntia ficus-indica (Cactaceae). University of Cape Town.