Browsing by Author "Hoffmann, John H"

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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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    Host plant associations of two cochineal insect species, Dactylopius ceylonicus and D. opuntiae (Dactylopiidae: Hemiptera), on the invasive cactus species Opuntia monacantha, O. ficus-indica and a possible hybrid cactus, in South Africa
    (2014) De Souza, Nadia R; Hoffmann, John H
    Several cochineal species have been used with great success for biological control of invasive cactus species, both in South Africa and elsewhere. Although most cochineal species are oligophagous, they are all exclusively associated with opuntioid cacti and therefore pose minimal risk of non-target effects in the Old World where there are no native Cactaceae. However, the ability of cochineal to use more than one host plant species has validated reports of inadvertent displacement of Dactylopius ceylonicus (the original agent released in 1913) by D. opuntiae (which was originally released on Opuntia ficus-indica in 1938) on O. monacantha in South Africa. There are also reports that the two cactus hosts, O. monacantha and O. ficus-indica, have hybridized in South Africa, but there are uncertainties as to which of the two cochineal species is associated with the alleged hybrid. The primary aim of this study was to determine the present status of D. ceylonicus and D. opuntiae on O. monacantha and on the possible hybrid. Cochineal samples were collected off cactus plants from sites in the Eastern Cape and Western Cape and identified using bioassays and morphological features. The performance of each cochineal species on each of the three cactus hosts (O. monacantha, O. ficus-indica and the hybrid) was determined through measures of female developmental rates, survival and mass at reproductive maturity. Results showed that D. ceylonicus remains present on O. monacantha at each of seven study sites, while D. opuntiae did not occur on O. monacantha at any of the sites. However, D. opuntiae is capable of establishing on O. monacantha, O. ficus-indica and the possible hybrid cactus under laboratory conditions. Dactylopius ceylonicus is also compatible with both O. monacantha and the possible hybrid, although it did not establish on O. ficus-indica. These findings showed that D. ceylonicus has not been displaced by D. opuntiae on O. monacantha in South Africa and it is expected that O. monacantha will continue to be kept under excellent biological control into the future. Results also indicated that possible hybridisation between the two Opuntia species has not produced a new taxon that is immune to either of the two control agents. However, further research is required to improve our understanding of this new plant taxon and confirm it will be suppressed by one or both of the cochineal species.
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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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    Patterns of parasitism and emergence in the gall midge Dasineura Dielsii (Diptera: Cecidomyiidae) : a biological control agent of Acacia cyclops in South Africa
    (2004) Wheat, Nicola M; Richardson, David Mark; Hoffmann, John H
    Acacia cyclops A. Cunn. ex G. Don is an invasive alien plant that invades fynbos and coastal dunes. Several acacia species are grown commercially in South Africa and this has limited biocontrol agents to those that reduce only reproductive capacity. Dasineura dielsii was released in 2002 as a biocontrol agent for A. cyclops. This gall-forming midge destroys inflorescences and prevents seedpods from forming, but allows continued harvesting. Insects overwinter as larvae within their galls. This study examined the levels of parasitism experienced by dormant D. dielsii larvae, as well as the trigger that causes them to break dormancy. Gall clusters were sampled over autumn and early winter, and were dissected to determine occupancy. The effects of temperature and light on dormant larvae were also examined. Observations showed that as the season progressed, more D. dielsii larvae entered dormancy. At the same time, a greater proportion of dormant larvae were parasitized. Parasitism was highest, at 18.5%, at the end of the study period, but this level of parasitism is not enough to reduce the biocontrol power of D. dielsii. A positive relationship between mass of gall clusters and the number of galls they contain was established. Larval dormancy could not be artificially broken by either light or temperature, and it appears that neither factor alone can trigger a break in dormancy.
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    Resource partitioning in buds and insect induced galls in the biocontrol of Acacia pycnantha
    (2000) Maguire, Victoria; Hoffmann, John H
    Two Trichilogaster sp. (gall-forming wasps) have been introduced against Acacia longifolia and Acacia pycnantha in the Cape. Trichilogaster acaciaelongifoliae has reduced both seed production (by between 85 and 100% in the Cape) and vegetative growth in Acacia longifolia. The aim of this project was to determine whether the galls and buds of Acacia pycnantha have significantly different energy values, and to determine what affect this has on the plant. There was no significant difference between gall and bud energy values (p=0.029, df-=7, alpha=0.05) but gall energy values were, on average, 9.35% lower than those of bud material. The mass of galls increased steadily throughout the year while bud material remains at the same weight. The resource loss to the plant caused by the galls is dependent on the size and number of galls present on the tree. Even with similar energy values per unit dry mass there were many more grams of gall material on a tree than bud material and thus they would demand more resources to be invested in the gall material. An important factor concerning the effect galling has on A. pycnantha is the time during the trees reproductive cycle that galling occurs. It has been shown that when galling coincides with the season when the most energy is channelled into reproduction that the damage to the tree is heaviest.
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    Why do wasp induced galls of Acacia longifolia photosynthesise?
    (2011) Haiden, Sarah; Cramer, Michael D; Hoffmann, John H
    While many stem and bud galls contain chlorophyll, and have the potential to photosynthesise, these insect-induced growths are generally thought to act as strong carbon sinks, manipulating the normal phloem transport of the host plant in order to serve the demands of the galling herbivore. This study investigated the photosynthetic capacity of bud galls induced by the wasp, Trichilogaster acaciae longifoliae (Pteromalidae) in the invasive Acacia longifolia. The role of this photosynthetic activity was examined in terms of its ability to subsidise carbon budgets, as well as to provide O₂ to the larvae and consume CO₂ in the dense gall tissue, thereby maintaining O₂ and CO₂ concentrations within the range of larval tolerance. Galls were found to contain an overall chlorophyll concentration that was less than half that of subtending phyllodes and a maximum stomata! conductance only 16% that of phyllodes. Gas exchange measurements indicated that while photosynthesis never fully compensated for the respiratory costs of the galls, light-induced carboxylation within galls contributed substantially to the maintenance and growth of galls, especially in the early stages of their development. Very low levels of O₂ were found within the larval chamber and internal tissues of galls, and these levels responded only marginally, if at all, to light, suggesting that the photosynthetic activity of galls does not play a critical role in providing 0 2 to the larvae. The percentage mortality and metabolic response of larvae in reaction to various atmospheres of reduced O₂ and elevated CO₂ indicated that larvae were tolerant of hypoxia and capable of rapidly reducing their respiratory rates to cope with hypercarbia, at least over the short term. Sustained metabolic arrest may, however, have toxic consequences for insects, causing cell damage or even death. The photosynthetic activity of galls substantially reduced internal CO₂ concentrations, thus preventing CO₂ from accumulating within galls over prolonged periods. Hence, the capacity of galls to photosynthesise has significant implications for the survival of the developing larvae by reducing the risk of hypercarbic_toxicity and supplying additional carbohydrates to the gall and its inhabitants, thereby creating a favourable microhabitat in which to live.