The interactions between plants and their insect herbivores are one of the main generators of biological diversity. A fundamental process generating this outstanding diversity is diet expansion to novel host plants. During the last four decades scientists accumulated evidence showing that co-adaptation between plants and herbivores is a major process assembling plant-herbivore interactions. However, rescent research suggests that adaptation is not always a prerequisite to generate novel plant-herbivore interactions. Novel associations between plants and insect herbivores may be assembled by ecological fitting - an ecological process whereby herbivores colonize novel host plants as a result of the suites of preadapted traits that they carry at the time of colonization. A widespread assumption concerning the architecture of insect herbivore genotypes is the "Jack of all trades master of none" principle. This principle proposes that there is a trade-off in genotype performances between host plants. The main prediction of this principle is that genotype performance will be negatively correlated among hosts. Genotypes displaying high performance on a given host will perform poorly on other hosts. This constraint of adaptation to multiple host plants implies that diet specialization will be selected over generalization. Contrary to these theoretical expectations, in most cases, genotypes that perform well in one host will also perform well in other host plants. Positive correlations in cross-host performance represent ecological and evolutionary dynamics opposite to the "Jack of all trades" principle. In this scenario genotypes with high performance on one host plant also have high performance on other plants, promoting generalization. The predictions of the current theory on the assemblage of novel plant-herbivore interactions focus on the fact that most insect herbivores are specialists. However, to fully understand the processes underlying the assembly of novel plant-insect interactions, it is necessary to study diet expansions in both specialist and generalist insect herbivores. This dissertation was performed at La Selva Biological Station, a tropical rain forest in Costa Rica, Central America. We studied a group of neotropical herbivores, the "rolled-leaf beetles" (Cephaloleia, Chrysomelidae: Cassidinae) and their host plants, neotropical plants in the order Zingiberales. Cephaloleia beetles have evolved with neotropical Zingiberales for the last 40-60 MY. Four paleotropical and one South American members of the Zingiberales have been introduced to La Selva during the last decade. After these introductions, currently seven Cephaloleia beetles are expanding their diets to exotic Zingiberales. These incipient diet expansions represent an opportunity to understand: 1) the relative roles of adaptation vs ecological fitting on the demography and colonization success of novel hosts in generalist and specialist herbivores and 2) whether genotypic performance across original and novel hosts are negatively correlated, as predicted by the "Jack of all trades" principle, or genotype performances across original and novel hosts are positively correlated. For most of the experiments included in this dissertation, I focused on the performance of larvae and adults of two generalist (Cephaloleia belti, C. dilaticollis) and two specialist beetles (Cephaloleia dorsalis, C. placida) reared in the laboratory on native or exotic Zingiberales. Generalist and specialist species display similar responses when changing their diets to novel hosts. Larvae preferred and performed better in the original than in the novel hosts. Adults usually displayed the opposite pattern, i.e. higher preference for and longevity on the exotic than on the novel hosts. In most novel interactions between Cephaloleia beetles and exotic Zingiberales, larval performance required adaptation, but adult performance was pre-adapted to the novel hosts. Therefore, both adaptation and ecological fitting are playing a role during diet expansions to novel hosts. Vital rates estimated through experimental demography show that population growth is reduced on novel host plants for both generalist and specialist Cephaloleia. Although in some cases population growth on the novel hosts is negative, suggesting the potential outcome of extinction after colonization or source-sink dynamics, several beetle species displayed positive population growth in the novel host plants. Positive instantaneous population growth rates in novel hosts supports diet expansions without substantial initial evolutionary change through ecological fitting. In quantitative genetics experiments testing for cross-host genetic correlations in performance between the original and novel host plants, we did not find evidence for negative genetic correlations, as predicted by the "Jack of all trades" principle. Most genetic correlations in performance between original and novel hosts were either not significant or they were positive. These results represent very different ecological and evolutionary dynamics than those predicted by the "Jack of all trades" principle. In this case, genotypes with high performance on original hosts also displayed high performance on novel hosts, promoting generalization. In conclusion, interactions between Cephaloleia beetles and plants from the order Zingiberales are labile. In some cases diet expansions may occur without substantial evolutionary change. In addition the genetic architecture of genotypes promotes generalization during diet expansions to novel hosts.

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The suitability of sunflower (Heliantus annuus L.) as a host of Ophraella communa Le Sage (Coleoptera: Chrysomelidae) was evaluated under greenhouse and field conditions. Population dynamics of O. communa on sunflower and on ragweed (Ambrosia artemisiifolia L.) were determined using a life table approach. Sixty percent of O. communa females died during the first 30 days on sunflower while only 14% died on ragweed plants. Only 20% of fertile females laid eggs on sunflower plants as compared with 100% on ragweed plants. Fecundity, life duration, egg viability, and other biological parameters were significantly higher on ragweed plants than on sunflower plants. Ragweed is the main host plant for O. communa. Nevertheless, in specific situations (no-choice) adults and larvae of O. communa can damage sunflower plants. However, O. communa cannot complete its life cycle or increase its population on sunflower plants.

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The objective of this study was to investigate endophagous insect species richness in Fynbos. The influences of plants as determinants of insect occurrence were given special attention. The endophagous insects associated with Proteaceae in Fynbos were compared to endophage assemblages from northern, non-Capensis Proteaceae. The Cape Fynbos genus Protea is utilized by many more insect taxa than the non-Fynbos species. The high diversity of host plants in Fynbos appears to have contributed to generating high, local endophagous insect diversity. Influences of regional climate, biotope and host-plant characteristics on the frequency of occurrence of insect borers exploiting Protea species was investigated in Fynbos. Distinct differences in frequency of encounter of the various insect taxa were recorded for the various host-plants studied. This variability was primarily accounted for by physical host-plant characteristics (infructescence and seed-set variables). These findings have important implications for evolution of insects associated with these plants, as well as for the conservation of insects and in pest control programmes on indigenous cut flowers. The relative species richness of endophagous and ectophagous insects in Fynbos was compared. Gall-forming insects (Diptera: Cecidomyiidae), were found to be considerably more speciose than other feeding guilds, showing that the ratio of endophages to ectophages in sclerophyllous vegetation types is high. The intimate relationship that endophagous insects have with their host plants tends to habitat specialization. These insects are therefore likely to undergo radiation together with their host-plants. Species richness of gall-insects in Fynbos was investigated to establish whether insect richness was proportional to plant species richness. The relationship between gall-insect species richness and plant-species richness was investigated. Fynbos harboured more gall-insect species than other Cape Floristic Region vegetation types. Gall-insect species richness was positively correlated with plant-species richness. Plant species richness appears to have contributed to the evolution of a rich gall-insect fauna in the region. Fynbos gall-insect species richness is comparable to other sclerophyllous vegetation types globally, underscoring the importance of this vegetation type as a centre of galler diversification. Finally, the importance of plant species richness as a determinant of gall-insect species richness was investigated by comparing different sclerophyllous vegetation types under the same climatic conditions. Gall were sampled from Fynbos and Karoo vegetation. Fynbos had higher gall-insect species richness, correlated with plant-species richness. Plant-species richness, or the distal factors that generated it, appear to have contributed significantly to the radiation of gall-insects in this region. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1995.

Mistletoes intimately connect to their host trees with a haustorium that allows them to access nutrients and water. Mistletoes in South Africa vary greatly in their degree of host specificity. Most species occur on a wide range of host families, while others are restricted to a single host family or—at the extreme—to a single host species. Mistletoes that are host generalists at a larger spatial scale may become host-specific at a local scale. One of the challenges in mistletoe biology is determining the factors that maintain local host specificity. Birds potentially reinforce the mistletoe–host interactions by direct dispersal. However, many mistletoe species coexist while parasitising different co-occurring host species. This suggests that host trees may impose more selection pressure than birds in determining host specificity. Thus, my thesis examines the role of host trees as ecological and physiological filters that influence the infection patterns and determine host specificity of mistletoes in South Africa. The second chapter of this thesis synthesises the literature on host specificity in mistletoes. I then present the results of four field and laboratory experiments that were used examine the features affecting host specificity in representatives of two families of mistletoes (Loranthaceae and Viscaceae) in South Africa. My main research objectives focus on host abundance and morphology, host compatibility, host water and nutrient content, abiotic influences on mistletoe seedling survival and growth and mistletoe–host stomatal morphology in relation to water potential that affect nutrient acquisition by mistletoes from their host trees. The geographic mosaic approach was explored as a potential explanation for the mistletoe–host interactions that direct host specificity in mistletoes. I synthesised the available literature on the mechanisms and factors that direct mistletoe host specificity. This was supported by data analysed from South African herbarium collections, books describing the South African flora and field observations in South Africa. I suggest that host abundance (host availability through time and space) and host compatibility (as determined by genetic, morphological, physiological and chemical factors) play a primary role in determining host specificity in South African mistletoes, while differential bird dispersal strengthens or weakens mistletoe–host interactions. Analysis of the network structure of mistletoe–host interactions at different levels (e.g., at the level of population, species and genus) followed by genetic and reciprocal germination experiments may reveal the patterns and mechanisms of host specificity in mistletoes. I quantified the mistletoe–host composition, height of potential host trees and nutrient and water content of mistletoes and their hosts at Pniel Estates. Surveys of the study site revealed a single mistletoe species, Viscum rotundifolium, parasitising only Ziziphus mucronata and Ehretia rigida. Both parasitised host species were not the most abundant trees, were not the tallest trees and did not have the highest water or nutrient content of trees in the area, although these factors have been found to be good predictors for mistletoe parasitism in other studies. Subsequently, I tested mistletoe–host compatibility by conducting a germination experiment in the greenhouse by inculcating seeds of V. rotundifolium on freshly cut branches of nine available potential host trees. I found that mistletoe seeds had a greater chance of attachment and subsequent survival on branches of E. rigida and Z. mucronata as compared with seeds on co-occurring Acacia and other potential host species. This suggests that host compatibility plays a role in directing the host specificity of V. rotundifolium at Pniel Estates. I found that individuals of V. rotundifolium had more negative water potentials than their host trees and, by doing so, they passively maintain the flow of nutrients. In addition, I found evidence that the mistletoe uses active uptake to access nutrients from host phloem because the leaf tissue of a mistletoe had a nitrogen-to-calcium ratio (N:Ca) >1. Conventionally, a high N:Ca ratio (>1) in the leaf tissue of a mistletoe is taken as evidence of active uptake from host phloem because N is highly phloem-mobile while Ca is a large molecule and is phloem-immobile. This method has shortcomings discussed at greater length in the chapter but my findings suggest that the mistletoe V. rotundifolium uses a combination of passive and active nutrient uptake. I quantified the mistletoe–host community composition and host physical features (height and diameter at breast height) in two sites in KwaZulu-Natal, South Africa—Highover and Mtontwane. The mistletoe Agelanthus natalitius (Loranthaceae) is common at both sites, parasitising the most abundant host species—Acacia karroo—and the second most abundant host tree—Acacia caffra. Prevalence of mistletoe infection (percentage of trees parasitised) was positively correlated with tree size (height and diameter at breast height). The two host species did not differ significantly in height. At Highover the host species A. caffra and A. karroo had a similar prevalence of mistletoe infection but at Mtontwane a significantly higher percentage of A. caffra trees was parasitised in comparison with A. karroo. However, the intensity of mistletoe infection (mean number of mistletoes per tree) was lower for A. caffra (Highover: 0.66 ± 0.01, Mtontwane: 0.89 ± 0.04) than for A. karroo (Highover: 0.73 ± 0.04, Mtontwane: 1.03 ± 0.64). There were two highly infected big trees in Highover and one in Mtontwane where many mistletoe-dispersing birds were nesting which inflated the numbers for intensity of mistletoe infection in A. caffra, however. I tested mistletoe–host compatibility by conducting a reciprocal transplant experiment in the two study sites. I applied a paired design, using one local and one non-local mistletoe seed in each pair, with seed pairs placed on the two main host species at the different sites. Except in Highover where an unidentified pathogen retarded growth and survival, mistletoe seeds placed on the same substrate and in the same site as their source host grew a longer hypocotyl and had greater survival. Regardless of source, mistletoes placed on A. karroo had longer hypocotyls and greater survival than mistletoes on A. caffra. These results suggest that there may be adaptation of the mistletoe Agelanthus natalitius to the most frequently encountered host species, Acacia karroo. To simulate the conditions encountered by mistletoes during the dry and cold South African winter, mistletoe seedlings were monitored at different levels of microclimate (light, temperature and moisture) in a growth chamber. I found that higher light availability (20% and 40% shade versus 80% shade), cool temperatures (15°C and 20°C versus 25°C) and continuous moisture availability improved seedling development and subsequent survival of two mistletoe species (Viscum rotundifolium and Agelanthus natalitius). I studied the leaf stomata of two host–mistletoe pairs (Acacia karroo–Agelanthus natalitius and Vitex obovata–Erianthemum dregei) using a scanning electron microscope to investigate some of the underlying mechanisms that enable mistletoes to maintain more negative water potentials than their host trees and at the same time control water loss. In addition, I examined the response of mistletoes to the application of abscisic acid (ABA), a plant growth regulator that controls stomatal closure. I found that the mistletoes had a higher density of stomata and had larger stomata than their host trees. In addition, both mistletoe and host leaves closed their stomata during midday and in response to exogenous ABA. The ability of mistletoes to control water loss in this way may be one reason why mistletoes rarely kill their host trees, which would be maladaptive. The mistletoes used in my studies are known to be host generalists at a larger spatial scale but I found that they were host specific at a local scale. The results of my research suggest that host abundance and compatibility play a role in directing host specificity, while host nutrient and water status have little effect on host specificity at this local scale. The interactions between the generalist mistletoes used in my studies and their hosts are likely to vary over the geographic ranges of the mistletoe and alternate among different hosts. This may create multiple locally host–specific mistletoe populations and produce a complex geographic mosaic of mistletoe–host combinations across space and time. I suggest that mistletoe populations in South Africa may comprise numerous lineages incapable of parasitising the full range of host species, which could potentially lead to the formation of distinct host races over time. In the future, it would be interesting to document the infection patterns of these generalist mistletoe species across their entire geographic ranges in southern Africa, with particular focus on the patterns of mistletoe infection in places where the host abundance changes among sites. Host preferences may vary with changes in host frequency and host community composition. This could be paired with reciprocal transplant germination experiments in several sites to ascertain whether the mistletoe species have higher fitness on the most locally abundant hosts. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.

The following body of research provides a detailed overview of the interactive effects of biocontrol agents and environmental factors and how these influence both the host plant and pathogen populations within hydroponic systems. Pythium and other zoosporic fungi are pathogens well suited to the aquatic environment of hydroponics. Motile zoospores facilitate rapid dispersal through fertigation water, resulting in Pythium becoming a yield reducing factor in most hydroponic systems and on most crops. With increasing trends away from pesticide use, biocontrol is becoming an ever more popular option. Unfortunately, much of our knowledge of biocontrol agents and their formulation can not be directly transferred to the widely differing environments of hydroponic systems. Paulitz (1997) was of the opinion that if biocontrol was to be successful anywhere, it would be in hydroponics. This is primarily due to the increased ability, in hydroponics, to control the growing environment and to differentiate between the requirements of the pathogen versus those of the host plant and biocontrol agent. Key environmental factors were identified as soil moisture, root zone temperature, form of nitrogen and pH. A review of the literature collated background information on the effects of biocontrol agents and environmental manipulation on plant growth and disease severity in hydroponic systems. A commercial formulation of Trichoderma (Eco-T(R1)) was used as the biocontrol agent in all trials. Dose responses in Pythium control and plant growth stimulation in lettuce were first determined using a horizontal trough system (closed system). In such systems optimum application rates were found to be lower than in field application (1.25x10[to the power of 5] spores/ml). This is probably because Trichoderma conidia are not lost from the system, but re-circulate until being transported into the root zone of a host plant. No significant growth stimulation was observed, although at high doses (5x10[to the power of 5] and 2.5x10[to the power of 5] spores/ml) a significant reduction in yield was recorded. Possible reasons for this growth inhibition are suggested and a new theory is proposed and investigated later in the thesis. In an open system of cucumber production (drip irrigated bag culture) no statistically significant results were initially obtained, however, general trends still showed the occurrence of positive biocontrol activity. The initial lack of significant results was mostly due to a poor knowledge of the horticulture of the crop and a lack of understanding of the epidemiology behind Trichoderma biocontrol activity. These pitfalls are highlighted and, in a repeat trial, were overcome. As a result it could be concluded that application rates in such systems are similar to those used in field applications. Management of soil moisture within artificial growing media can aid in the control of Pythium induced reductions in yield. A vertical hydroponic system was used to determine the interactive effects of soil moisture and Trichoderma. This system was used because it allowed for separate irrigation regimes at all 36 stations, controlled by a programmable logic controller (PLC). With lettuce plants receiving optimum irrigation levels, no significant reduction in yield was observed when inoculated with Pythium. However, after Pythium inoculation, stresses related to over- or under-watering caused significant yield losses. In both cases, Trichoderma overcame these negative effects and achieved significant levels of disease control, especially under higher soil moisture levels. Growth stimulation responses were also seen to increase with increasing soil moisture. Similar results were obtained from strawberry trials. These results show that Pythium control is best achieved through the integration of Trichoderma at optimum soil moisture. However, where soil moisture is above or below optimum, Trichoderma serves to minimize the negative effects of Pythium, providing a buffering capacity against the effects of poor soil moisture management. Pythium, root zone temperature and form of nitrogen interact significantly. In greenhouse trials using horizontal mini troughs with facilities for heating or cooling recirculating water, nitrate fertilizer treatments resulted in statistically significant results. Lettuce growth was highest at 12°C, although no significant differences in yield were observed between 12-24°C. Pythium was effective in causing disease over the same temperature range. Pythium inoculation did not result in yield reduction at 6 and 30°C. Trichoderma showed a slight competitive advantage under cooler temperatures (i.e., 12 degrees C), although significant biocontrol occurred over the 12-24 degrees C range. Ammonium fertilizer trials did not generate statistically significant data. This is possibly due to complex interactions between root temperature, ammonium uptake, and competitive exclusion of nitrification bacteria by Trichoderma. These interactions are difficult to replicate over time and are probably influenced by air temperature and available light which are difficult to keep constant over time in the system used. However, the data did lead to the first clues regarding the effects of Trichoderma on nitrogen cycling as plants grown with a high level of ammonium at high temperatures were seen to suffer more from ammonium toxicity when high levels of Trichoderma were added. In further trials, conducted in the recirculating horizontal mini trough system, it was determined that Trichoderma applications resulted in an increase in the percentage ammonium nitrogen in both the re-circulating solution and the growing medium. This was a dose-related response, with the percentage ammonium nitrogen increasing with increasing levels of Trichoderma application. At the same time an increase in ammonium in the root tissue was observed, corresponding with a decrease in leaf nitrate levels and an increase in levels of Cu, Na, Fe and P in leaf tissue. In independent pot trials, populations of nitrifying bacteria in the rhizosphere were also seen to decrease with increasing Trichoderma application rates. This led to the conclusion that the increase in ammonium concentration was as a result of decreased nitrification activity due to the competitive exclusion of nitrifying bacteria by Trichoderma. The possibility that Trichoderma functions as a mycorrhizal fungus and so increases the availability of ammonium for plant uptake is not discarded and it is thought that both mechanisms probably contribute. Water pH provides the most powerful tool for enhancing biocontrol of Pythium by Trichoderma. Trichoderma shows a preference for more acidic pHs while Pythium prefers pHs between 6.0 and 7.0. In vitro tests showed that Trichoderma achieved greater control of Pythium at pH 5.0, while achieving no control at pH 8.0. In greenhouse trials with the recirculating horizontal mini trough system, yield losses resulting from Pythium inoculation were greatest at pH 6.0 and 7.0, with no significant reduction in yield at pH 4.0. Biocontrol activity showed an inverse response with greatest biocontrol at pH 5.0. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2003.

High, intermediate and low virulent strains of Xanthomonas campestris pv. manihotis were isolated from cassava fields in Nigeria. Resistance to cassava bacterial blight (CBB) increased with age. CBB was established by as little as 10('4) CFU/mL inoculum, but general symptom severity and lesion length differences between resistant, intermediate and susceptible cultivars were greatest with 10('6) CFU/mL and leaf wilting with 10('8) CFU/mL. CBB increased shoot dry weight and foliar ion leakage; this increase was greater for potassium and magnesium than for sodium and calcium. Pathogen multiplication in leaves was least in the resistant cultivar and it spread to the stem only in the susceptible one. CBB reduced liquid flow in stem of resistant, intermediate and susceptible cultivars by 43, 35 and 96%, respectively. Flow in the healthy susceptible cultivar was double that in the more resistant cultivars. The number of non-functional vascular bundles in diseased plants was negatively correlated with liquid absorption and translocation.

Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2008.