The role of the mite Orthogalumna terebrantis in the biological control programme for water hyacinth, Eichhornia crassipes, in South Africa

Water hyacinth (Eichhornia crassipes) is an aquatic macrophyte originating from the Amazon basin. Due to its beautiful appearance it has been introduced into numerous countries across the world as an ornamental pond plant. It was introduced into South Africa in the early 1900s and has since reached pest proportions in many of the country’s fresh water bodies, causing significant economic and ecological losses. It is now considered to be the worst aquatic weed in South Africa. Efforts to control the spread of the weed began in the early 1970s and there have been some successes. Biological control has been used widely as an alternative to mechanical and chemical controls because it is cost-effective, self-sustaining and environmentally friendly. To date, six biological control agents have been introduced onto water hyacinth in South Africa. However, due to factors such as cold winter temperatures and interference from chemical control, the agent populations are occasionally knocked-down and thus the impact of biological control on the weed population is variable. In addition, many South African water systems are highly eutrophic, and in these systems the plant growth may be accelerated to such an extent that the negative impact of the agents’ herbivory is mitigated. One of the agents established on the weed is the galumnid mite Orthogalumna terebrantis, which originates from Uruguay. In South Africa, the mite was initially discovered on two water hyacinth infestations in the Mpumalanga Province in 1989 and it is now established at 17 sites across the country. Many biological control researchers believe that the mite is a good biological control agent but, prior to this thesis, little quantitative data existed to confirm the belief. Thus, this thesis is a post-release evaluation of O. terebrantis in which various aspects of the mite-plant relationship were investigated to determine the efficacy of the mite and thus better understand the role of the mite in the biological control programme of water hyacinth in South Africa. From laboratory experiments, in which mite densities were lower than densities occurring in the field, it was found that water hyacinth growth is largely unaffected by mite herbivory, except possibly at very high mite densities. When grown in high nutrient conditions the growth of the plant is so great that any affect the mite has is nullified. Plant growth is thus more affected by nutrients than by mite herbivory. However, mite feeding was also influenced by water nutrient levels and mite herbivory was greatest on plants grown in high nutrient conditions. The presence of the mite had a positive effect on the performance of the mirid Eccritotarsus catarinensis, such that the interactions of the two agents together had a greater negative impact on the plant’s growth than the individual agents had alone. Furthermore, water hyacinth physiological parameters, such as the plant’s photosynthetic ability, were negatively impacted by the mite, even at the very low mite densities used in the study. Plant growth rate is dependent on photosynthetic ability i.e. the rate of photosynthesis, and thus a decrease in the plant’s photosynthetic ability will eventually be translated into decreased plant growth rates which would ultimately result in the overall reduction of water hyacinth populations. In addition, temperature tolerance studies showed that the mite was tolerant of low temperatures. The mite already occurs at some of the coldest sites in South Africa. Therefore, the mite should be able to establish at all of the water hyacinth infestations in the country, but because it is a poor disperser it is unlikely to establish at new sites without human intervention. It is suggested that the mite be used as an additional biological control agent at sites where it does not yet occur, specifically at cold sites where some of the other, less cold-tolerant, agents have failed to establish. Finally, conditions of where, how many and how often the mite should be distributed to water hyacinth infestation in South Africa are discussed

Insights into the pollination requirements of the only African wild tobacco, Nicotiana africana (Solanaceae) from the Namib Desert

Nicotiana species (Solanaceae) are widely distributed, mainly in the Americas and Australia. Nicotiana africana is the only species indigenous to Africa; its populations are confined to isolated mountains in the Namib Desert and thus little is known about this species' reproductive strategy or reliance on pollinators, if any. Plants grow in a sheltered environment among granite boulders and wind pollination is therefore unlikely. Our aim was to use a controlled hand-pollination experiment to identify the pollination requirements of N. africana and thereby infer the level of reliance on pollinators. One of five treatments was applied to flowers: either self- or cross-pollination, with half the flowers being emasculated and half not, and the fifth treatment consisted of unmanipulated flowers. Fruit set, seed set and seed weight were measured to determine pollinator reliance. Fruit set and seed set were similar for crossed and selfed flowers. Our findings show that N. africana is self-compatible but also partially dependant on pollinators for reproduction. Using floral traits, the plant's natural distribution and comparisons with other Nicotiana species, we predict that sunbirds (Nectariniidae) are the most likely pollinators of this species in its natural habitat.The National Research Foundation of South Africa (grant 73671) and the University of Pretoria.http://www.elsevier.com/locate/jaridenv2017-02-28hb2016Zoology and Entomolog

The effect of herbivory by the mite Orthogalumna terebrantis on the growth and photosynthetic performance of water hyacinth (Eichhornia crassipes)

Eutrophication of fresh water systems is one of the most important factors contributing to the invasion of fresh water bodies by water hyacinth, Eichhornia crassipes. The South American mite, Orthogalumna terebrantis, established on the weed in South Africa in the late 1980s, but the impact of mite herbivory on the weed has never been quantified. Water hyacinth was grown under low, medium and high nitrogen and phosphorus nutrient conditions and the effect of mite herbivory on the weed's growth was examined. Additionally, the impact of different mite herbivory intensities on the weed's photosynthetic performance was examined because herbivory may have more subtle effects on the plant than can be seen from changes in plant growth parameters. Water nutrient content had a great impact on plant growth, but growth was unaffected by mite herbivory in all levels of nutrients tested. Photosynthetic performance of water hyacinth leaves exposed to varying levels of mite herbivory was assessed by measuring net photosynthetic rate (A), leaf conductance (gl), transpiration rate (E) and intercellular CO2 concentration (Ci), and by measuring specific fluorescence parameters including maximal fluorescence (Fm), efficiency of photosystem II (Fv/Fm) and certain JIP-test parameters. Photosynthesis decreased as mite herbivory increased, but there was a positive correlation between gl, E and Ci, and the amount of leaf tissue damaged through mite feeding. The efficiency of photosystem II (PSII) decreased as mite herbivory increased, as seen in the altered fluorescence emission of mite-damaged plants, but this was not the consequence of decreased chlorophyll content. Feeding by O. terebrantis thus decreased water hyacinth photosynthetic rate and the light reaction performance, even at relatively low mite densities. These results show that the impact of a biological control agent on its host plant may not be obvious at a plant growth level, but may nonetheless affect the plant at a physiological level

The only African wild tobacco, Nicotiana africana: Alkaloid content and the effect of herbivory

Herbivory in some Nicotiana species is known to induce alkaloid production. This study examined herbivore-induced defenses in the nornicotine-rich African tobacco N. africana, the only Nicotiana species indigenous to Africa. We tested the predictions that: 1) N. africana will have high constitutive levels of leaf, flower and nectar alkaloids; 2) leaf herbivory by the African bollworm Helicoverpa armigera will induce increased alkaloid levels in leaves, flowers and nectar; and 3) increased alkaloid concentrations in herbivore-damaged plants will negatively affect larval growth. We grew N. africana in large pots in a greenhouse and exposed flowering plants to densities of one, three and six fourth-instar larvae of H. armigera, for four days. Leaves, flowers and nectar were analyzed for nicotine, nornicotine and anabasine. The principal leaf alkaloid was nornicotine (mean: 28 µg/g dry mass) followed by anabasine (4.9 µg/g) and nicotine (0.6 µg/g). Nornicotine was found in low quantities in the flowers, but no nicotine or anabasine were recorded. The nectar contained none of the alkaloids measured. Larval growth was reduced when leaves of flowering plants were exposed to six larvae. As predicted by the optimal defense theory, herbivory had a localized effect and caused an increase in nornicotine concentrations in both undamaged top leaves of herbivore damaged plants and herbivore damaged leaves exposed to one and three larvae. The nicotine concentration increased in damaged compared to undamaged middle leaves. The nornicotine concentration was lower in damaged leaves of plants exposed to six compared to three larvae, suggesting that N. africana rather invests in new growth as opposed to protecting older leaves under severe attack. The results indicate that the nornicotine-rich N. africana will be unattractive to herbivores and more so when damaged, but that potential pollinators will be unaffected because the nectar remains alkaloid-free even after herbivory

Physiological mechanisms of dehydration tolerance contribute to the invasion potential of ceratitis capitata (Wiedemann) (Diptera: Tephritidae) relative to its less widely distributed congeners

CITATION: Weldon, C. W., et al. 2016. Physiological mechanisms of dehydration tolerance contribute to the invasion potential of ceratitis capitata (Wiedemann) (Diptera: Tephritidae) relative to its less widely distributed congeners. Frontiers in Zoology, 13:15, doi:10.1186/s12983-016-0147-z.The original publication is available at http://frontiersinzoology.biomedcentral.comBackground: The Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) is a highly invasive species now with an almost cosmopolitan distribution. Two other damaging, polyphagous and closely-related species, the marula fruit fly, Ceratitis cosyra (Walker), and the Natal fly, Ceratitis rosa Karsch, are not established outside of sub-Saharan Africa. In this study, adult water balance traits and nutritional body composition were measured in all three species at different temperatures and levels of relative humidity to determine whether tolerance of water stress may partially explain their distribution. Results: Adult C. capitata exhibited higher desiccation resistance than C. rosa but not C. cosyra. Desiccation resistance of C. capitata was associated with lower rates of water loss under hot and dry conditions, higher dehydration tolerance, and higher lipid reserves that were catabolised during water stress. In comparison with C. capitata, C. cosyra and C. rosa lost water at significantly higher rates under hot, dry conditions, and did not catabolise lipids or other sources of metabolic water during water stress. Conclusions: These results suggest that adult physiological traits permitting higher tolerance of water stress play a role in the success of C. capitata, particularly relative to C. rosa. The distribution of C. cosyra is likely determined by the interaction of temperature with water stress, as well as the availability of suitable hosts for larval development.http://frontiersinzoology.biomedcentral.com/articles/10.1186/s12983-016-0147-zPublisher's versio

Additional file 3: of Physiological mechanisms of dehydration tolerance contribute to the invasion potential of Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) relative to its less widely distributed congeners

Table S3. General linear model for the relationship between species, sex, temperature (Temp) and relative humidity (RH) on the dehydration tolerance of three Ceratitis species. Estimated body water (determined from initial body mass using the equations in Table S1) was included as a covariate. Significant effects (P < 0.05) are indicated by bold type. (DOC 37 kb

Additional file 1: of Physiological mechanisms of dehydration tolerance contribute to the invasion potential of Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) relative to its less widely distributed congeners

Table S1. Linear regression for the relationship between body mass (mg) and body water content (mg) for cohorts of three Ceratitis species that were subsequently tested for desiccation resistance and water loss rate at two temperatures. The equation for each relationship was used to estimate initial body water content from initial body mass for flies subjected to desiccation and water loss rate assays. (DOC 41 kb

Results of generalized linear model (GLM) analyses for alkaloid concentrations in <i>Nicotiana africana</i>.

1<p>Data for undamaged leaves from control plants as well as damaged leaves from plants with six larvae were not included in the analysis because no damaged and undamaged middle leaves, respectively, were available.</p

Effect of herbivory on leaf alkaloid concentrations.

<p>Effect of herbivory by <i>Helicoverpa armigera</i> fourth-instar larvae on (A) nicotine, (B) nornicotine and (C) anabasine concentrations (mean ± SE) in undamaged (open bars) and herbivore damaged (black bars) middle leaves of flowering <i>Nicotiana africana</i> plants. <i>n</i> = 4–30.</p