Toxic effect of herbicides used for water hyacinth control on two insects released for its biological control in South Africa

The integrated control of water hyacinth, Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae) has become necessary in South Africa, as biological control alone is perceived to be too slow in controlling the weed. In total, seven insect biological control agents have been released on water hyacinth in South Africa. At the same time, herbicides are applied by the water authorities in areas where the weed continues to be troublesome. This study investigated the assumption that the two control methods are compatible by testing the direct toxicity of a range of herbicide formulations and surfactants on two of the biological control agents released against water hyacinth, the weevil, Neochetina eichhorniae Warner (Coleoptera: Curculionidae) and the water hyacinth mirid, Eccritotarsus catarinensis (Carvalho) (Hemiptera: Miridae). A number of the formulations used resulted in significant mortality of the mirid and the weevil. Products containing 2,4-D amine and diquat as active ingredients caused higher mortality of both agents (up to 80% for the mirid) than formulations containing glyphosate. Furthermore, when surfactants were added to enhance herbicide efficiency, it resulted in increased toxicity to the insects. We recommend that glyphosate formulations should be used in integrated control programmes, and that surfactants be avoided in order to reduce the toxic nature of spray formulations to the insect biological control agents released against water hyacinth.     

Assessing the true status of the fish species Labeo cylindricus (Peters 1868) (Teleostei: Cyprinidae) in Lake Baringo,Kenya

Eutrophication contributes to the proliferation of alien invasive weed species such as water hyacinth Eichhornia crassipes. Although the South American moth Niphograpta albiguttalis was released in South Africa in 1990 as a biological control agent against water hyacinth, no post-release evaluations have yet been conducted here. The impact of N. albiguttalis on water hyacinth growth was quantified under low-, medium- and high-nutrient concentrations in a greenhouse experiment. Niphograpta albiguttalis was damaging to water hyacinth in all three nutrient treatments, but significant damage in most plant parameters was found only under high-nutrient treatments. However, E. crassipes plants grown in high-nutrient water were healthier, and presumably had higher fitness, than plants not exposed to herbivory at lower-nutrient levels. Niphograpta albiguttalis is likely to be most damaging to water hyacinth in eutrophic water systems, but the damage will not result in acceptable levels of control because of the plant's high productivity under these conditions. Niphograpta albiguttalis is a suitable agent for controlling water hyacinth infestations in eutrophic water systems, but should be used in combination with other biological control agents and included in an integrated management plan also involving herbicidal control and water quality management.     

The effect of two biological control agents, the weevil Neochetina eichhorniae and the mirid Eccritotarsus catarinensis on water hyacinth, Eichhornia crassipes , grown in culture with water lettuce, Pistia stratiotes

We assessed the effect of two biological control agents, the mirid Eccritotarsus catarinensis (Carvalho) and the weevil Neochetina eichhorniae (Warner), singly or in combination, on the competitive ability of their host plant, water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub., grown in a screen house, in competition with another aquatic plant (Pistia stratiotes L.). Water hyacinth plant growth characteristics measured included fresh weight, leaf and petiole lengths, number of inflorescences produced, and new shoots. Without herbivory, water hyacinth was 18 times more competitive than water lettuce (across all experimental combinations of initial plant densities), as estimated from fresh weights. Both insect species, singly or in combination, reduced water hyacinth plant growth characteristics. E. catarinensis alone was less damaging than the weevil and under normal conditions, i.e., floating water hyacinth, is not expected to increase control of water hyacinth beyond that of the weevil. When combined with the weevil, half the inoculum of weevils and half the inoculum of mirids produced the same growth reduction as the full inoculum of the weevil. Under conditions where the weevils are not effective because water hyacinths are seasonally rooted in mud, the mirid, which lives entirely on leaves, should become a useful additional biological control agent. Handling Editor: John Scott.     

Feeding impact of the planthopper Taosa longula (Hemiptera: Dictyopharidae) on water hyacinth,Eichhornia crassipes (Pontederiaceae)

Taosa longula Remes Lenicov (Hemiptera: Dictyopharidae), a planthopper native to South America, is a candidate for the biological control of water hyacinth, Eichhornia crassipes (Mart.) Solms-Laubach (Pontederiaceae), a serious weed worldwide. Biological control requires agents that are not only specific but also effective. Damage caused by sap-sucking insects is difficult to assess. In this work we designed an experimental and analytical procedure to evaluate the potential damage of T. longula on water hyacinth. The damage that T. longula causes to the clonal reproduction, biomass production, and growth of water hyacinth was studied through a paired greenhouse trial with floating cages. The performance of the plant, starting from two plants per treatment, was evaluated at different insect densities (5, 10, 15 and 20 nymphs per cage) until all the nymphs moulted to adults. The tests showed that individual growth and biomass production of water hyacinth was reduced due to the effect of the insect feeding above five nymphs per cage. The number of new plants produced by clonal reproduction was only significantly different above 15 nymphs per cage. These results suggest that this planthopper could be an effective agent for the biological control of water hyacinth.     

Biological control of water hyacinth in California’s Sacramento–San Joaquin River Delta: observations on establishment and spread

Water hyacinth (Eichhornia crassipes (Martius) Solms-Laubach) is a serious invasive weed in the Sacramento–San Joaquin River Delta of California. Three insects: Neochetina eichhorniae Warner and Neochetina bruchi Hustache (Coleoptera: Curculionidae) and Niphograpta (=Sameodes) albiguttalis (Warren) (Lepidoptera: Crambidae) were released during 1982–1987 at four locations for the biological control of water hyacinth. Observations in 1985 suggested that all three species had established. By 2002, water hyacinth populations in the Delta still required an aggressive chemical control campaign and the status of the biological control agents was in question. In late 2002, a field survey to determine the distribution and abundance of the released insects was performed. Water hyacinth plants were collected by boat in the main water channels and from land at smaller sloughs and examined for insects. In total, 27 sites with water hyacinth distributed across the Delta were examined of which 21 had weevils. Weevil abundance ranged from 0 to 10.9 weevils per plant, with an average of 0.93 (±0.47 SEM) adult weevils per plant. All weevils (n?=?518) were identified as N. bruchi. No N. eichhorniae were recovered and no larvae or evidence of larval feeding by N. albiguttalis were observed. A total of 322 weevils were examined for microsporidia and none was found infected, indicating an infection rate of less than 1%. These results suggest that N. bruchi may be the only established biological control agent of water hyacinth in the Delta and that infection by microsporidia does not appear to be limiting its population abundance.     

Cryptic species of a water hyacinth biological control agent revealed in South Africa: host specificity,impact, and thermal tolerance

The discovery that cryptic species are more abundant than previously thought has implications for weed biological control, as there is a risk that cryptic species may be inadvertently released with consequences for the safety of the practice. A cryptic species of a biological control agent released for the control of the invasive alien macrophyte, water hyacinth, Eichhornia crassipes (C. Mart.) Solms. (Pontederiaceae), was recently discovered in South Africa. The two species were considered a single species prior to genetic analysis and interbreeding experiments. The original biological control agent retains the name Eccritotarsus catarinensis (Carvalho) (Heteroptera: Miridae) whereas the new species has been described as Eccritotarsus eichhorniae Henry. In this study, we compared the host specificity, efficacy, and thermal physiologies of the two species. The host specificity of the two species within the Pontederiaceae was very similar and both are safe for release in South Africa. Comparison of the per capita impact of the two species indicated that E. eichhorniae was the more damaging species but this is likely to be influenced by temperature, with E. catarinensis being more effective under lower temperatures and E. eichhorniae being more effective under higher temperatures. Releasing the correct species for the thermal environment of each release site will improve the level of control of water hyacinth in South Africa. This example highlights the need to keep populations of biological control agents from different native range collection localities separate, and to screen for host specificity and efficacy.     

The decline of water hyacinth on Lake Victoria was due to biological control by Neochetina spp.

There has been some debate recently about the cause of the decline of water hyacinth on Lake Victoria. While much of this evidence points to classical biological control as the major factor, the El Niño associated weather pattern of the last quarter of 1997 and the first half of 1998 has confused the issue. We argue first that the reductions in water hyacinth on Lake Victoria were ultimately caused by the widespread and significant damage to plants by Neochetina spp., although this process was increased by the stormy weather associated with the El Niño event; second that increased waves and current on Lake Victoria caused by El Niño redistributed water hyacinth plants around the lake; and third that a major lake-wide resurgence of water hyacinth plants on Lake Victoria has not occurred and will not occur unless the weevil populations are disrupted. We conclude that the population crash of water hyacinth on Lake Victoria would not have occurred in the absence of the weevils, but that it may have been hastened by stormy weather associated with the El Niño event.     

Fungi associated with Eichhornia crassipes in South Africa and their pathogenicity under controlled conditions

Eichhornia crassipes Mart. Solms-Laubach (Pontederiaceae), water hyacinth, continues to be the world's worst aquatic weed. In South Africa, considerable research has been conducted on biological control agents associated with water hyacinth, with the release of six arthropods and one fungus, but little is known about the occurrence and impacts of native phytopathogenic fungi. Nation-wide surveys were conducted in 2010 and 2011 on various aquatic bodies of South Africa to identify the fungal pathogens associated with water hyacinth. Diseased plant parts were collected and fungi were isolated and identified. Some 250 isolates belonging to more than 25 genera were collected. Some of these represent new host records, as well as undescribed taxa. Isolates of Acremonium zonatum (Sawada) Gams, Alternaria eichhorniae Nag Raj and Ponnappa, Bipolaris hawaiiensis (M.B. Ellis) Uchida and Aragaki, Fusarium Link, Myrothecium roridum Tode ex Fr. and Ulocladium sp., showed the highest pathogenicity and have the potential to be useful in complementing the ongoing biocontrol programme on water hyacinth in South Africa.     

Biology and host preference of the planthopper Taosa longula (Hemiptera: Dictyopharidae), a candidate for biocontrol of water hyacinth

Taosa longula Remes Lenicov (Hemiptera: Dictyopharidae) is a planthopper from the South American tropics that feeds on water hyacinth, Eichhornia crassipes (Mart.) Solms-Laubach (Pontederiaceae). The biology of T. longula was studied in the laboratory and field to evaluate it as a potential biological control agent for this widespread aquatic weed. The developmental time of nymphs was recorded at different temperatures (15, 19, 23, 25, 27 and 30 °C), and developmental threshold temperatures were obtained for the different instars. The host range was evaluated in terms of development and feeding preference. Development from instar I to adult was recorded in two no-choice trials, one with cut leaves of Pontederiaceae, and a second with growing whole plants. In the cut-leaf tests, adults were obtained from Pontederia cordata var. cordata, P. rotundifolia and water hyacinth. In the whole plant test, T. longula adults were obtained only from water hyacinth. Feeding preference was evaluated by means of a paired-choice test with 10 T. longula first instars on whole plants of P. c. cordata, P. rotundifolia and water hyacinth. The number of insects that fed on water hyacinth was significantly higher than on P. c. cordata and P. rotundifolia. Taosa longula showed a clear preference for water hyacinth and exhibited warm climate requirements, making it an attractive candidate for water hyacinth biological control in tropical and subtropical areas.     

Should the mirid,Eccritotarsus catarinensis (Heteroptera: Miridae), be considered for release against water hyacinth in the United States of America?

Between one and seven biological control agents have been released against water hyacinth (Eichhornia crassipes (Mart.) Solms) in at least 30 countries, with varied success. A mirid, Eccritotarsus catarinensis (Carvalho) (Heteroptera: Miridae), the most recent agent released, is damaging to the plant on the African continent. It could be useful in the USA where water hyacinth remains a problem, but its introduction remains in doubt because during host specificity trials, it developed on Pontederia cordata L. (pickerelweed), indigenous to the USA. However, it did not establish on pickerelweed monocultures during South African field trials, and only light spillover feeding occurred where the two plants coexisted suggesting that the use of P. cordata as a host is a laboratory artefact and it may be suitable for use in the USA, if its thermal physiology allows establishment. We reran models developed for South Africa using CLIMEX to predict whether the mirid will establish where water hyacinth and pickerelweed co-occur, but not where pickerelweed occurs in the absence of water hyacinth. The models suggest that the mirid's distribution will be limited by cold winter temperatures and insufficient thermal accumulation to the southern states of the USA, within the main distribution of water hyacinth. Even though some spillover feeding on pickerelweed might result where the two plants co-occur, the risk of population level effects seems minimal and the risk to more northern pickerelweed negligible. The benefits, including improved habitat for pickerelweed, associated with further suppression of water hyacinth, outweigh the minimal risk of collateral damage to pickerelweed.     

The role of eutrophication in the biological control of water hyacinth, <Emphasis Type="Italic">Eichhornia crassipes</Emphasis>, in South Africa

South Africa has some of the most eutrophic aquatic systems in the world, as a result of the adoption of an unnecessarily high 1 mg l⁻¹ phosphorus (P) standard for all water treatment works in the 1970 s. The floating aquatic macrophyte, water hyacinth (Eichhornia crassipes (Mart.) Solms (Pontederiaceae)), has taken advantage of these nutrient rich systems, becoming highly invasive and damaging. Despite the implementation of a biological control programme in South Africa, water hyacinth remains the worst aquatic weed. A meta-analysis of published and unpublished laboratory studies that investigated the combined effect of P and nitrogen (N) water nutrient concentration and control agent herbivory showed that water nutrient status was more important than herbivory in water hyacinth growth. Analysis of long-term field data collected monthly from 14 sites around South Africa between 2004 and 2005 supported these findings. Therefore the first step in any water hyacinth control programme should be to reduce the nutrient status of the water body.     

Determinants and patterns of population growth in water hyacinth

Mathematical models of the growth of water hyacinth are developed to provide a sound basis for assessing existing and potential control options. We show that under constant experimental conditions, water hyacinth shows logistic growth (r² of 0.69–1.00). The effects of nutrients and temperature on model parameters are explored using data from the literature. The model is verified against growth in a natural infestation. The resulting model incorporates two of the most important factors (temperature and water nutrient level) that determine whether water hyacinth is an important environmental problem at a given site. These relationships form a robust basis for further model development, and can be readily used to evaluate how the plant will respond to changes in nutrient inputs.     

Economic evaluation of water loss saving due to the biological control of water hyacinth at New Year’s Dam,Eastern Cape province,South Africa

Water hyacinth Eichhornia crassipes is considered the most damaging aquatic weed in the world. However, few studies have quantified the impact of this weed economically and ecologically, and even fewer studies have quantified the benefits of its control. This paper focuses on water loss saving as the benefit derived from biological control of this plant between 1990 and 2013 at New Year’s Dam, Alicedale, Eastern Cape, South Africa. Estimates of water loss due to evapotranspiration from water hyacinth vary significantly; therefore, the study used three different rates, high, medium and low. A conservative raw agriculture value of R 0.26 per m³ was used to calculate the benefits derived by the water saved. The present benefit and cost values were determined using 10% and 5% discount rates. The benefit/cost ratio at the low evapotranspiration rate was less than one, implying that biological control was not economically viable but, at the higher evapotranspiration rates, the return justified the costs of biological control. However, at the marginal value product of water, the inclusion of the costs of damage to infrastructure, or the adverse effects of water hyacinth on biodiversity, would justify the use of biological control, even at the low transpiration rate.     

Acarine regulators of water hyacinth in Kerala (India)

Screening of the mite fauna on water hyacinth, Eichhornia crassipes, at 25 localities throughout Kerala (India) yielded a list of 21 phytophagous species from nine families. Distribution of mites at these localities and type of injury produced suggested that Oligonychus biharensis and Orthogalumna terebrantis warrant further study. In an experimental set-up the injury caused by these mites was quantified. Infestation by Ol. biharensis lead to reduced photosynthesis by the host (significant loss in chlorophyll content). Injurious effects of Or. terebrantis involved leaf mining by the developing instars leading to wilting of the leaves resulting in reduced dry weight. The relative merits of the two species as potential control agents of water hyacinth are discussed.     

Two in one: cryptic species discovered in biological control agent populations using molecular data and crossbreeding experiments

There are many examples of cryptic species that have been identified through DNA‐barcoding or other genetic techniques. There are, however, very few confirmations of cryptic species being reproductively isolated. This study presents one of the few cases of cryptic species that has been confirmed to be reproductively isolated and therefore true species according to the biological species concept. The cryptic species are of special interest because they were discovered within biological control agent populations. Two geographically isolated populations of Eccritotarsus catarinensis (Carvalho) [Hemiptera: Miridae], a biological control agent for the invasive aquatic macrophyte, water hyacinth, Eichhornia crassipes (Mart.) Solms [Pontederiaceae], in South Africa, were sampled from the native range of the species in South America. Morphological characteristics indicated that both populations were the same species according to the current taxonomy, but subsequent DNA analysis and breeding experiments revealed that the two populations are reproductively isolated. Crossbreeding experiments resulted in very few hybrid offspring when individuals were forced to interbreed with individuals of the other population, and no hybrid offspring were recorded when a choice of mate from either population was offered. The data indicate that the two populations are cryptic species that are reproductively incompatible. Subtle but reliable diagnostic characteristics were then identified to distinguish between the two species which would have been considered intraspecific variation without the data from the genetics and interbreeding experiments. These findings suggest that all consignments of biological control agents from allopatric populations should be screened for cryptic species using genetic techniques and that the importation of multiple consignments of the same species for biological control should be conducted with caution.     

Plant community dynamics relative to the changing distribution of a highly invasive species, <Emphasis Type="Italic">Eichhornia crassipes</Emphasis>: a remote sensing perspective

Eradicating or controlling invasive alien species has frequently had unintended consequences, such as proliferation of other invasive species or loss of ecosystem function. We explore this problem using a case study of a highly invasive floating aquatic macrophyte, water hyacinth (Eichhornia crassipes), in the Sacramento-San Joaquin Delta of California. We used 5 years of remote sensing data to perform change detection analysis to study plant community dynamics contemporaneous with changes in water hyacinth cover. Our results show that as water hyacinth cover decreased, submerged aquatic plant (SAP) cover increased and vice versa. This effect was strongest in large patches of water hyacinth. We found no evidence that the native floating aquatic species, pennywort (Hydrocotyle umbellata), benefitted from reducing cover of water hyacinth. In most years, pennywort cover either showed no trend or followed the same trajectory as water hyacinth cover. In this study a decrease in cover of water hyacinth most often resulted in colonization by SAP species with some habitat returning to open water.     

Influence of weather on emergency transport events coded as stroke: population-based study in Japan

Studying the relation between incidence of stroke and weather is difficult because it requires large-scale community-based data collection. Despite the lack of strong evidence that weather conditions influence stroke incidence, many clinicians feel that meteorological conditions influence the onset of stroke. This study examined whether emergency events related to stroke are influenced by meteorological factors and was based on computerized records of emergency medical transport services in a Japanese city during the period January 1992–December 2003. A total of 53,585 patients transported for an event coded as stroke were analyzed in relation to meteorological factors such as temperature, humidity, and barometric pressure. Poisson regression analysis was applied to clarify the influence of daily meteorological conditions on the daily incidence of emergency transport due to events coded as stroke. Ordinary least squares regression analysis was used to evaluate the influence of weather, defined as the combination of meteorological parameters, on the occurrence of emergency transport due to events coded as stroke. Daily mean ambient temperature and daily mean relative humidity showed a statistically significant negative effect on the incidence of the emergency transport events for both men and women (P<0.001). Daily mean barometric pressure was not significantly related to these events. The occurrence of a holiday was negatively related to the incidence (P<0.001). Dry weather and cool weather were likely to shift the circadian curve of the incidence upward. Thus, occurrence of emergency transport due to events coded as stroke is likely to be associated with weather conditions.     

Predicting the distribution of Eccritotarsus catarinensis, a natural enemy released on water hyacinth in South Africa

Water hyacinth [Eichhornia crassipes (Mart.) Solms (Pontederiaceae)] is the most damaging aquatic weed in South Africa, where five arthropod biological control agents have been released against it. The most recent introduction of Eccritotarsus catarinensis (Carvalho) (Heteroptera: Miridae) has failed to establish permanent populations at a number of sites in South Africa where water hyacinth is a problem. Cold winter temperatures at these sites are assumed to be the reason for these establishment failures. This assumption was tested by investigating the thermal physiology of the mirid, then incorporating these data into various predictive distribution models. Degree‐day models predict 3–14 generations per year at different localities in South Africa, and five generations at a Johannesburg site where the mirid failed to overwinter. The inability to develop sufficiently rapidly during winter months may hinder overwintering of this insect, which was predicted to develop through only one generation during the winter months of April to August in Johannesburg. A CLIMEX model also showed that cold stress limits the mirid's ability to overwinter in the interior of the country, while determination of the lower lethal limit (–3.5 °C) and critical thermal minimum (1.2 ± 1.17 °C) also indicated that extreme temperatures will limit establishment at certain sites. It is concluded that E. catarinensis is limited in its distribution in South Africa by low winter temperatures.     

Die küstenvegetation ostkanadas: D. Thannheiser. Münstersche Geographische Arbeiten. No. 10. Schöningh,Paderborn, 1981, 201 pp. 166 figs., 41 tables,DM. 41.50. ISBN 3-506-73210-2

Temperatures 10 and 2 cm above water level, in leaves of Salvinia molesta Mitchell, and 2 and 10 cm below water level, were measured in the field at hourly intervals over a total of 125 days. Temperature cycles of leaves and air had mean diurnal amplitudes of 8°C in summer and 17°C in winter; cycles in water lagged behind and had amplitudes which decreased with depth. Most parts of S. molesta were warmer than the air at a nearby weather station most of the time and there was temperature stratification in the water during the warm part of each day.Standard meteorological variables were selected, using stepwise regression, to predict daily maximum and minimum temperatures of S. molesta. Thermal inertia of water in the lake seemed to elevate S. molesta temperatures in autumn and depress them in spring compared with temperatures at the weather station. Better predictors were obtained by adding to meteorological variables a function based on the annual cycle of temperatures 10 cm below water level. Hourly temperatures experienced by S. molesta were predicted using curves fitted to diurnal cycles. The reliability of predictions was tested for each season of the year with independent data for a total of 78 days. Predicted temperatures were close to observed temperatures both in absolute terms and in terms of temperature-dependent growth rates of S. molesta.     

First record of an indigenous South African parasitoid wasp on an imported biological control agent,the water hyacinth hopper

ABSTRACT

Water hyacinth, Pontederia crassipes (Martius) [≡Eichhornia crassip es (Martius) Solms-Laubach] (Pontederiaceae), is native to South America, but has expanded its range to many other regions of the world including South Africa. Megamelus scutellaris Berg (Hemiptera: Delphacidae) was released as a biological control agent and has established in several regions. Recently, the indigenous species Echthrodelphax migratorius Benoit, (Hymenoptera: Dryinidae) was discovered in South Africa parasitising M. scutellaris. This newly discovered relationship might have repercussions for the efficacy of biological control of water hyacinth by the delphacid. The wasp may negatively impact M. scutellaris populations making it difficult for the agent to successfully manage the invasive weed. Contrarily, the parasitoid may be beneficial by keeping the M. scutellaris populations stable, serving as a natural enemy.