Biological Control Of Water Hyacinth Research Articles

Water turbidity affects the establishment of Neochetina eichhorniae (Warner) (Coleoptera: Curculionidae): Implications for biological control of water hyacinth

Water hyacinth is the target of nine biological control agents in South Africa including Neochetina eichhorniae (Warner) and Neochetina bruchi (Hustache) (Coleoptera: Curculionidae). These two weevils have also been released against water hyacinth in Rwanda, but failed to control the weed invasion, possibly due to high turbidity in the country's water bodies. This study therefore aimed to investigate the effect of water turbidity on the establishment and performance of N. eichhorniae in Rwanda. Turbidity levels were measured over two seasons in four Rwandan rivers and two lakes. The results were then used to benchmark laboratory trials to test the effect of turbidity on the weevils' development. Water hyacinth plants were maintained at four turbidity levels: Clear water (2 Nephelometric Turbidity Units (NTU): low (85 NTU), medium (600 NTU) and high (1500 NTU). Each treatment plant was inoculated with three N. eichhorniae larvae, while control plants were free of larvae. Plant growth was measured weekly for three months, while adult weevil emergence was recorded from the 56th day of the experiment. The number of adults emerging from the treatment plants grown in the clear water, low, medium and high turbidity levels were 24, 21, 12 and 0, respectively. Larval feeding was greater on plants growing in clear water and the low turbidity, compared to the medium and high turbidity treatments. These results indicate that N. eichhorniae may not establish or perform well in water bodies with high levels of turbidity, which in turn enhances the growth of water hyacinth, allowing compensatory growth for weevil feeding.

It's a numbers game: inundative biological control of water hyacinth (Pontederia crassipes), using Megamelus scutellaris (Hemiptera: Delphacidae) yields success at a high elevation, hypertrophic reservoir in South Africa.

ABSTRACT Classical biological control of water hyacinth in South Africa has been constrained by cool winter temperatures that limit population growth of the biological control agents, and highly eutrophic waters which enhance plant growth. However, inundative releases of the control agent, Megamelus scutellaris (Hemiptera: Delphacidae), at the Hartbeespoort Dam, South Africa, suggest that water hyacinth can be managed successfully using biological control as a standalone intervention for the first time in the absence of herbicide operations, despite eutrophication and a temperate climate. Sentinel-2 satellite images were used to measure the reduction in water hyacinth cover from over 37% to less than 6% over two consecutive years since M. scutellaris was first released on the dam in 2018, while site surveys confirmed a corresponding increase in M. scutellaris population density from fewer than 500 insects/m2 in October 2019, to more than 6000 insects/m2 by March 2020. Inundative release strategies are recommended for the control of water hyacinth in South Africa at key stages of its invasion, particularly after winter, and flooding events.

Host spectrum, reproduction, and survival of Neochetina eichhorniae (Coleoptera: Curculionidae) - A potential natural enemy for controlling water hyacinth (Eichhornia crassipes)

Neochetina eichhorniae (Coleoptera: Curculionidae) is a natural enemy of water hyacinth and its morphology as well as biology has been investigated previously. In this study, the host range of N. eichohorniae was examined using the following 7 groups of plants: Pontederiaceae (water hyacinth, Eichhornia crassipes; hastate-leaf pondweed, Monochoria hastata); food crops (rice, Oryza sativa; maize, Zea mays; sweet potato, Ipomoea batatas and cassava, Manihot esculenta); vegetables (cucumber, Cucumis sativus; mustard greens, Brassica juncea; turnip, Raphanus sativus; water spinach, Ipomoea aquatica; pepper elder, Peperomia pellucida; cabbage, Brassica oleracea var. capitata); herbaceous (boat lily, Tradescantia discolor, gotu kola, Centella asiatica, purple-heart, Tradescantia pallida); fruits (mango, Mangifera indica; longan, Dimocarpus longan; rambutan, Nephelium lappaceum); water plants (sacred lotus, Nelumbo nucifera; red water lily, Nymphaea rubra, yellow bur-head, Limnocharis flava) and industrial plants (sugarcane, Saccharum ssp.; peanut, Arachis hypogaea). The N. eichohorniae was found to be the only survivor that developed and completed its life cycle on water hyacinth. The N. eichohorniae caused only minimal damage to hastate-leaved pondweed and its life cycle was not completed on this weed. When fed with water hyacinth, N. eichohorniae female produced 358.9 eggs on average and the egg-laying period was 16 weeks. Forty three percent of eggs were laid from the 5th to the 8th weeks after females emerged from cocoons. The hatchability of N. eichohorniae eggs was 75.2% and 66.8% of the larvae could survive and became pupae. Approximately 79.1% of the pupae emerged as adults. Among the adults, the female ratio was 48.7%. These results indicated that N. eichohorniae could be considered as a natural enemy to be used for biological control of water hyacinth.

Biological control of water hyacinth, Pontederia crassipes (C. Mart.) Solms (Pontederiaceae), in California: release and re-distribution of biological control agents 1987–2006

Water hyacinth, Pontederia crassipes (C. Mart.) Solms (Pontederiaceae), is a serious aquatic weed in lakes, ponds, and waterways in California and around the world. Three insects, the weevils Neochetina eichhorniae Warner, 1970, Neochetina bruchi Hustache, 1926 (Coleoptera: Brachyceridae), and a leaf moth Niphograpta albiguttalis (Warren, 1889) (Lepidoptera: Crambidae) were released at four sites in 1982–1987 for the biological control of water hyacinth in the Sacramento-San Joaquin River Delta. Here we report on statewide efforts to distribute the biological control agents outside the Delta and subsequent surveys to determine insect establishment. From 1987–2006, the three insects were redistributed to seven locations in five counties (Fresno, Los Angeles, Merced, Sacramento, and Santa Cruz) in California. Surveys in 2018–2020 found both Neochetina weevils at two locations in Merced County and one location in Sacramento County. No weevils were found at release locations in the upper San Joaquin River and associated waterways in western Fresno County or at the release location in Santa Cruz County. Plants at the release locations in Los Angeles County and eastern Fresno County had been eradicated and not sampled in this survey. These recoveries expand the known locations of Neochetina weevils in central California. The hyacinth moth, N. albiguttalis, was not recovered.

BIOLOGICAL CONTROL OF WATER HYACINTH

Water hyacinth (Eichhornia crassipes) is a floating aquatic weed and native of Amazon River. Water hyacinth is one of the fastest growing plants they primarily reproduce from runners or stolons. Each plant of E. crassipes can produce thousands of seeds each year and these seeds can remain viable for more than 28 years. Water hyacinth caused water loss through evapotranspiration which is more significant than indigenous weeds. Water hyacinth caused many problems in canals, ponds, lakes, rivers likes they are blocking of canals and causing floods, reduction of water quality, oxygen depletion, increased evapotranspiration rate, fish production problems, the beauty of ponds and effects on human health. Controlling methods of water hyacinth includes physical, chemical and biological but the biological method is effective and environment friendly. Neochetina bruchi, N. eichhorniae, and water hyacinth borer (Sameodes albiguttalis) are found effective biological control on water hyacinth. These weevils are feed on water hyacinth and reducing the size of water hyacinth, its vegetative propagation, and seed production. Semi-aquatic grasshopper Cornops aquaticum is also found effective control on water hyacinth.

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

ABSTRACTWater hyacinth, Pontederia crassipes (Martius) [≡Eichhornia crassipes (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.

Variation in cool temperature performance between populations of Neochetina eichhorniae (Coleoptera: Curculionidae) and implications for the biological control of water hyacinth, Eichhornia crassipes, in a temperate climate

Biological control of water hyacinth, Eichhornia crassipes, has resulted in variable outcomes in temperate regions where cool climates are thought to limit population growth and performance of the biological control agents. The weevil, Neochetina eichhorniae (Coleoptera: Curculionidae), originating from Argentina, was introduced into northern California, USA, in 1982. The realized distribution and abundance of this weevil is limited, and the exotic weed remains a problem. In this study, we tested populations of N. eichhorniae from northern California, Australia, South Africa, and Uruguay to examine the effects of low temperature on life-history performance to determine if cold hardiness differs between populations. We measured the development time, fecundity, survivorship, and thermal tolerance (chill coma: CTmin and supercooling point: SCP) of the four N. eichhorniae populations under two temperature treatments simulating fall and winter seasons of northern California. Results suggest that immature stages of all populations tested failed to survive and females did not reproduce in the winter treatment. In the fall treatment, all populations showed similar performance in most of the measured life history traits. The Australian population had the highest intrinsic rate of increase, net reproductive rate and doubling time, due to its longer oviposition period, and higher daily fecundity (2.1 ± 0.2 eggs per day), twice that of the California population (1.0 ± 0.2 eggs per day). Thus, the introduction of N. eichhorniae from Australia into northern California may increase weevil densities, distribution, and improve biological control of water hyacinth.

The contributions of biological control to reduced plant size and biomass of water hyacinth populations

Water hyacinth is invasive in many countries, where it reduces aquatic biodiversity and limits water resource utilisation. Biological control of water hyacinth has been successful in South Africa, but has suffered from a lack of empirical data to prove causation. Insect exclusion trials were conducted to quantify the contribution of Neochetina eichhorniae and N. bruchi to the integrated control of water hyacinth on the Nseleni River, South Africa. Insecticide was not expected to induce phytotoxicity, but would prevent weevil damage in water hyacinth plants; and weevil herbivory was predicted to reduce plant petiole length, and above/below surface biomass. Results showed that insecticide had no phytotoxic effects and excluded weevils for 3 weeks, providing a baseline for field applications. Biological control on the Nseleni River directly affected water hyacinth biomass and petiole length, but did not affect plant cover. Plants subject to weevil herbivory demonstrated reductions in above and below surface biomass and had shorter petioles compared to insect-free plants. Dead biomass was also higher in biological control treatments. Biological control strongly affects plant size, biomass and vigour; however, further integrated control is required to facilitate reduction in mat cover, which is the goalpost for successful control of floating aquatic plants.

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

ABSTRACTWater 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.

Naturally occurring phytopathogens enhance biological control of water hyacinth (Eichhornia crassipes) by Megamelus scutellaris (Hemiptera: Delphacidae), even in eutrophic water

Insect biological control agents directly damage target weeds by removal of plant biomass, but herbivorous insects have both direct and indirect impacts on their host plants and can also facilitate pathogen infection. Megamelus scutellaris Berg (Hemiptera: Delphacidae) was recently released into South Africa to help control invasive water hyacinth (Eichhornia crassipes, Pontederiaceae). We compared the impact of fungicide surface-sterilised and unsterilised M. scutellaris individuals and water hyacinth leaves on growth of the weed at two nutrient levels. The survival and reproduction of adult M. scutellaris was not reduced by sterilisation. Under high nutrient conditions, unsterilised M. scutellaris with unsterilised leaves reduced water hyacinth daughter plant production by 32%, length of the second petiole by 15%, chlorophyll content by 27% and wet weight biomass by 48%, while also increasing leaf chlorosis 17-fold, in relation to control plants under the same nutrient regime. Surface sterilisation of the insect and/or plant surfaces led to a general reduction in these impacts on water hyacinth growth and health. This contrast was less evident under low nutrient conditions. Megamelus scutellaris facilitated infection by fungal and other pathogens, thus its biology is compatible with pathogens that could be developed into mycoherbicides. This integrated approach may be ideal for management of infestations of water hyacinth in eutrophic water systems where control has been problematic, both in South Africa and elsewhere.

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.

First establishment of the planthopper,Megamelus scutellarisBerg, 1883 (Hemiptera: Delphacidae), released for biological control of water hyacinth in California

Water hyacinth (Eichhornia crassipes (Martius) Solms-Laubach) is a non-native, invasive floating aquatic weed in the Sacramento San Joaquin Delta and associated river watersheds of northern California. Prior releases of biological control agents have not led to sustained control. The South American planthopper, Megamelus scutellaris Berg, 1883, permitted and released first in the southeastern U.S., was released at three sites in this region from 2011 to 2013, leading to establishment at one site in a pond off Willow Creek in Folsom in the American River watershed. Planthopper populations consisting of nymphs (two-thirds or more of total counts) peaked in late summer each year between 2013 and 2015, reaching densities of six to nine planthoppers per plant by 2015. Megamelus scutellaris dispersed 50 m per year from the point of release between 2013 and 2015 and, based on degree-day estimation, were capable of producing four generations per year at the Folsom site. Proportion live leaves per plant declined by 27% in the Folsom pond between 2012 and 2015. In 2015, plants in the release pond had 40% less live above-water biomass than plants 200 meters away in a canal, into which planthoppers had dispersed in 2014–2015. This early impact of the planthopper could, however, be obscured by inter-annual and within-site variability in plant growth. This study documents the first establishment of M. scutellaris on water hyacinth in the western U.S.

Adaptability of two weevils (Neochetina bruchi and Neochetina eichhorniae) with potential to control water hyacinth in the Rift Valley of Ethiopia

Neochetina weevils have potential as biocontrol agents for water hyacinth, an aquatic weed which seriously affects irrigation water supply in sugarcane, vegetables and other horticultural crop production in the Rift Valley of Ethiopia. A study was conducted on (i) the adaptability and duration of developmental stages of Neochetina weevils and (ii) the damage they cause to water hyacinth in the Rift Valley of Ethiopia. The Rift Valley weather conditions appeared to be suitable for the two weevil species studied, enabling them to produce four generations per year. The egg hatching period of Neochetina bruchi ranged from 4 to 10 days, while Neochetina eichhorniae took 8–12 days. Larvae of N. bruchi took a comparatively shorter period (32–38 days) than N. eichhorniae (52–60 days) to complete their development. N. bruchi laid a maximum of 10.4 eggs female−1 day−1, whereas N. eichhorniae laid 8.2 eggs female−1 day−1. The intrinsic rate of increase of N. bruchi was found to be 0.06 with a generation time of 74.8 days and a population doubling period of 14.3 days. The intrinsic rate of increase of N. eichhorniae was 0.046 accompanied by a generation time of 94.8 days and a doubling period of 18.7 days. Feeding by adult weevils and tunnelling by larvae significantly impacted the vigour and reproduction of water hyacinth plants. N. bruchi and N. eichhorniae affected plants had about 72% (76%) and 66% (58%) reduction in fresh (and dry) weight, respectively. Thus, based on their damage potential and difference in rate of population growth, it is concluded that N. bruchi could be considered as a promising candidate for biological control of water hyacinth under Ethiopian conditions.

The effect of water pollution on biological control of water hyacinth

South Africa has one of the world’s biggest gold mining regions with an associated problem of acid mine drainage (AMD), which increases the bioavailability of heavy metal contaminants in water. The prevalence of water hyacinth (Eichhornia crassipes) in South African water systems, despite the release of seven biocontrol agents since 1974, is often attributed to high levels of eutrophication. Metal concentration in plant shoots is known to affect insect herbivory. Nevertheless, little is known about the effect of heavy metals or AMD on Neochetina eichhorniae and Neochetina bruchi, which are the most widely established biocontrol agents on E. crassipes in South Africa. Herein, the effect of eight different heavy metals common in AMD (arsenic (As), gold (Au), copper (Cu), iron (Fe), mercury (Hg), manganese (Mn), uranium (U) and zinc (Zn)), as well as three different simulated AMD concentration treatments (low, medium and high), on the performance of Neochetina weevils were investigated by releasing adults on plants growing in tubs and pools, three weeks after the addition of individual metal or AMD treatments. After six weeks, the number of weevil larvae per plant, the number of adult survivors per plant, the number of adult feeding scars on leaves, and the number of larval mines per plant were recorded. Two females of N. eichhorniae and N. bruchi from each tub were dissected and the number of ovariole follicles was counted. Adult feeding in Neochetina was significantly reduced on plants exposed to both Cu and As while larval feeding was significantly reduced on plants exposed to Hg, Zn, As and Cu, with Cu causing the greatest effect. Similarly, weevil feeding and reproduction were reduced in the medium and high concentration AMD treatments. Larval development was significantly impaired by both metal and AMD treatments. These negative effects disagree with published data which showed no effect of metals on Neochetina weevils. The disparity is explained by long exposure of the weevils on whole plants, rather than short exposure to excised leaves, as recorded in the literature. Finally these findings provide evidence that some heavy metals and AMD might be constraining biocontrol agents of water hyacinth in South Africa.

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.

The role of eutrophication in the biological control of water hyacinth, Eichhornia crassipes, 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.

Biological control of water hyacinth – the South African experience1

Water hyacinth remains one of the worst aquatic weeds worldwide, and its presence in South Africa since the early twentieth century prompted research into biological control options. The first control agent released in South Africa was the weevil Neochetina eichhorniae in 1974, but the project was terminated three years later, and resumed in 1985. Since then, five arthropod biocontrol agents have been released in South Africa, more than anywhere else in the world, including another weevil, N. bruchi, a moth, Niphograpta albiguttalis, a mite, Orthogalumna terebrantis and a mirid, Eccritotarsus catarinensis. To date, the success of biological control of water hyacinth in South Africa has been variable, and this has been ascribed to, amongst other factors, climatic incompatibility of control agents, and eutrophication of water hyacinth impoundments. Successful control is achieved in warm subtropical sites which are not affected by frost in winter, and in water bodies that are not heavily polluted with nitrates and phosphates. The biocontrol programme is the result of collaborative research with scientists from the USA, Australia and Argentina, and the experience obtained and lessons learned in these regions and South Africa can be used to initiate a biocontrol programme in Europe.

Impact of nutrients and herbivory by Eccritotarsus catarinensis on the biological control of water hyacinth, Eichhornia crassipes

Many water hyacinth infestations in South Africa are the symptom of eutrophication, and as a result, biological control of this weed is variable. This study examined the effects of herbivory by the mirid, Eccritotarsus catarinensis, on water hyacinth grown at high, medium and low nitrogen (N) and phosphorus (P) nutrient concentrations. Water nutrient concentration appears to be the overriding factor affecting plant growth parameters of water hyacinth plants—at high nutrient concentrations, leaf and daughter plant production were more than double than at low nutrient concentrations, while stem length was twice as great at high nutrient concentrations compared to low concentrations. Chlorophyll content was also twice as high at high nutrient concentrations than low concentrations. Conversely, flower production at high nutrient concentrations was less than half that at low concentrations. Herbivory by E. catarinensis did not have as great an effect on water hyacinth vigour as nutrient concentration did, although it significantly reduced the production of daughter plants by 23 ± 9%, the length of the second petiole by 13 ± 5%, and chlorophyll content of water hyacinth leaves by 15 ± 6%. In terms of insect numbers, mirids performed better on plants grown under medium nutrient conditions (99 ± 28 S.E.), compared to high nutrient concentrations (52 ± 27 S.E.), and low nutrient concentrations (25 ± 30 S.E.). Thus, these results suggest that the fastest and most significant reduction in water hyacinth proliferation would be reached by lowering the water nutrient concentrations, and herbivory by E. catarinensis alone is not sufficient to reduce all aspects of water hyacinth vigour, especially at very high nutrient concentrations.

Biomass and photosynthetic productivity of water hyacinth (Eichhornia crassipes) as affected by nutrient supply and mirid (Eccritotarus catarinensis) biocontrol

The biological control of water hyacinth is affected by water nitrogen and phosphorus content and this was investigated experimentally at five levels of nutrient supply by measuring plant photosynthetic and growth responses, and mirid reproduction and herbivory of nutrient treated plants. Low nitrogen (2–0.2 mg L −1 ) and phosphorus (0.2–0.01 mg L −1 ) supply decreased hyacinth photosynthesis, growth and biomass accumulation relative to plants supplied 200 mg L −1 N and 20 mg L −1 P. This effect depended more on nitrogen supply than phosphorus supply. Chlorophyll fluorescence showed that the photosynthetic light reactions of low nutrient plants were affected and leaves had decreased chlorophyll content, density of functional photosystems II and dissipated a greater proportion of absorbed energy as heat. Gas exchange parameters showed reduced carboxylation efficiency, rates of RuBP regeneration and light saturated photosynthetic rates, but not quantum yields. Effects on photosynthesis translated into lower plant dry biomass. Mirid herbivory exacerbated the effects of low nutrients noted for chlorophyll fluorescence, gas exchange parameters and biomass accumulation, however, these effects were not always significant and there was no obvious correlation between the level of nutrients supplied and the effect of mirid herbivory. Low nutrient supply did, however, affect mirid performance reducing the number of adult insects, nymphs and herbivory intensity suggesting that in the long-term mirid populations would be significantly affected by water nutrient status.

Impact of the biocontrol agent Eccritotarsus catarinensis, a sap-feeding mirid, on the competitive performance of waterhyacinth, Eichhornia crassipes

The mirid, Eccritotarsus catarinensis , was released in South Africa to aid in the biological control of waterhyacinth ( Eichhornia crassipes ). Post-release evaluations are needed to quantify the mirid’s impact on the weed in South Africa. The subtle feeding damage that it causes is not easily measured, but studies have shown that interactions with other plant stresses, e.g., plant competition, can often magnify impacts of plant-feeding insects. The impact of the mirid was therefore evaluated using an additive series analysis of competition between waterhyacinth and waterlettuce ( Pistia stratiotes ), as influenced by mirid herbivory. Competitive abilities of waterlettuce and waterhyacinth were determined using an inverse linear model with plant weight as the yield variable. In the absence of herbivory, waterhyacinth was 23 times more competitive than waterlettuce, but only 10 times more competitive when exposed to mirid feeding. Waterlettuce was only 0.9 times as aggressive as waterhyacinth that was free of herbivory, but 1.5 times as competitive when mirids were impacting waterhyacinth. Most importantly, in the presence of herbivory on waterhyacinth, interspecific competition coefficients from waterhyacinth on waterlettuce were no longer statistically significant. These results show that the mirid destabilizes waterhyacinth’s competitive interactions between these two floating plant species, although impacts were subtle. This insect is unlikely to be an effective agent by itself, but it will be a useful complement to the existing biological control agents in South Africa.