About the Author(s)


Martin P. Hill Email symbol
Department of Zoology and Entomology, Rhodes University, South Africa

Julie Coetzee symbol
Department of Botany, Rhodes University, South Africa

Citation


Hill, M.P. & Coetzee, J., 2017, ‘The biological control of aquatic weeds in South Africa: Current status and future challenges’, Bothalia 47(2), a2152. https://doi.org/10.4102/abc.v47i2.2152

Note: This paper was initially delivered at the 43rd Annual Research Symposium on the Management of Biological Invasions in South Africa, Goudini Spa, Western Cape, South Africa on 18-20 May 2016.

Original Research

The biological control of aquatic weeds in South Africa: Current status and future challenges

Martin P. Hill, Julie Coetzee

Received: 15 Aug. 2016; Accepted: 06 Dec. 2016; Published: 31 Mar. 2017

Copyright: © 2017. The Author(s). Licensee: AOSIS.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background: Aquatic ecosystems in South Africa are prone to invasion by several invasive alien aquatic weeds, most notably, Eichhornia crassipes (Mart.) Solms-Laub. (Pontederiaceae) (water hyacinth); Pistia stratiotes L. (Araceae) (water lettuce); Salvinia molesta D.S. Mitch. (Salviniaceae) (salvinia); Myriophyllum aquaticum (Vell. Conc.) Verd. (parrot’s feather); and Azolla filiculoides Lam. (Azollaceae) (red water fern).

Objective: We review the biological control programme on waterweeds in South Africa.

Results: Our review shows significant reductions in the extent of invasions, and a return on biodiversity and socio-economic benefits through the use of this method. These studies provide justification for the control of widespread and emerging freshwater invasive alien aquatic weeds in South Africa.

Conclusions: The long-term management of alien aquatic vegetation relies on the correct implementation of biological control for those species already in the country and the prevention of other species entering South Africa.

Introduction

Aquatic ecosystems in South Africa have been prone to invasion by introduced macrophytes since the late 1800s, when water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub. (Pontederiaceae), was first recorded as naturalised in KwaZulu-Natal (Cilliers 1991). Several other species of freshwater aquatic plants, all notorious weeds in other parts of the world, have also become invasive in many of the rivers, man-made impoundments, lakes and wetlands of South Africa (Hill 2003). These are Pistia stratiotes L. (Araceae) (water lettuce); Salvinia molesta D.S. Mitch. (Salviniaceae) (salvinia); Myriophyllum aquaticum (Vell. Conc.) Verd. (parrot’s feather); and Azolla filiculoides Lam. (Azollaceae) (red water fern) (Hill 2003), which along with water hyacinth comprise the ‘Big Bad Five’ (Henderson & Cilliers 2002). Recently, new invasive aquatic plant species have been recorded which are still at their early stages of invasion, including the submerged species, Egeria densa Planch. (Hydrocharitaceae) (Brazilian water weed) and Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae); the emergent species, Sagittaria platyphylla (Engelm.) J.G.Sm. and S. latifolia Willd. (Alismataceae); Lythrum salicaria L. (Lythraceae) (purple loosestrife), Nasturtium officinale W.T. Aiton. (Brassicaceae) (watercress); Iris pseudacorus L. (Iridaceae) (yellow flag); and Hydrocleys nymphoides (Humb. & Bonpl. ex Willd.) Buchenau (Alismataceae) (water poppy); and the new floating weeds, Salvinia minima Baker (Salviniaceae) and Azolla cristata Kaulf. (Azollaceae) (Mexican azolla); and the rooted floating Nymphaea mexicana Zucc. (Nymphaeceae) (Mexican water lily) (Coetzee et al. 2011a; Coetzee, Bownes & Martin 2011b). The mode of introduction of these species is mainly through the horticultural and aquarium trade (Martin & Coetzee 2011), and two issues contribute to the invasiveness of these macrophytes following establishment: the lack of co-evolved natural enemies in their adventive range (McFadyen 1998); and disturbance, the presence of nitrate- and phosphate-enriched waters, associated with urban, agricultural and industrial pollution that promotes plant growth (Coetzee & Hill 2012).

Aquatic weeds in South Africa are found throughout the country including the winter rainfall areas of the western part of the country, the more subtropical eastern parts and the cool, temperate areas of the Highveld plateau (Henderson 2001). Although the alteration of hydrological flows in South African river systems through the construction of impoundment walls, gauging weirs, culverts and low-water bridges where constant slow-flowing waters have facilitated population build-up and thus problems caused by aquatic weeds (Hill & Olckers 2001), infestations are also found in unimpacted habitats, such as A. filiculoides infestations in wetlands in the southern Free State and I. pesudocorus in wetlands of the Cape Peninsula. Here, we review the current status of aquatic weeds in South Africa, their socio-economic and environmental impacts and the benefits of their control.

Drivers of invasive aquatic plant invasions

It is important to understand the invasion biology of an organism, if effective control measures are to be implemented. Several authors (e.g. Bauer 2012; MacDougall & Turkington 2005) have grouped invasive alien species into three broad categories viz. (1) passengers, which are solely dependent on a disturbance for establishment and proliferation, and if the disturbance is removed, the invasion and associated impacts cease; (2) drivers of biodiversity loss, which include species that do not need any disturbance to establish; and (3) back-seat drivers whereby an initial disturbance is required for an invasive alien plant species to establish, but once established, even if the disturbance is removed, the invasion continues. Aquatic weed invasions in South Africa are examples of back-seat drivers. These invasive species rely on the broad ecosystem disturbance of slow-flowing permanent waters caused by impoundments and eutrophication which facilitates establishment and, linked with enemy release, allows them to proliferate, thereby gaining a competitive advantage over indigenous aquatic plants (Coetzee & Hill 2012). The resulting large continuous mats significantly impact all aspects of aquatic biodiversity and ecosystem functioning (see below).

Impacts of aquatic invasive plant invasions

Aquatic weeds cause various environmental (or ecological) and socio-economic impacts (which are in their majority negative), affecting floral and faunal diversity and ecosystem functioning and services. The impact mechanisms and effects of aquatic weeds differ between species, which is largely based on differences in their growth form and the habitat that they have invaded. We applied the generic impact scoring system (GISS) presented by Nentwig et al. (2016) to assess the impacts of eight water weed species in South Africa before and after biological control. This is not the intended use of GISS, which was designed to prioritise invasive alien species for control, but does allow a comparison and ranking of the impacts of water weeds in South Africa and an assessment of the success of the biological control programmes. GISS relies on published evidence and comprises 12 impact categories divided evenly between environmental and socio-economic impacts. Under the environmental impacts, the effect that the invasive alien species has on native fauna and flora, either directly or through competition, disease transmission, hybridisation and the ecosystem services, is rated. Under the socio-economic categories, the impact that the invasive alien species has on agriculture, forestry, infrastructure, human health and social well-being is scored. A six-level scoring system is applied (Nentwig et al. 2016):

0 –no data available, no impacts known, not detectable or not applicable

1 –minor impacts, only locally, only on common species and negligible economic loss

2 –minor impacts, more widespread, also on rarer species and minor economic loss

3 –medium impacts, large-scale, several species concerned, relevant decline, relevant ecosystem modifications and medium economic loss

4 –major impact with high damage, major changes in ecosystem functions, decrease of species and major economic loss

5 –major large-scale impact with high damage and complete destruction, threat to species including local extinctions and high economic costs

Table 1 presents an analysis of the socio-economic and environmental impacts caused by eight of the most invasive and well-studied species of aquatic weeds established in South Africa. The list of weed species chosen for this study was not exhaustive, but represents three of the main habits, free-floating species, emergent species and submerged species. Scoring systems have their flaws, in that they are constrained by time and locality, but provide valuable benchmarks. We, therefore, scored aquatic weeds in South Africa based on both the worst-case scenario and the current status (i.e. before and after biological control for the weeds that have biocontrol programmes), thereby presenting a measure of the value of biological control. The scores were based on published studies using South African data captured in review papers or chapters, except for S. platyphylla, E. densa and H. verticillata, where little data exist on their impacts in South Africa, and thus, we relied on data from elsewhere in the world (e.g. Adair et al. 2012; Langeland 1996; Yarrow et al. 2009). Where the impact was unknown, largely because it was unstudied, the impact was assigned a neutral score of zero. Further, confidence limits were based on Nentwig et al. (2016) where 1 = low confidence (no empirical data or literature to support the impact score), 2 = medium confidence (no empirical data from South Africa, but literature from elsewhere to support the impact score) and 3 = high confidence (empirical and published data from South Africa support the impact score) (Appendix 1).

TABLE 1: The impact scores with level of confidence per impact categories of the GISS (Nentwig et al. 2016) for eight water weeds in South Africa, presenting the worst-case scenario in the absence of any biological control, and the current situation in South Africa, post biological control, where applicable.

This analysis shows that of the floating macrophytes, E. crassipes had the biggest impact on aquatic ecosystems in South Africa, followed by A. filiculoides, P. stratioes and S. molesta. Although based on literature, this result is supported by annual field surveys throughout South Africa and is probably because of the fact that water hyacinth is the largest of the macrophytes that warrant control, it is the most widespread, we have studied its impacts (e.g. Coetzee, Jones & Hill 2014; Fraser, Martin & Hill 2016; Midgley, Hill & Villet 2006) and it has historically been the most difficult of the water weeds to control (e.g. Coetzee et al. 2011a; Hill 2003; Hill & Cilliers 1999). Although not considered to be under complete biological control (Klein 2011), the ecological and socio-economic impact of the weed has been significantly reduced through the introduction of eight biological control agents (Coetzee et al. 2011a; Paterson et al. 2016). On the contrary, the impacts of A. filiculoides on South African freshwater systems were quantified by Ashton and Walmsley (1984) and McConnachie et al. (2003). Based on this evidence, this weed achieved a score of 40 on the GISS, just below E. crassipes, and was considered more damaging than either P. stratiotes or S. molesta in the absence of biological control. Following the introduction of the highly successful agent, Stenopelmus rufinasus Gyllenhal (Coleoptera: Curculionidae), A. filiculoides no longer poses a threat to aquatic ecosystems of the country (Hill & McConnachie 2009; McConnachie et al. 2003; McConnachie, Hill & Byrne 2004); indeed, we could not find a single negative impact of this weed in this country and thus scores 0. Furthermore, biological control has significantly reduced the impact scores of P. stratiotes, S. molesta and M. aquaticum, highlighting the ecological and economic benefits of biological control.

Interestingly, the two submerged species analysed, E. densa and H. verticillata, recorded the two of the highest impact scores. This is largely because of their fairly recent invasion status in South Africa, and thus, we relied heavily on the published literature. Although H. verticillata is only confined to one site in South Africa (Coetzee et al. 2009b), and its impact at this site has not been quantified, its impact in the United States suggests that it should be given a very high priority in terms of impact and thus the need for control (Balciunas et al. 2002; Langeland 1996). The emergent species, S. platyphylla, scored the lowest in comparison with the other macrophytes possibly because it is a new invader still in the lag phase, not yet dominating the riparian zone, and is also not yet considered a major weed elsewhere in the world.

Impact of water weeds on biodiversity loss

Although the socio-economic impacts of water weeds have been fairly well reported (reviewed in Villamagna & Murphy 2010), there are very few specific examples that have documented their impacts on biodiversity. Below we present two case studies of the direct impact of water hyacinth on aquatic biodiversity in South Africa.

Case study 1: Midgley et al. (2006)

In this first case study, the benthic invertebrate community and algal biomass were sampled under water hyacinth mats and in water hyacinth-free water over a 13-month period, using artificial substrates in New Year’s Dam, Eastern Cape Province, a cool temperate region of the country. The number of families and the number of individuals per substrate were significantly lower under the mats. Further, measures of biodiversity, including Shannon-Weiner diversity index, Margalef’s richness index, Pielou’s evenness index and chlorophyll a, were all significantly lower under water hyacinth mats than in water hyacinth-free zones, demonstrating the impact of water hyacinth on benthic biodiversity.

Case study 2: Coetzee et al. (2014)

Although similar to the previous study, this study aimed to determine whether the presence of water hyacinth altered the diversity and assemblage structure of benthic macroinvertebrates in a conservation area in a subtropical region of the country, the Nseleni Nature Reserve near Richard’s Bay. The benthic macroinvertebrate assemblage was sampled over 1 year at five sites under water hyacinth mats and at five sites without water hyacinth in the Nseleni River. Once again, artificial substrates were placed beneath water hyacinth mats or in the open water to allow for colonisation by freshwater macroinvertebrates, and left for a period of 6 weeks, repeated on seven occasions over 10 months. Twenty-nine families comprising 18 797 individuals were collected, 817 (13 families) individuals were from under water hyacinth mat sites compared with 17 980 (27 families) individuals from open water sites. However, 98% of individuals collected were the invasive snail, Tarebia granifera L. (Thiaridae). This study again highlights that the presence of water hyacinth has a significantly negative impact on aquatic macroinvertebrate biodiversity, but in a conservation area.

Control of aquatic invasive plant invasions

In South Africa, water weeds have been controlled through the use of mechanical and manual removal, herbicide application and biological control. Although manual removal using rakes and pitchforks can be successful, it is labour intensive. Although one of the pillars of the Working for Water Programme of the Natural Resources Management Programmes of the Department of Environmental Affairs is job creation through alien plant removal, this method is really ineffective for water weeds and this work force is better used on controlling terrestrial weeds in South Africa. Manual removal of submerged aquatic species such as H. verticillata and E. densa invariably leads to fragmentation of the weed mat and subsequent dispersal and increased infestation of the weed (Dayan & Netherland 2005).

Herbicidal control, using formulations containing the active ingredient glyphosate, is still used to control water hyacinth in some of the larger dams and river systems in South Africa. Herbicidal control of water hyacinth depends on skilled operators who maintain a long-term follow-up programme continually to control re-infestation from scattered plants and those germinating from seed. Therefore, any herbicide programme against the weed requires a commitment to an ongoing operation of unlimited duration. It is the lack of a follow-up regime that has often led to the failure of herbicidal control programmes (Hill & Olckers 2001). Although herbicide application is often used as part of an integrated management approach (Hill & Coetzee 2008), Hill, Coetzee and Ueckermann (2012) showed that a number of herbicide formulations used in South Africa were toxic to some of the biological control agents that have been released against this weed.

The biological control programme against water weeds in South Africa was initiated in 1973 and the weevil, Neochetina eichhorniae Warner, was released in 1974 (Cilliers 1991). Since that time, 13 agent species (11 insects, one mite and one pathogen) have been released against five weeds (Table 2). The biological control programme against water weeds in South Africa has been highly successful with four of the five weeds targeted (water lettuce, salvinia, parrot’s feather and azolla) considered to be under complete control whereby no other control methods are required to keep the weed populations at a level where they no longer impact the aquatic biodiversity and water utilisation (see above) (Coetzee et al. 2011a). Although water hyacinth is not considered to be under complete biological control, in some areas biological control has controlled the weed, whereas in other areas it has reduced populations and impact such that alternative control methods such as herbicide applications are required far less frequently (Coetzee et al. 2011a; Hill & Cilliers 1999).

TABLE 2: Biological control agents released for the control of freshwater alien aquatic weed species in South Africa (after Klein 2011).

The biological control programme on water weeds in South Africa is co-ordinated through Rhodes University in collaboration with University of the Witwatersrand and the Plant Protection Research Institute of the Agricultural Research Council. This programme comprises about 7 research staff, 14 support and technical staff, and 12 postgraduate students and postdoctoral fellows. The activities carried out by this research group include pre-release studies on new agents for several species, including S. platyphylla, I. pseudocorus and E. densa, and qualitative post-release evaluation studies on all of the weeds on which agents have been released. The most significant aspect of the post-release evaluation studies is an annual country-wide survey of all water weed sites (~450 infested water bodies) assessing weed and agent populations. These surveys that have been carried out since 2008 provide the guidance for the water weed biological control programme and a measure of success or failure.

Results of the surveys show that since 2008, there has been a substantial increase in the number of recorded invaded sites, but more importantly, the percentage of these sites where the respective biocontrol agents are present has increased significantly because of enhanced efforts to release agents from mass rearing centres (Figure 1). This has led to an increase in the control of the weeds and a reduction in their ecological and environmental impacts (Table 1). Another unintended benefit of these surveys is that they have served as an ideal early detection platform for additional freshwater invasive macrophytes. For example, since 2008, the number of locality records for E. densa, S. platyphylla and I. pseudacorus has increased (from 0 to 14, 16 and 14 sites, respectively) to the point that these species are no longer considered eradication targets (Wilson et al. 2013). All three species are now targets for biological control. On the contrary, H. verticillata remains confined to one system, Jozini Dam (KZN).

FIGURE 1: Results of the first (2008) and most recent (2015) nationwide surveys on aquatic weeds in South Africa.

Some new developments arising from these field surveys since the 2011 review paper (Coetzee et al. 2011a) are presented below. Mass rearing and implementation also forms an important part of the research programme as in many areas of the country that are prone to cold winters and eutrophic waters, classical biological control is not as effective as an augmentive programme whereby high numbers of healthy agents are released at the onset of summer when field populations of the agents are low. Part of the mass rearing programme involves the employment of people living with disabilities (Weaver et al. 2016).

The biological control programme against A. filiculoides in South Africa using the Azolla specialist S. rufinasus has been highly successful (McConnachie et al. 2003, 2004). However, field surveys showed that the agent utilised another Azolla species, thought to be the native Azolla pinnata subsp. africana (Desv.) Baker, which contradicted the host specificity trials (Hill 1998). However, molecular analysis showed that what we thought was the native species, A. pinnata subsp. africana, was a new invasive species, A. cristata Kaulfuss, a close relative of A. filiculoides (Madeira et al. 2016). Field surveys have shown that S. rufinasus is capable of establishing populations on A. cristata in the warmer, eastern part of the country and will likely result in this plant never becoming highly invasive.

Most of the biological control research on water weeds is centred around water hyacinth, and the plant hopper, Megamelus scutellaris Berg (Hemiptera: Delphacidae), is the most recent agent to have been released in 2013. This agent has now established and is impacting the plant in the cooler areas of the country where the other agents have traditionally struggled to establish and have an effect (Coetzee, Byrne & Hill 2007). Recent molecular work has revealed that two separate populations of the mirid, Eccritotarsus catarinensis Carvahlo (Hemiptera: Miridae), collected from Brazil (collected in 1994) and Peru (collected in 1999), respectively, are in fact cryptic species (Paterson et al. 2016). Fortunately, these populations were kept separate and both subjected to impact and host specificity testing. However, this finding does show that the importation of multiple consignments of the same species for biological control should be conducted with caution.

Benefits of biological control of water hyacinth

Weed biological control has traditionally suffered from a lack of quantitative post-release evaluation studies that show economic or ecological benefit. Where the benefits of a biological control programme have been measured, it has focussed on economic benefits (e.g. Van Wilgen et al. 2004). For aquatic weeds, McConnachie et al. (2003) quantified the benefits of the biological control programme against red water fern using the weevil, S. rufinasus Gyllenhal, in South Africa and showed that the agent removed the impact of the weed on water supply, stock health and recreational activities (see above). Further, De Groote et al. (2003) demonstrated that the successful biological control of water hyacinth in southern Benin significantly increased the yearly income of the population of this region through increased crop and fish production. Also, Van Wyk and Van Wilgen (2002) compared the costs and benefits of three control interventions for E. crassipes and showed that biological control, along with integrated control, offered the best return on investment.

New threats to the aquatic environment

Coetzee et al. (2011b) highlighted the significance that the delays in promulgating appropriate legislation against a suite of new aquatic invaders could have in allowing their unmitigated establishment and spread in South African water bodies. In 2014, however, the promulgation of the National Environmental Management: Biodiversity Act (10/2004) (NEM:BA) and the publication of the Alien and Invasive Species List in 2014 resulted in the listing of 10 Category 1a aquatic plant species, including H. verticillata, I. pseudacorus and S. platyphylla; 16 Category 1b aquatic plant species, including A. cristata and S. minima; and one Category 2 aquatic plant species. This legislation will provide much needed impetus to curb the spread and impacts of this suite of invaders in South Africa. As these five species are no longer considered targets for eradication (Coetzee et al. 2011a, 2011b; Jaca & Mkhize 2015), biological control programmes have been initiated against these species and are currently at various stages of development: from surveying for potential natural enemies, in the case of I. pseudacorus; screening for host specificity in quarantine, in the case of S. platyphylla and S. minima; pending release of a suitable agent in the case of H. verticillata; to assessing the impact of an agent already released against A. filiculoides that has subsequently been found on A. cristata (Table 2).

Despite the NEM:BA legislation, there are a number of additional unlisted aquatic plant species whose introduction and establishment must be prevented at all costs. The role that pet traders, aquarists, boating enthusiasts and fishermen play in the spread of invasive aquatic species has been highlighted from around the world (Cohen et al. 2007; Maki & Galatowitsch 2004; Padilla & Williams 2004) and is a significant channel for the introduction and spread of aquatic plants throughout South Africa too (Martin & Coetzee 2011). Species such as Cabomba caroliniana Gray (Cabombaceae), Alternanthera philoxeroides Griseb. (Amaranthaceae) and Stratiotes aloides L. (Hydrocharitaceae) are widespread invaders elsewhere in the world (e.g. Julien et al. 2012; Schooler, Cabrera-Walsh & Julien 2009; Thiebaut 2007) and pose a threat to South African waterways, should they be introduced. Awareness and publicity programmes on potential new threats could go a long way in preventing their introduction and trade, as well as improved phytosanitory efforts and border control.

Discussion

Hill and Olckers (2001) critiqued the biological control programme on water hyacinth in South Africa. Although their emphasis was water hyacinth, the points made in that paper are pertinent to all invasive alien water weeds in South Africa. Hill and Olckers stated that there were four issues that mitigated against the sustainable biological control of water hyacinth; the injudicious use of herbicides that was antagonistic to the biological control agents; the cold winters in the temperate regions of the country that was deleterious to the build-up of agent populations; eutrophic waters that allowed the weeds to compensate for herbivory; and the fact that many of the systems infested by these weeds were small and lacked the necessary wind fetch to break up mats of agent infested weed. In the 15 years since the publication of Hill and Olckers, a considerable amount of research has been undertaken to better understand these four issues (summarised and reviewed in Byrne et al. 2010; Coetzee et al. 2011a, 2011b; Coetzee & Hill 2012). The implementation of this research has resulted in the release of additional agents that are better adapted to the diversity of habitats in South Africa (e.g. M. scutellaris which is able to establish on water hyacinth in cooler regions), and an emphasis on inundative releases of high numbers of agents at appropriate times of year (e.g. in spring and after herbicide application). This has been made possible through the construction of three mass-rearing facilities (City of Cape Town, Rhodes University and the South African Sugar Research Institute) that produce agents on demand.

The biological control programme against water weeds in South Africa has been highly successful, as measured by an increase in the number of sites under biological control, coupled with a significant reduction in the percentage cover of these weeds and a recovery of ecosystem services. However, unless the primary driver of disturbance (i.e. eutrophication by nitrates and phosphates) in aquatic ecosystems is addressed, we anticipate, rather than control, a succession of invasions by a suite of water weeds (Coetzee et al. 2011a, 2011b). Although we have shown that biological control has played a significant role in the recovery of aquatic biodiversity, these biodiversity benefits will be short-lived in impacted ecosystems unless an integrated catchment management approach is adopted which addresses eutrophication.

Acknowledgements

This research was funded through the Department of Environmental Affairs, Natural Resource Management Programme’s Working for Water programme. Further funding for this work was provided by the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation of South Africa. Any opinion, finding, conclusion or recommendation expressed in this material is that of the authors, and the National Research Foundation does not accept any liability in this regard.

Competing interests

The authors declare that they stand to gain no financial benefit from the publication of this article.

Authors’ contributions

J.C. and M.P.H. contributed equally to the conception, analysis and writing of the article.

References

Adair, R.A., Keener, B.R., Kwong R.M., Sagliocco, J.L. & Flower, G.E., 2012, ‘The Biology of Australian weeds, 60. Sagittaria platyphylla (Engelmann) J.G. Smith and Sagittaria calycina Engelmann’, Plant Protection Quarterly 27(2), 47–58.

Ashton, P.J. & Walmsley, R.D., 1984, ‘The taxonomy and distribution of Azolla species in southern Africa’, Botanical Journal of the Linnean Society 89, 239–247. https://doi.org/10.1111/j.1095-8339.1984.tb02198.x

Balciunas, J.K., Grodowitz, M.J., Cofrancesco, A.F. & Shearer, J.F., 2002, ‘Hydrilla’, in R. Van Driesch, B. Blossey, M. Hoddle, S. Lyon & R. Reardon (eds.), Biological control of invasive plants in the Eastern United States, pp. 91–114, USDA Forest Service, Morgantown, WV.

Bauer, J., 2012, ‘Invasive species: “Back-seat drivers” of ecosystem change?’, Biological Invasions 14, 1295–1304. https://doi.org/10.1007/s10530-011-0165-x

Byrne, M.J., Hill, M.P., Robertson, M., King, A., Jadhav, A., Katembo, N. et al., 2010, Integrated Management of Water Hyacinth in South Africa: Development of an integrated management plan for water hyacinth control, combining biological control, herbicidal control and nutrient control, tailored to the climatic regions of South Africa, Report to the Water Research Commission. WRC Report No. No TT 454/10, Water Research Commission, Pretoria, South Africa.

Cabrera Walsh, G., Magal Dalto, Y., Mattioli, F.M., Carruthers, R.I. & Anderson, L.W., 2013, ‘Biology and ecology of Brazilian elodea (Egeria densa) and its specific herbivore, Hydrellia sp., in Argentina’, Biocontrol 58, 133–147. https://doi.org/10.1007/s10526-012-9475-x

Center, T.D., Hill, M.P., Cordo, H. & Julien, M.H., 2002, ‘Waterhyacinth’, in R. G. van Driesche, S. Lyon, B. Blossey, M. S. Hoddle & R. Reardon (eds.), Biological control of invasive plants in the Eastern United States, pp. 41–64, USDA Forest Service, Morgantown, WV.

Chapman, M. & Dore, D., 2009, Sagittaria strategic plan 2009, Unpublished report to Goulburn Murray Water, Boulburn Broken Catchment Management Authority and the tri-state Sagittaria taskforce, RuralPlan Pty Ltd, Goomalibee, VIC.

Cilliers, C.J., 1991, ‘Biological control of water hyacinth, Eichhornia crassipes in South Africa’, Agriculture, Ecosystems and Environment 37, 207–217. https://doi.org/10.1016/0167-8809(91)90149-R

Cilliers, C.J., 1999, ‘Biological control of parrot’s feather, Myriophyllum aquaticum (Vell.) Verdc. (Haloragaceae), in South Africa’, in T. Olckers & M.P. Hill (eds.), Biological control of weeds in South Africa (1990–1998), African Entomology Memoir 1, pp. 113–118.

Cilliers, C.J., Hill, M.P., Ogwang, J.A. & Ajuonu, O., 2003, ‘Aquatic weeds in Africa and their control’, in P. Neuenschwander, C. Borgemeister & J. Langewald (eds.), Biological control in IPM systems in Africa, pp. 161–178, CAB International, Wallingford, CT.

Coetzee, J.A., Bownes, A. & Martin, G.D., 2011b, ‘Prospects for the biological control of submerged macrophytes in South Africa’, African Entomology 19(2), 469–487. https://doi.org/10.4001/003.019.0203

Coetzee, J.A., Byrne, M.J. & Hill, M.P., 2007, ‘Predicting the distribution of Eccritotarsus catarinensis, a natural enemy released on water hyacinth in South Africa’, Entomologia Experimentalis et Applicata 125, 237–247. https://doi.org/10.1111/j.1570-7458.2007.00622.x

Coetzee, J.A., Center, T.D., Byrne, M.J. & Hill, M.P., 2005, ‘Impact of the biological control agent Eccritotarsus catarinesis, a sap-feeding mirid, on the competitive performance of water hyacinth, Eichhornia crassipes’, Biological Control 32, 90–96. https://doi.org/10.1016/j.biocontrol.2004.08.001

Coetzee, J.A. & Hill, M.P., 2012, ‘The role of eutrophication in the biological control of water hyacinth, Eichhornia crassipes, in South Africa’, BioControl 57, 247–261. https://doi.org/10.1007/s10526-011-9426-y

Coetzee, J.A., Hill, M.P., Byrne, M.J. & Bownes, A., 2011a, ‘A review of the biological control programmes on Eichhornia crassipes (C. Mart.) Solms (Pontederiacaeae), Salvinia molesta D.S. Mitch. (Salviniaceae), Pistia stratiotes L. (Araceae), Myriophyllum aquaticum (Vell.) Verdc. (Haloragaceae) and Azolla filiculoides Lam. (Azollaceae) in South Africa’, African Entomology 19(2), 451–468. https://doi.org/10.4001/003.019.0202

Coetzee, J.A., Hill, M.P., Julien, M.H., Center, T.D. & Cordo, H.A., 2009b, ‘Eichhornia crassipes (Mart.). Solms-Laub. (Pontederiaceae)’, in R. Muniappan, G.V.P. Reddy & A. Raman (eds.), Biological control of tropical weeds using arthropods, pp.183–210, Cambridge University Press, Cambridge, UK.

Coetzee, J.A., Jones, R.W. & Hill, M.P., 2014, ‘Water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub. (Pontederiaceae), reduces benthic macroinvertebrate diversity in a protected subtropical lake in South Africa’, Biodiversity and Conservation 23, 1319–1330. https://doi.org/10.1007/s10531-014-0667-9

Coetzee, J.A., Schlange, D. & Hill, M.P., 2009a, ‘Potential spread of the invasive plant Hydrilla verticillata in South Africa based on anthropogenic spread and climate suitability’, Biological Invasions 11, 801–812. https://doi.org/10.1007/s10530-008-9294-2

Cohen, J., Mirotchnick, N. & Leung, B., 2007, ‘Thousands introduced annually: The aquarium pathway for non-indigenous plants to the St. Lawrence Seaway’, Frontiers in Ecology and the Environment 5, 528–532. https://doi.org/10.1890/060137

Dayan, F.E. & Netherland, M.D., 2005, ‘Hydrilla, the perfect aquatic weed, becomes more noxious than ever’, Outlooks on Pest Management 16, 277–282. https://doi.org/10.1564/16dec11

De Groote, H., Ajuonu, O., Attignon, S., Djessou, R. & Neuenschwander, P., 2003, ‘Economic impact of biological control of water hyacinth in Southern Benin’, Ecological Economics 45, 105–117. https://doi.org/10.1016/S0921-8009(03)00006-5

Diop, O., Coetzee, J.A. & Hill, M.P., 2010, ‘Impact of different densities of Neohydronomus affinis (Coleoptera: Cuculionidae) on Pistia stratiotes (Araceae) under laboratory conditions’, African Journal of Aquatic Science 32, 267–271. https://doi.org/10.2989/16085914.2010.538505

Doeleman, J.A., 1989, Biological control of Salvinia molesta in Sri Lanka: An assessment of costs and benefits, Australian Centre for International Agricultural Research, Technical Report 12, 14 pp, Union Offset, Canberra, Australia.

Fraser, G., Martin, J. & Hill, M.P., 2016, ‘Economic evaluation of water loss saving due to the biological control of water hyacinth at New Year’s Dam, Eastern Cape Province, South Africa’, African Journal of Aquatic Science 41(2), 227–234. https://doi.org/10.2989/16085914.2016.1151765

Henderson, L., 2001, Alien weeds and invasive plants: A complete guide to declared weeds and invaders in South Africa, Plant protection research institute handbook No. 12, Agricultural Research Council, Pretoria, South Africa.

Henderson, L. & Cilliers, C.J., 2002, Invasive aquatic plants, Plant protection research institute handbook no. 16, pp. 1–88, Agricultural Research Council, Pretoria, South Africa.

Hill, M.P., 1998, ‘Life history and laboratory host range of Stenopelmus rufinasus, a natural enemy for Azolla filiculoides in South-Africa’, BioControl 43, 215–224. https://doi.org/10.1023/A:1009903704275

Hill, M.P., 2003, ‘The impact and control of alien aquatic vegetation in South African aquatic ecosystems’, African Journal of Aquatic Science 28, 19–24. https://doi.org/10.2989/16085914.2003.9626595

Hill, M.P. & Cilliers, C.J., 1999, ‘A review of the arthropod natural enemies, and factors that influence their efficacy, in the biological control of water hyacinth, Eichhornia crassipes (Mart.) Solms-Laubach (Pontederiaceae), in South Africa’, in T. Olckers & M.P. Hill (eds.), Biological control of weeds in South Africa (1990–1998), African Entomology Memoir 1, 103–112.

Hill, M.P. & Coetzee J.A., 2008, ‘Integrated control of water hyacinth (Eichhornia crassipes) in Africa’, EPPO Bulletin/Bulletin OEPP 38(3), 452–457. https://doi.org/10.1111/j.1365-2338.2008.01263.x

Hill, M.P., Coetzee, J.A. & Ueckermann, C., 2012, ‘Toxic effect of herbicides used for water hyacinth control on two insects released for its biological control in South Africa’, Biocontrol Science and Technology 22, 1321–1333. https://doi.org/10.1080/09583157.2012.725825

Hill, M.P. & McConnachie, A.J., 2009, ‘Azolla filiculoides Lamarck (Azollaceae)’, in R. Muniappan, G.V.P. Reddy & A. Raman (eds.), Biological control of tropical weeds using arthropods, pp. 74–87, Cambridge University Press, Cambridge, UK.

Hill, M.P. & Olckers, T., 2001, ‘Biological control initiatives against water hyacinth in South Africa: Constraining factors, success and new courses of action’, in M.H. Julien, M.P. Hill, T.D. Center & D. Jianqing (eds.), Biological and integrated control of water hyacinth, Eichhornia crassipes, ACIAR Proceedings, pp. 33–38, Australian Centre for International Agricultural Research, Canberra.

Jaca, T. & Mkhize, V., 2015 ‘Distribution of Iris pseudacorus (Linnaeus, 1753) in South Africa’, BioInvasions Records 4, 249–253. https://doi.org/10.3391/bir.2015.4.4.03

Julien, M., Sosa, A., Chan, R., Schooler, S. & Traversa, G., 2012, ‘Alternanthera philoxeroides (Martius) Grisebach – Alligator weed’, in M. Julien, R. McFadyen & J. Cullen (eds.), Biological control of weeds in Australia, pp. 43–51, CSIRO Publishing, Melbourne.

Julien, M.H., Hill, M.P. & Tipping, P.W., 2009, ‘Salvinia molesta D. S. Mitchell (Salviniaceae)’, in R. Muniappan, G.V.P. Reddy & A. Raman (eds.), Biological control of tropical weeds using arthropods, pp. 378–407, Cambridge University Press, Cambridge, UK.

Klein, H., 2011, ‘A catalogue of the insects, mites and pathogens that have been used or rejected, or are under consideration, for the biological control of invasive alien plants in South Africa’, African Entomology 19, 515–549. https://doi.org/10.4001/003.019.0214

Lambertini, C., Riis, T., Olesen, B., Clayton, J.S., Sorrell, B.K. & Brix, H., 2010, ‘Genetic diversity in three invasive clonal aquatic species in New Zealand’, BMC Genetics 11, 52–70. https://doi.org/10.1186/1471-2156-11-52

Langa, S.D.F., 2013, ‘Impact and control of waterweeds in the Southern Mozambique Basin rivers’, PhD thesis, Rhodes University, 283 pp.

Langeland, K.A., 1996, ‘Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), “the perfect aquatic weed”’, Castanea 61, 293–304.

MacDougall, A.S. & Turkington, R., 2005, ‘Are invasive species the drivers or passengers of change in degraded ecosystems?’, Ecology 86, 42–55. https://doi.org/10.1890/04-0669

Madeira, P.T., Center, T.D., Coetzee, J.A., Pemberton, R.W., Purcell, M.A. & Hill, M.P., 2013, ‘Identity and origins of introduced and native Azolla species in Florida’, Aquatic Botany 111, 9–15. https://doi.org/10.1016/j.aquabot.2013.07.009

Madeira, P.T., Hill, M.P., Dray, F.A., Jr., Coetzee, J.A., Paterson, I.D. & Tipping, P.W., 2016, ‘Molecular identification of Azolla invasions in Africa: The Azolla specialist, Stenopelmus rufinasus proves to be an excellent taxonomist’, South African Journal of Botany 105, 299–305. https://doi.org/10.1016/j.sajb.2016.03.007

Mailu, A.M., 2001, ‘Preliminary assessment of the social, economic and environmental impacts of water hyacinth in the Lake Victoria Basin and the status of control’, in M.H. Julien, M.P. Hill, T.D. Center & D. Jianqing (eds.), Biological and integrated control of water hyacinth, Eichhornia crassipes, ACIAR Proceedings, pp. 130–139, Australian Centre for International Agricultural Research, Canberra.

Maki, K. & Galatowitsch, S., 2004, ‘Movement of invasive aquatic plants into Minnesota through the horticultural trade’, Biological Conservation 118, 389–396. https://doi.org/10.1016/j.biocon.2003.09.015

Martin, G.D. & Coetzee, J.A., 2011, ‘Fresh water aquatic plant invasion risks posed by the aquarium trade, aquarists and the internet trade in South Africa’, Water SA 37, 371–380.

McConnachie, A.J., de Wit, M.P., Hill, M.P. & Byrne, M.J., 2003, ‘Economic evaluation of the successful biological control of Azolla filiculoides in South Africa’, Biological Control 28, 25–32. https://doi.org/10.1016/S1049-9644(03)00056-2

McConnachie, A.J., Hill, M.P. & Byrne, M.J., 2004, ‘Field assessment of a frond-feeding weevil, a successful biological control agent of red water fern, Azolla filiculoides, in southern Africa’, Biological Control 29, 326–331. https://doi.org/10.1016/j.biocontrol.2003.08.010

McFadyen, R.E., 1998, ‘Biological control of weeds’, Annual Review of Entomology 43, 369–393. https://doi.org/10.1146/annurev.ento.43.1.369

Midgley, J.M., Hill, M.P. & Villet, M.H., 2006, ‘The effect of water hyacinth, Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae), on benthic biodiversity in two impoundments on the New Year’s River, South Africa’, African Journal of Aquatic Science 31(1), 25–30. https://doi.org/10.2989/16085910609503868

Moore, G.R. & Hill, M.P., 2012, ‘A quantitative post-release evaluation of biological control of water lettuce, Pistia stratiotes L. (Araceae) by the weevil Neohydronomus affinis Hustache (Coleoptera: Curculionidae) at Cape Recife Nature Reserve, Eastern Cape Province, South Africa’, African Entomology 20(2), 380–385. https://doi.org/10.4001/003.020.0217

Morris, M.J., Wood, A.R. & den Breeÿen, A., 1999, ‘Plant pathogens and biological control of weeds in South Africa: A review of projects and progress during the last decade’, in T. Olckers & M.P. Hill (eds.), Biological control of weeds in South Africa (1990–1998), African Entomology Memoir 1, 129–137.

Nentwig, W., Bacher, S., Pyšek, P., Vilà, M. & Kumschick, S., 2016, ‘The generic impact scoring system (GISS): A standardization tool to quantify the impacts of alien species’, Environmental Monitoring and Assessment 188, 315. https://doi.org/10.1007/s10661-016-5321-4

Neuenschwander, P., Julien, M.H., Center, T.D. & Hill, M.P., 2009, ‘Pistia stratiotes L. (Araceae)’, in R. Muniappan, G.V.P. Reddy & A. Raman (eds.), Biological control of tropical weeds using arthropods, pp. 332–352, Cambridge University Press, Cambridge, UK.

Padilla, D.K. & Williams, S.L., 2004, ‘Beyond ballast water: Aquarium and ornamental trades as sources of invasive species in aquatic ecosystems’, Frontiers in Ecology and the Environment 2, 131–138, https://doi.org/10.1890/1540-9295(2004)002[0131:BBWAAO]2.0.CO;2

Paterson, I., Mangan, R., Downie, D., Coetzee, J., Hill, M., Burke, A. et al., 2016, ‘Two in one: Cryptic species discovered in biological control agent populations using molecular data and crossbreeding experiments’, Ecology and Evolution 6, 6139–6150. https://doi.org/10.1002/ece3.2297

Pieterse, A.H., 1981, ‘Hydrilla verticillata – A review’, Abstracts on Tropical Agriculture 7, 9–34.

Schooler, S., Cabrera-Walsh, W. & Julien, M., 2009, ‘Cabomba caroliniana Gray (Cabombaceae)’, in R. Muniappan, G.V.P. Reddy & A. Raman (eds.), Biological control of tropical weeds using arthropods, pp. 88–107, Cambridge University Press, Cambridge, UK.

Souza, W.T.Z., 2011, ‘Hydrilla verticillata (Hydrocharitaceae), a recent invader threatening Brazil’s freshwater environments: A review of the extent of the problem’, Hydrobiologia 669, 1–20. https://doi.org/10.1007/s10750-011-0696-2

Thiebaut, G., 2007, ‘Non-indigenous aquatic and semiaquatic plant species in France’, in F. Gherardi (ed.), Biological invaders in inland waters: Profiles, distribution, and threats, vol. 2, pp. 209–229, Invading Nature – Springer Series in Invasion Ecology, Berlin.

Thomas, P.A. & Room, P.M., 1986, ‘Taxonomy and control of Salvinia molesta’, Nature 320(6063), 581–584.

Van Driesche, R.G., Carruthers, R.I., Center, T., Hoddle, M.S., Hough-Goldstein, J., Morin, L. et al., 2010, ‘Classical biological control for the protection of natural ecosystems’, Biological Control 54, S2–S33. https://doi.org/10.1016/j.biocontrol.2010.03.003

Van Wilgen, B.W., De Wit, M.P., Anderson, H.J., Le Maitre, D.C, Kotze, I.M., Ndala, S. et al., 2004, ‘Costs and benefits of biological control of invasive alien plants: Case studies from South Africa: Working for Water’, South African Journal of Science 100, 113–122.

Van Wyk, E. & Van Wilgen, B.W., 2002, ‘The cost of water hyacinth control in South Africa: A case study of three options’, African Journal of Aquatic Science 27, 141–149. https://doi.org/10.2989/16085914.2002.9626585

Villamagna, A.M. &. Murphy, B.R., 2010, ‘Ecological and socio-economic impacts of invasive water hyacinth (Eichhornia crassipes): A review’, Freshwater Biology 55, 282–298. https://doi.org/10.1111/j.1365-2427.2009.02294.x

Weaver, K., Hill, M.P., Hill, J., Coetzee, J.A., Paterson, I.D. & Martin, G.D., 2016, ‘Project and practice narrative: Using insects to bridge the gap between science and the community’, Journal for New Generation Sciences (JNGS).

Wilson, J.R.U., Ivey, P., Manyama, P. & Nänni, I., 2013, ‘A new national unit for invasive species detection, assessment and eradication planning’, South African Journal of Science 109(5/6), 1–13. https://doi.org/10.1590/sajs.2013/20120111

Yarrow, M., Marin, V.H., Finlayson, M., Tironi, A., Delgado, L.E. & Fischer, F., 2009, ‘The ecology of Egeria densa Planchon (Liliopsida: alismatales): A wetland ecosystem engineer?’, Revista Chilena de Historia Natural 82, 299–313. https://doi.org/10.4067/S0716-078X2009000200010

Appendix 1


TABLE 1-A1: Summary of GISS detailed impact levels (Nentwig et al. 2016), for eight freshwater invasive alien aquatic weeds, in South Africa. Scoring was conducted in both the absence of biological control (BC) (worst-case scenario – no BC) and the presence of biological control (current status – BC), where applicable. References are included. Where there is a paucity on impact data and no implementation of biological control to date in South Africa, impacts realised elsewhere have been considered (i.e. for Sagittaria platyphylla, Egeria densa and Hydrilla verticillata).
TABLE 1-A1 (Continues…): Summary of GISS detailed impact levels (Nentwig et al. 2016), for eight freshwater invasive alien aquatic weeds, in South Africa. Scoring was conducted in both the absence of biological control (BC) (worst-case scenario – no BC) and the presence of biological control (current status – BC), where applicable. References are included. Where there is a paucity on impact data and no implementation of biological control to date in South Africa, impacts realised elsewhere have been considered (i.e. for Sagittaria platyphylla, Egeria densa and Hydrilla verticillata).
TABLE 1-A1 (Continues…): Summary of GISS detailed impact levels (Nentwig et al. 2016), for eight freshwater invasive alien aquatic weeds, in South Africa. Scoring was conducted in both the absence of biological control (BC) (worst-case scenario – no BC) and the presence of biological control (current status – BC), where applicable. References are included. Where there is a paucity on impact data and no implementation of biological control to date in South Africa, impacts realised elsewhere have been considered (i.e. for Sagittaria platyphylla, Egeria densa and Hydrilla verticillata).
TABLE 1-A1 (Continues…): Summary of GISS detailed impact levels (Nentwig et al. 2016), for eight freshwater invasive alien aquatic weeds, in South Africa. Scoring was conducted in both the absence of biological control (BC) (worst-case scenario – no BC) and the presence of biological control (current status – BC), where applicable. References are included. Where there is a paucity on impact data and no implementation of biological control to date in South Africa, impacts realised elsewhere have been considered (i.e. for Sagittaria platyphylla, Egeria densa and Hydrilla verticillata).

 

Crossref Citations

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