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The Contributions of Biological Control to Reduced Plant Size and Biomass of Water Hyacinth Populations

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The Contributions of Biological Control to Reduced Plant Size and Biomass of Water Hyacinth Populations Hydrobiologia (2018) 807:377–388 https://doi.org/10.1007/s10750-017-3413-y PRIMARY RESEARCH PAPER The contributions of biological control to reduced plant size and biomass of water hyacinth populations Roy W. Jones . Jaclyn M. Hill . Julie A. Coetzee . Martin P. Hill Received: 15 June 2017 / Revised: 22 September 2017 / Accepted: 7 October 2017 / Published online: 20 October 2017 Ó Springer International Publishing AG 2017 Abstract Water hyacinth is invasive in many coun- plant cover. Plants subject to weevil herbivory tries, where it reduces aquatic biodiversity and limits demonstrated reductions in above and below surface water resource utilisation. Biological control of water biomass and had shorter petioles compared to insect- hyacinth has been successful in South Africa, but has free plants. Dead biomass was also higher in biological suffered from a lack of empirical data to prove control treatments. Biological control strongly affects causation. Insect exclusion trials were conducted to plant size, biomass and vigour; however, further quantify the contribution of Neochetina eichhorniae integrated control is required to facilitate reduction and N. bruchi to the integrated control of water in mat cover, which is the goalpost for successful hyacinth on the Nseleni River, South Africa. Insecti- control of floating aquatic plants. cide was not expected to induce phytotoxicity, but would prevent weevil damage in water hyacinth Keywords Eichhornia crassipes Á Weevil plants; and weevil herbivory was predicted to reduce herbivory Á Petioles Á Neochetina spp. Á Plant fitness Á plant petiole length, and above/below surface bio- Invasion mass. Results showed that insecticide had no phyto- toxic effects and excluded weevils for 3 weeks, providing a baseline for field applications. Biological control on the Nseleni River directly affected water Introduction hyacinth biomass and petiole length, but did not affect Eichhornia crassipes (Martius) Solms-Laubach (Pont- ederiaceae) (water hyacinth), is a free-floating inva- Handling editor: Andrew Dzialowski sive aquatic macrophyte of South American origin R. W. Jones Á J. M. Hill (&) Á M. P. Hill (Edwards & Musil, 1975; Barrett & Forno, 1982), that Department of Zoology and Entomology, Rhodes has been introduced to and become invasive in many University, PO Box 94, Grahamstown 6140, South Africa countries around the world, where it reduces aquatic e-mail: [email protected] biodiversity and limits water resource utilisation J. A. Coetzee (Center, 1994; Midgley et al., 2006; Villamagna & Department of Botany, Rhodes University, Murphy, 2010; Coetzee et al., 2014; Getsinger et al., PO Box 94, Grahamstown 6140, South Africa 2014). Water hyacinth’s ability to reproduce both sexually and asexually, along with its rapid reproduc- R. W. Jones Ezemvelo KZN Wildlife, PO Box 10416, Meerensee, tion rate, leads to the formation of dense interlocking Richards Bay 3901, South Africa mats with complex root structures (Villamagna & 123 378 Hydrobiologia (2018) 807:377–388 Murphy, 2010). While it is limited to freshwaters, it by the continual cloudy weather during the El Nin˜o flourishes in systems with high nutrient loading (Heard event of 1997/1998. & Winterton, 2000; Xie et al., 2004; Villamagna & In rebuttal, Wilson et al. (2007) suggested that Murphy, 2010; Coetzee & Hill, 2012) and through while much of the evidence points to classical efficient nutrient utilisation, successfully outcompetes biological control as the major factor, the El Nin˜o native flora for space and sunlight (Cilliers, 1991). associated weather pattern of the last quarter of 1997 Globally, it is highly pervasive in the southeastern and the first half of 1998 confused the issue. Wilson USA, Southeast Asia, Central America and central, et al. (2007) argued that the reductions in water western and southern Africa (Te´llez et al., 2008; hyacinth on Lake Victoria were ultimately caused by Villamagna & Murphy, 2010) and although the the widespread and significant damage to plants by historical pathways of invasion are not clear, its Neochetina spp., although the increased wind and spread is attributed predominately to human actions wave action associated with El Nin˜o would have (Telle´z et al., 2008; Villamagna & Murphy, 2010). increased the mortality of insect-stressed plants. Although mechanical and chemical methods are Furthermore, there is mounting evidence that abiotic available for the control of water hyacinth, classical factors such as climate, extreme weather events and biological control has been applied in multiple coun- local environmental conditions play a substantial role tries around the world as a more sustainable option in the success or failure of biological control pro- (Julien & Griffiths, 1998), with varying degrees of grammes (see Cuda et al., 2008 for a comprehensive reported success (Hill & Cilliers, 1999; Jimenez et al., review). This example illustrates how the lack of 2001; Spencer & Ksander, 2004; Center & Dray, sound empirical data in the field undermines the 2010). The first record of water hyacinth in South scientific acceptance and validity of biological Africa was in 1900 (Jacot Guillarmod, 1979) and it is control. currently the most widespread aquatic plant invader in Quantifiable follow-up field evaluations and empir- the country (Hill & Coetzee, 2017). The control ical tests are thus imperative to improve the science programme in South Africa however, has largely supporting biological control (Cuda et al., 2008; relied on the use of biological control (Coetzee & Hill, Morin et al., 2009). By necessity, successful post- 2011). This programme, which started in the 1970s, release evaluations should have a long-term monitor- has reduced water hyacinth populations at a number of ing focus, identifying underlying causes of success or sites around the country (Coetzee & Hill, 2012). failure, which will promote the development of better Funding for post-release monitoring in biological management strategies (Cuda et al., 2008; Morin et al., control programmes has been limited, with funds 2009). Standardised procedures and objective evalu- largely restricted to screening, introduction and ations of success are needed to confirm the establish- establishment (Cuda et al., 2008; Morin et al., 2009). ment of agents and establish their effects on the target As a result, historically, weed biological control has weed and associated ecological community (Cuda suffered from a lack of empirical data to prove et al., 2008), which is particularly challenging at a causation of impact, relying on before and after landscape scale. One method of evaluating the efficacy scenarios (Ehler, 1976; Thomas & Reid, 2007; Carson of weed biological control agents is through insect et al., 2008; Cuda et al. 2008; Hajek et al., 2016). This exclusion experiments. While there has been some approach has been regularly criticised (e.g. Thomas & research carried out on insecticide exclusion trials Reid, 2007; Carson et al., 2008;Mu¨ller-Scha¨rer & (Tipping & Center, 2002; Tipping et al., 2008), this Schaffner, 2008) and most recently so in Africa has mainly been on terrestrial weeds. regarding the biological control programme on water In this study, we use the biological control hyacinth on Lake Victoria. Williams et al. (2005) programme on the Nseleni River in the KwaZulu- suggest that the Neochetina spp. weevils alone were Natal province of South Africa as a case study. Water not responsible for the rapid reduction in weed hyacinth was first recorded on the river in the early biomass on Lake Victoria, as the rapid decrease in 1970s and by 1982 there was a 16 km plug of the weed water hyacinth abundance in only 2 years was extending from the confluence of the Nseleni and extraordinarily fast. They suggested the decline was Mposa Rivers down to Lake Nsezi. Ad hoc control more likely due to decreased light availability caused efforts, which comprised aerial and boat side herbicide 123 Hydrobiologia (2018) 807:377–388 379 spraying at peak infestation levels, to manage water contributions of biological control to this management hyacinth populations, were practiced on the Nseleni plan have been hard to quantify and possibly under River between the late 1970s and 1994 (Jones, 2009) estimated. and in 1984 heavy floods alleviated the problem, as Therefore, the aim of this study was to investigate most of the water hyacinth was washed away. the following hypotheses (1) Actara SC insecticide Thereafter, little was done to the remaining island application will not have phytotoxic effects on water populations of water hyacinth, because the decreased hyacinth plants, but will be effective at preventing level of infestation was no longer considered a threat weevil damage, and (2) herbivory by Neochetina spp. (Jones, 2001). By 1995 however, the water hyacinth weevils will reduce petiole length, as well as above infestation had returned, with between 40 and 100% and below surface biomass of water hyacinth plants cover across the entire river. The response to this due to physical damage. These hypotheses were infestation in 1995 was an integrated control manage- investigated through a series of insect exclusion trials ment programme which included widespread chemi- to quantify the contribution of biological control to the cal application and biological control. Steel cables integrated control of water hyacinth on the Nseleni were erected across the river to prevent water hyacinth River system. mats being blown upstream and to accumulate mats at the cables, making
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