Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Booting Aquatics with Biocontrol

New Zealand has a serious problem with invasive alien aquatic weeds, including rooted, submerged macrophytes such as lagarosiphon (Lagarosiphon major), hornwort (Ceratophyllum demersum) and Brazilian waterweed (Egeria densa).

Aquatic and wetland weeds are one of the worst groups of invasive species because they negatively affect the quality, quantity and accessibility of irrigation and potable water, and they are easily spread by water birds and downstream flows to new, uninvaded areas. They are typically characterised by rapid vegetative growth and the ability to easily regenerate via fragmentation (the production of new plants from small broken segments) and dormant vegetative organs such as tubers or turions. Aquatic and wetland weeds critically threaten the unique and sensitive biota associated with these ecosystems, and they negatively affect farming and hydro-electric operations and recreational water use, usually with high costs of control. In freshwater systems, the negative impacts of these weeds are often enhanced by other drivers, such as nutrient enrichment, which further degrade these systems and the ecosystem services they provide.

While several control methods are routinely used to help manage aquatic and wetland weeds, they are costly, particularly once a weed has established and eradication is not possible. They are usually non-selective and can lead to other unwanted, non-target impacts. For example, manual and mechanical control causes plant fragmentation, which can worsen the problem by establishing new infestations in previously uninvaded areas. If misused, chemical control using herbicides can be toxic to non-target native fauna and flora, and the rapid decay of large amounts of plant biomass can lead to anoxic (low oxygen) conditions in the water, or algal blooms from the release of plant nutrients.

For this reason, biological control, which has the advantage of being selective, self-sustaining and of low environmental risk, is a viable option for helping to managing aquatic plant problems. “Biocontrol of floating, emergent and submerged aquatic weeds has been studied and applied worldwide since the 1960s, often with great success,” said Angela Bownes. “With the expanding threat of aquatic and wetland weeds to our freshwater ecosystems, we need another weapon in our arsenal to help reduce the negative ecological and economic impacts of these weeds. We see this is an important new focus area for weed biocontrol in New Zealand,” said Angela.

Shoot-mining midge (adult left, lava right) and damage (centre)

Lagarosiphon, which is widely distributed throughout the North and South Islands and is considered a major pest by stakeholders managing New Zealand lakes, was identified as the most promising first target. Lagarosiphon, or oxygen weed, is native to South Africa and is also highly invasive in several parts of Europe. Biocontrol options for lagarosiphon were well studied in Ireland in the early 2010s, with two candidate biocontrol agents – a leaf-mining fly (Hydrellia lagarosiphon) and a shoot-mining midge (Polypedilum tuburcinatum) – prioritised based on their narrow host range and their damage potential. Host specificity testing of the leaf-mining fly was opportunistically done while collaborators from the University of Dublin were working on the fly. Their research included key native aquatic plants in New Zealand and showed that the fly is sufficiently host specific for release here.

The host range of the shoot-mining midge in a New Zealand context is currently being researched on our behalf by Ben Miller from the Centre for Biological Control (CBC) at Rhodes University in South Africa. Aquatic plant species in the Hydrocharitaceae and native aquatic plants in the order Alismatales will be tested in South Africa to assess the midge’s host range and safety for release.

However, before embarking on any new application to release a weed biocontrol agent, it is important to conduct pre-release research to guide the programme and maximise the chances of success. “This is essential baseline research to ensure the candidate agents are not already present on the target weed, and to assess the risk of parasitism of the agents, which could affect their establishment and efficacy,” said Quentin Paynter who conducted a feasibility study on biocontrol options for lagarosiphon in New Zealand.

“Equally important for this particular programme is to conduct further underpinning research to assess whether biological control is likely to be an effective management approach for lagarosiphon. Biocontrol of a similar submerged aquatic macrophyte, hydrilla (Hydrilla verticillata) in the USA had variable success, so we need to assess quantitative scientific data to help us predict an outcome for a biocontrol programme against lagarosiphon,” said Quentin.

This will be done by comparing plant biomass and growth rates of lagarosiphon in the native range of South Africa, where the candidate biocontrol agents are present, and in New Zealand, where lagarosiphon (to our knowledge) does not have any significant natural enemies. “This will test the hypothesis that lagarosiphon is a serious weed here because of a lack of natural enemies to keep it in check, and that this can be reversed using biocontrol,” said Quentin.  This work will be done in collaboration with aquatic weed experts at the CBC in South Africa and is planned to commence this coming summer.

Although there is much work to be done prior to proceeding with an application to release any biocontrol agents for lagarosiphon, early indications are that it will be a valuable tool to help manage the weed. Angela explains that “the fact that we don’t have any native plants in the same plant family (Hydrocharitaceae) as lagarosiphon greatly improves our chances of finding host-specific natural enemies, and since the invasive biotype in New Zealand is dioecious (having male and female flowers on separate plants), and we only have female plants, there is no seed bank to contribute to reinvasion.  Also, since lagarosiphon typically occurs in the high-lying areas of South Africa, the agents should be cold-tolerant enough to thrive in the New Zealand climate.

Although biocontrol can be a highly effective control method, it is not expected to cause rapid, large-scale declines of lagarosiphon populations that could jeopardise the integrity of our aquatic ecosystems. “In fact, we expect that an integrated approach, using different control methods at different times and locations, will be necessary to provide a long-term, environmentally sound solution to the management of lagarosiphon and other aquatic and wetland weeds in New Zealand,” said Angela.

This project is currently supported with funding from the Ministry of Business, Innovation and Employment as part of Manaaki Whenua – Landcare Research’s Beating Weeds programme.


Angela Bownes -