It’s one of our most ambitious weed biocontrol projects ever.
Lagarosiphon is a rooted, perennial, submerged aquatic plant native to southern Africa that has invaded other regions of the world, most notably Ireland and New Zealand, but also several other European countries and Australia. Commonly known as oxygen weed, lagarosiphon was spread around the world as an aquarium plant. Perversely, large infestations of the weed cause local deoxygenation of water through changing water chemistry.
Outside of its native range, lagarosiphon spreads via broken stem fragments which produce roots, giving rise to new plants and new infestations. In New Zealand, it forms dense and extensive underwater mats in lakes and slow-flowing rivers throughout the country, outcompeting native aquatic plant species, disrupting water flows, increasing the risk of flooding, interfering with recreational activities such as fishing and boating, and reducing the aesthetic value of lakes. Lagarosiphon is also a significant pest in the South Island’s hydro lakes, clogging intakes and blocking power generation.
Currently available control methods include herbicides (mainly Diquat), mechanical and suction dredging, and the application of weed matting to shade out the plant. However, these are costly, time-consuming, labour intensive, potentially have adverse environmental impacts and only provide a short-term solution.
Biocontrol offers the potential for sustained long-term management of lagarosiphon with low environmental risk. The Hydrellia leaf-mining fly proposed for release is multivoltine, which means it has multiple generations per year, with many overlapping generations throughout the warmer months. But worldwide, biocontrol of submerged aquatic plants has only been attempted three times.
Will it work? Research done in conjunction with Manaaki Whenua by Dr Nompumelelo Baso, from Rhodes University in South Africa, suggest that it will.
Her work showed that there is no leaf-mining fly in New Zealand associated with submerged aquatic plants, which strongly suggests that the risk of parasitism of the lagarosiphon leaf-mining fly is very low. Without specialist parasitoids and predators, the leaf-mining fly could reach high population densities that could bring about a decline in lagarosiphon.
Degree-day modelling predicted that the leaf-mining fly could complete between 4.5 and 9.3 generations per year in New Zealand, depending on local climate, again suggesting that the fly could establish well and sustain viable populations in most parts of the country invaded by lagarosiphon.
Nompumelelo also tested whether female flies could deposit eggs on artificial substrates in cases where lagarosiphon shoot tips don’t reach the water surface. The answer was yes – the fly larvae would hatch and then seek the submerged plant.
Is it safe for our native plants? Host range testing of the leaf-mining fly was completed in Ireland in the early 2010s and included representatives of New Zealand’s most closely related native aquatic flora – so we know that the fly won’t attack them instead.
We now have all the information and data required to build a strong case for release approval for the leaf-mining fly for lagarosiphon. However, before submitting the application to the EPA, work is needed to consult and engage with stakeholders and Māori, and to conduct a full assessment of the economic, environmental and social risks, costs and benefits.
But as we know from other very successful and highly cost-effective weed biocontrol projects we have run – good things are well worth waiting for.
