Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Manchurian Wild Rice, Coming to a Wetland near You?

Manchurian wild rice. Image - Northland Regional Council

Manchurian wild rice. Image - Northland Regional Council

Manchurian wild rice (MWR; Zizania latifolia) is a valued food crop in its native range where it grows symbiotically with a fungus (Ustilago esculenta) that causes the stem to enlarge, forming a sizable vegetable.

As its name suggests, MWR is a member of the grass family, Poaceae. The Zizania genus includes four species of wild rice. Apart from MWR, which is native to Taiwan, Eastern China and South-east Asia, the other three species are native to North America. The species most commonly harvested and eaten as grain is the annual species Z. palustris. MWR, gathered from the wild, was once an important grain in ancient China, but gradually lost importance with increasing population density and its habitat was converted for use in raising rice (Oryza sativa).

The plant was accidentally introduced into New Zealand via soil ballast from ships, and it is now considered a weed around waterways in Northland where it grows up to 4 m tall. MWR grows in fresh water, on river banks, in roadside ditches and poorly drained paddocks. It can even tolerate salt-water conditions, growing in lagoons and on tidal flats. Key infestations are around Dargaville, but the plant has recently established in Whangarei and the Far North. There are also small populations present in the Auckland, Waikato and Wellington regions. MWR is clearly a very invasive plant and one to be wary of. It can invade pastures, blocking or impeding drainage, and the rhizomes of the plant can also penetrate into and through stopbanks, opening them up and eventually destroying them. The effects of the plant are not limited to agricultural land of course, and many of the wetlands in New Zealand, which support rare biodiversity, could potentially be affected. Until now, herbicides have provided an effective control measure, but once the plant has become widespread it becomes logistically difficult to use them as a control tool. Also, herbicide use on a large scale is costly and can negatively affect waterways. MWR seed is spread by birds and spread can also occur when rhizome fragments break away, or are transported between water bodies by machinery and other human activities. Large floating mats can take root, forming new infestations.

Stan Bellgard and his colleagues recently prepared a study on the feasibility of biocontrol options for this weed. The natural enemies of wild rice and rice species – being commercially important crops – are well known. “There are a number of very promising host-specific insect agents that could be considered for biocontrol of wild rice in New Zealand,” said Simon Fowler, who has studied rice pests in Sri Lanka and India. Insect options include a stem sucker (Saccharosydne procerus), stem borer (Chilo suppressalis), and a stem and head borer (Apamea apamiformis). “There are also several pathogens that could lend themselves to biocontrol of wild rice including the smut mentioned earlier (Ustilago esculenta) and a rust fungus (Uromyces coronatus), as well as a range of stem rots that might lend themselves to inundative control options,” explained Stan.

The feasibility study also found that MWR is considered a good biocontrol target when run through the scoring system developed by Quentin Paynter. Attributes in its favour include the fact that it is not a weed its native range, and it is an aquatic plant, which have proven to be easier biocontrol targets than terrestrial weeds. Host-testing would also be relatively straightforward and cheap. Although in New Zealand we have some native, endemic and exotic plants in the Ehrhartoideae they are in a different sub-tribe (Ehrharteae) to MWR (Oryzeae), and there are unlikely to be any contentious issues of economic importance. The key question now is whether investment in developing a biocontrol programme is justified, given the relatively limited distribution of MWR currently. It is likely that biocontrol could be developed for this target for around $500,000.

This feasibility study was funded by and prepared for Northland Regional Council on behalf of the Ministry for Primary Industries.