Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Key Hurdle Cleared for Alligator Weed Project

<em>Alternanthera nahui</em>, no longer a major impediment to developing biocontrol for alligator weed.

Alternanthera nahui, no longer a major impediment to developing biocontrol for alligator weed.

The only aquatic weed we have attempted to biocontrol in New Zealand is alligator weed (Alternanthera philoxeroides).

The alligator weed beetle (Agasicles hygrophila) and moth (Arcola malloi) provide good control of the weed on static water bodies in warmer parts of the country. However, they are not able to control terrestrial infestations of the weed, or aquatic infestations that are regularly flooded or situated in cooler parts of the country. So in the past 5 years we have been keen to find additional biocontrol agents that could potentially fill these key gaps, and have supported an Australian programme to screen for new agents. Unfortunately the results obtained by CSIRO were disappointing. All six potential insect agents they tested were able to feed and develop on other Alternanthera species. Because Australia has native Alternanthera species this has ruled out any plans to further consider these insects for release on that side of the Tasman. However, the situation was not quite so clear-cut for New Zealand.

In the past, it would not have been a problem if the other Alternanthera species in New Zealand (A. denticulata, A. pungens and A. sessilis) were attacked, as they were all considered to be exotic introductions. No one disputes that A. pungens is exotic, and many believed that A. denticulata might have been a recent arrival. But things got tricky when a recent paper re-described New Zealand A. sessilis as an endemic species A. nahui. The chances of making a successful case to release a biocontrol agent in New Zealand that could potentially harm an endemic plant were fairly slim, leaving us with only two potential avenues to follow. The first was to attempt to develop more sophisticated testing methods to show that while some attack was possible, any attack on non-targets in the field would be insignificant; and the second was to further investigate the status of A. nahui in New Zealand. Since the molecular data supporting the revision paper was limited, and Australian material in the Allan Herbarium at Lincoln appeared to resemble A. nahui, we decided to have a crack at both.

The taxonomy of Alternanthera is quite confused, which is not helped by the fact that the conditions plants are growing in can have a big impact on what they look like. We have confirmed that molecular methods are the only way to accurately identify some Alternanthera plants. Luckily our plant population geneticist, Gary Houliston, was quickly able to identify material in Australia that is identical to A. nahui here, and it appears we have a subset of the genetic material present in Australia. “This suggests that A. nahui is a recent introduction to New Zealand,” confirmed Gary. We also got a similar result for A. denticulata, clearing the way for a potential application for additional agents for alligator weed to be released in New Zealand in due course.

So what might be in this application? Of the six insects studied in recent times we have identified the two which look the most promising: a stem/root galling fly (Ophiomyia marellii) that attacks the nodes, and a foliage-feeding beetle (Systena nitentula). We have imported a shipment of flies from Australia and are attempting now to establish a rearing colony, and we will look at importing the beetle from its native range in Argentina when funds permit. We still hope to gain better information about what these insects might do in the field if released in New Zealand, but some damage to A. denticulata and A. nahui is no-longer likely to be a fatal impediment to this project.

This project is funded by the National Biocontrol Collective and the Ministry of Business, Innovation and Employment, under the Beating Weeds Programme. We thank everyone who sent in various Alternanthera specimens for our molecular studies, especially John Hosking (Department of Primary Industries, New South Wales) and Jo Palmer (CSIRO). We also acknowledge Shon Schooler and Richard Chan (CSIRO) for all their efforts seeking new insect agents.