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Individual study: Herbicide treatment to control curly water-thyme Lagarosiphon major, an invasive waterweed, in Lake Wanaka, Otago, New Zealand

Published source details

Clayton J. & Franklyn G. (2005) Assessment of the December 2004 Lagarosiphon major control programme in Lake Wanaka. NIWA Client Report: HAM2005-046 report.

Summary

Curly water-thyme Lagarosiphon major, a waterweed native to South Africa, is now present in many ponds and lakes throughout New Zealand where it is considered invasive. Since becoming established in Lake Wanaka (New Zealand's fourth largest lake) 30 years ago, its accelerated spread around the lake has become the cause of concern both for amenity value (the lake is a popular tourist destination), and native flora and fauna alike.

A Lagarosiphon management control programme was therefore prepared by the Lake Wanaka Managers' Committee, and two methods of control were decided upon, application of diquat herbicide (described here) and suction dredging (see Case 262).

Study area: In response to invasion by curly water-thyme Lagarosiphon major, a management control programme was undertaken in Lake Wanaka (South Island), New Zealand in December 2004.

Baseline surveys: Several baseline sites were selected in November 2004, a month prior to commencement of Lagarosiphon control measures. This was done to provide an objective description of representative sites selected for treatment by diquat (or suction dredging - see Case 262). These sites remained confidential to the National Institute of Water & Atmospheric Research (the agency undertaking the assessment of the control success) so that the contractors could not focus their efforts on these areas. An assessment of the Lagarosiphon control effectivness was carried out in the third week of April 2005 by scuba diving the fixed profiles recorded prior to treatment, using an underwater video camera.

Herbicide application: Application of the 'aquatic' herbicide diquat (formulated with Aquagel® at manufacturers recommended rates) was undertaken by helicopter (7 December 2004) and boat (14 December 2004). Favourable weather conditions prevailed at the time of treatment. Areas treated by helicopter were Paddock Bay (approx. 20 ha), Parkins Bay (8.5 ha), Quartz Creek (2 ha), Stevensons Island (0.03 ha), the head of Stevensons Arm (1 ha) and West Wanaka Bay (0.7 ha). Areas treated from boat were Bishop's Bay Bluff to Paddock Bay narrows (0.15 ha), Quartz Creek to the neck of Stevensons Arm (excluding Isthmus) (0.5 ha) and Colquhouns to West Wanaka Bay (0.1 ha).

Effects of diquat treatment on Lagarosiphon: In the third week of April 2005, an assessment of the effectiveness of the attempted control was carried out at the baseline sites. Results were variable.

The most effective outcome was within Paddock and Parkins Bays where the most expansive areas of Lagarosiphon were present and which contained around 80% of the total waterweed sprayed in the lake. Diquat worked well in these relatively enclosed bays, most of the weed having collapsed onto the lake bed. Much of the upper stem sections and leaves had decomposed leaving only more resistant 'woody' stem material. However, at the time of the inspection (four months after treatment), plant recovery was widespread with many new shoots emerging from the defoliated stems lying on the sediment. Recovery was variable with new shoot growth commonly between 0.2 and 0.5 m long. Without further treatment full recovery was predicted within one year.

Although the diquat treatment did not kill the Lagarosiphon, without it the waterweed beds would have reached the water surface (as in previous years), resulting in fragmentation due to boating activities, wildlife and wave action, leading to further dispersal of fragments out of these bays.

In contrast to Paddock and Parkins Bays, all other diquat treated areas had minimal to no discernable effect on Lagarosiphon. This was put down to inadequate contact time, a common problem when treating less extensive areas of waterweed, especially when lying alongside large areas of open water, where gradients are steep or where colonies are at an early stage of invasion. Other factors may have also contributed to lack of success, e.g. detritus accumulation on leaf surfaces preventing the diquat from being absorbed by the plant. However, even in patches where Lagarosiphon shoots were clean and had no detritus or algae coating the leaf surfaces, there was little or no damage.

The viscosity of Aquagel® used and application procedures therefore need to be evaluated to maximise contact time of diquat in target treatment areas, especially smaller ones.

Effects of diquat on other water plants: The diquat treatment in the two bays was very effective in control of Canadian pondweed Elodea canadensis (another introduced waterweed but less invasive than Lagarosiphon). In Lake Wanaka, Lagarosiphon out-competes Elodea, but Elodea commonly grows in a band below Lagarosiphon (which will not grow in water deeper than about 6.5 m). Elodea is much more susceptible to diquat than Lagarosiphon. The affected Elodea beds (previously 1 - 1.5 m tall at depths from c. 5 - 7 m) were very much reduced but the first stages of recovery were apparent four months after treatment and, as with Lagarosiphon, full recovery was expected within a year.

Both Paddock and Parkins Bays support substantial native plant communities growing in water too deep for Lagarosiphon and Elodea to persist. These native plants consist mostly of stoneworts or charophytes (several species of Chara and Nitella). These charophyte beds (ecologically important in clear water lakes such as Wanaka) were not impacted by diquat, even when directly exposed within treatment areas.

Follow-up treatment: A re-spray with diquat four months after the intial treatment (while recovering shoot growth is young, soft, clean, green and particularly susceptible to diquat) was recommended in order to kill vegetative buds that were re-growing from the collapsed woody stems.


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