Efficacy of diquat use on Salvinia molesta at the Te Henga wetlands

Efficacy of diquat use on Salvinia molesta at the Te Henga
wetlands, Waitakere, Auckland
Background:
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Salvina molesta (salvina) is an aquatic water weed - under the National Interest Plant
Response (NIPR) programme, the long-term goal is eradication from New Zealand.
In late 2014, salvinia was found again inthe wetlands at Te Henga (Bethells Beach),
Waitakare, Auckland. Previously salvinia had been absent after ground treatment at
Te Henga in 2011.Whether this was a re-introduction or a re-colonisation remains
unclear.
A response, in parallel with the NIPR programme, was initiated at Te Henga, with the
goal of eradication from this site.
To achieve this goal, the herbicide diquat was selected, in conjunction with physical
methods of containment, to limit the spread of salvinia.
Ground-based application:
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Ground based diquat treatment occurred on 25 March and 1 April 2015, in the
eastern area (figure 1).
Figure 1: Map showing location of diquat application – ground based application (25 March and 1 April
2015), helicopter based application (22 April 2015).
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Unmanned Aerial Vehicle (UAV) survey:
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A UAV survey of the wetlands was completed on 20 March 2015.
Sites of interest (i.e. sites suspected of harbouring salvinia, inaccessible via
conventional means) were identified for follow-up.
Survey, post ground based application:
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On 16 April 2015, an independent survey to determine the distribution of salvinia
within the Te Henga wetlands was carried out.
Findings of this survey emphasised the complex nature of salvinia from Te Henga
(figure 2).
The presence of salvina was detected at sites previously surveyed to be free.
Figure 2: Demonstration of the difficult nature of salvinia – this salvinia was located underneath willows,
near one of the western outlets.
Helicopter application:
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On 22 April 2015, helicopter based application of diquat was carried out over the
western and middle areas of the Te Henga wetlands.
Application went as planned, with ideal weather conditions, and with all areas
effectively treated.
The area flown and treated by helicopter was recorded (figure 3).
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Figure 3: Helicopter flight lines (white) and diquat application area (yellow). From on-board GPS flight
data.
Salvinia spectral signature and UAV survey:
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Spectral analysis of salvinia, to obtain a spectral signature for salvinia, was carried
out. The spectral signature of four other significant common wetland plans was also
obtained.
While obtaining the spectral signature of salvinia, in-field observations were reported,
which demonstrated activities to date had impacted on salvinia as expected (figure
4).
Due to unfavourable weather conditions, the UAV survey was unable to proceed.
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Figure 4: Sprayed salvinia amongst native Elocharis sphacelata (tall spike sedge) – as can be seen,
salvinia is killed by the diquat, whereas the sedge has regenerated from the rhizome.
Survey, post-helicopter based application:
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Over the days of 12 – 15 June 2015, an independent survey was carried out within
the Te Henga wetlands.
The primary purpose of this survey was to determine the distribution of live salvinia
within the wetlands.
The survey also assessed:
- the efficacy of diquat application to date;
- any visual signs of spray damage outside the application area (i.e. spray
drift).
Results for this survey showed the following:
- One infestation of salvinia was detected outside of the ground and helicopter
based applications (figure 5).
- Good control of most salvinia infestations has been achieved, following
ground and helicopter application of diquat (see figure 6 for an example).
- Within the area of helicopter application, salvinia protected by overhanging
vegetation will require ground based treatment (figure 7).
- Salvinia protected by other wetland plants (e.g. tall spike sedge) has been
exposed and a follow-up treatment is required (figure. 8).
- Regeneration of wetland plants from rhizomes is evident at many sites (e.g.
see figures 6 – 8).
- No evidence of spray drift damage to vegetation running parallel to the road
or other areas outside the treatment area (figure 7).
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Figure 5: Salvinia located outside area previously sprayed. The limited size enabled secure collection
and destruction of all scattered fragments at this site.
Figure 6: Eastern bay (ground sprayed). As expected, the majority of the salvinia has been killed.
(Note also the native sedge regenerating).
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Figure 7: Midway along southern shore – spot sprayed by helicopter. The majority of the salvinia has been
killed. Salvinia protected by overhanging vegetation is now exposed. In the background, healthy NZ flax can
be observed, adjacent to the application zone, confirming an absence of spray drift.
Figure 8: Dense salvinia in foreground protected by the native sedge (western area of clearwater) – now
exposed after helicopter application of diquat. The salvinia in the background was not protected by
vegetation, demonstrating visually the efficacy of diquat at Te Henga.
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Summary:
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Ground and helicopter treatment of salvinia, using diquat, is effective at reducing the
salvinia infestation at Te Henga.
Follow-up ground and helicopter operations will be required for the eradication of
salvinia at Te Henga. This is due to a number of factors, including:
- Overhanging vegetation (especially on the south shore), protecting salvinia
during helicopter treatment
- Native vegetation (e.g. tall spike sedge, raupo) sheltering salvinia –
knockback of this vegetation has exposed salvinia for upcoming treatments.
- Treatment of tertiary (mat-forming) salvinia will not get a 100% kill rate on first
application, but a high kill level is evident as expected.
- New infestations may be located during subsequent surveys.
Regeneration of native vegetation is evident post-treatment across multiple sites
within the wetlands.
With no permanent structures (e.g. rhizomes), no regeneration of salvinia is evident.
However, salvinia that survives may re-infest the wetlands.
A comparison of the survey results is presented in figure 9, this displays survey
points where salvinia was present at the time of the survey. As can be seen, the total
sites where salvinia is present has reduced overall, in particular in the western area
where helicopter treatment took place on 22 April 2015. Note, the points only indicate
presence of salvinia, not actual quantity and to not incorporate the outlying areas
where we are dependent on aerial surveys for assessment.
(Photos in this report are courtesy of Contractors employed by MPI)
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Figure 9: Comparison of survey results – Red points: Feb. 2015, Yellow points: Jun. 2015
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