The evaluation of aerially applied triclopyr mixtures for the control of dense infestations of wilding Pinus contorta in New Zealand

Exotic conifer species were planted for erosion control, research, shelter, landscaping
and production forests from the late 1880s onwards (Ledgard 2001]) throughout New Zealand. Most of these conifers are primary colonisers and have naturally
regenerated from these plantings. These conifers, known as wildings, have spread extensively
since then, and the total area in which wilding conifers occur in the South Island
of New Zealand is estimated to be in excess of 500,000 ha (Raal and Gous 2010]). Wilding conifers occur within more than 200,000 hectares of land administered by
the Department of Conservation (DOC), of which approximately two thirds is invaded
by Pinus contorta (Dougl.) (Ledgard 2001]).

Wilding conifers are controlled either by mechanical (brush cutters and chainsaws),
physical (hand pulling and slashing) or chemical (herbicides) methods. For dense infestations
(greater than 80% canopy cover), aerial boom spray applied chemical control is a cost
effective and practical solution (Raal and Gous 2010]). However, the feasibility of boom sprayed chemical control for invasive wilding
conifers in New Zealand has had variable success to date (Raal and Gous 2010]).

Potential herbicides for controlling wildings have been identified through pot trials
(Gous 2010a], 2010b]). The selective and systemic herbicides triclopyr ester (3,5,6-trichloro-2-pyridyloxyacetic
acid) and picloram (4-amino-3,5,6-trichloropicolinic acid), applied alone (triclopyr
ester) or in combination (triclopyr ester and picloram) and the non-selective systemic
combination of glyphosate (N-(phosphonomethyl)glycine) and metsulfuron (2-[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]-oxomethyl]sulfamoyl]benzoic
acid methyl ester) provided best control of small (height of ~0.3 m) P. contorta, P. mugo (Turra) and Pseudotsuga mensiesii (Mirb.).

Although these herbicides have been shown to be effective on young juvenile conifers
(Gous 2010a], 2010b]), little research has been conducted using these herbicides on mature conifers, growing
in dense infestations where trees can reach heights of up to 15 m. Recent research
shows that these herbicides have low efficacy (?64% mortality) on large wilding P. contorta (height range 1???15 m) and P. mugo (height range 0.5???5 m) when they are mixed with water and aerially applied at low
rates (150 L ha^?1) (Gous et al. 2014]). Gous et al. (2014]) concluded that application rates of water had to be high enough to ensure complete
coverage of the foliage. As a consequence of these results, the objective of this
research was to examine the efficacy of a range of systemic herbicides, applied in
high water rates (400 L ha^?1), with wetting agents, to control dense field infestations of P. contorta.