Jean-Marc Dufour-Dror1 and Tuvia Yaacoby2
1Ecologist, invasive plant specialist, consultant, Israel
2The Hebrew University of Jerusalem, Israel
Acacia saligna (Labill.) H.L. Wendl. (Port Jackson willow) is one of the worst invasive tree species in several countries of the Mediterranean basin (e.g. Israel, Cyprus, Italy, Portugal and Spain). It is also one of the main invasive plants in South Africa. This wattle species forms dense stands where all native plants are displaced while the thick litter produced increases the concentration of soil organic matter and soil nitrogen. Acacia saligna invades various natural ecosystems ranging from coastal sand dunes, dry rocky slopes to riparian habitats, where it completely modifies the basic characteristics of the ecosystems. Due to the nature and extent of its impacts, this wattle species is regarded as a major woody invasive transformer.
Several control techniques, either chemical or biological, have been tested and developed in order to stop and reduce the proliferation of Acacia saligna. While biocontrol attempts in South Africa have, to some degree, proven less successful than initially anticipated (Strydom et al. 2017; Impson & Hoffmann 2019), chemical control of Acacia saligna requires relatively large quantities of herbicides and the direct application procedures are time consuming and therefore expensive (Krupek et al 2016).
In order to improve the control of Acacia saligna significantly, several experiments based on the direct application of aminopyralid (Milestone©) were conducted at four different sites in Israel between 2016 and 2018. The trees were controlled with the hack and squirt technique using 1 ml of aminopyralid per 5 cm of trunk diameter. Undiluted volumes of aminopyralid were applied with a pipettor directly into the cambium immediately after the cuts. The experiment was carried out in five groups of 30 mature individuals (n=150) forming dense stands in natural sites within the Mediterranean region (Csa) of Israel. The trees were only treated once between March and October and each tree was tagged. The experimental sites included natural habitats among the most infested by this wattle in Israel: coastal sand dunes, riparian habitat along river banks in the alluvial plain, as well as dry habitats. The impact of the control was evaluated through systematic monitoring of each tree over a period of 13 months from the date of application. The results showed that 94.7% of the trees completely lacked signs of vitality and died as a result of the control; 3.3% of the trees had less than 5% green phyllodes at the end of the follow-up period, while 2% of the trees still had between 5% and 20% green phyllodes after the application. Among the trees that did not die, none produced flowers and therefore the seed production was stopped.
The outstanding results obtained during these experiments constitute a major breakthrough in the control management of Acacia saligna for the following reasons: (i) The control procedure is basic and extremely easy to apply as it requires only an axe and a pipettor or a small plastic squeeze bottle; (ii) the control operations take less than 30 seconds per tree; (iii) the volume of herbicide needed is at least 5 times less than when controlling A. saligna with common herbicides such as glyphosate or imazapyr; (iv) the ecotoxicological profile of aminopyralid is much better than that of commonly used herbicides (Cal-IPC 2015); (v) this control method can be applied in all types of habitats and terrains infested with A. saligna; and (vi) at last, the fact that large quantities of trees can be controlled within short time periods, while more than 94% of the trees do not require repeated action, make this control technique far cheaper compared to other chemical control methods currently in use.
Considering the recent ban of imazapyr by the EU, the public controversy over glyphosate, and the multiple advantages mentioned above, this control method opens new perspectives for the control of Acacia saligna, and by extension for the control of all invasive wattle species.