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Plantwise Technical Factsheet

Sri Lankan privet (Ligustrum robustum subsp. walkeri)

Host plants / species affected
Cryptomeria japonica (Japanese cedar)
L. robustum subsp. walkeri is an evergreen shrub or small tree to 10 m tall; however mature trees were observed with heights in excess of 15 m with trunks greater that 2 m in diameter in Sri Lanka, possibly due to their old age (estimated at over 100 years old). Young stems are lenticelate, glabrous or minutely puberulous. Leaves and petioles are glabrous; lamina lanceolate, glabrous, 3-9 x 5-2.5 cm, margin entire. base rounded or obtuse, attenuate into the petiole; apex acute-attenuate; venation somewhat obscure with 5-7 primary veins per side. Inflorescence terminal, pyramidal, paniculate, many-flowered, scented, 20 cm long, glabrate or minutely puberulous. Calyx campanulate, 1 mm long, more or less entire or with 4 very shallow triangular teeth. Corolla tube, white, 1 mm long, lobes 4, valvate, 2 mm long, more or less reflexed at anthesis. Stamens 2, exserted, filament inserted at top of the corolla tube, 1.5 mm long; anthers narrowly oblong, 1 mm long. Ovary rounded, 0.5 mm; style 1.5 mm long, stigma terminal, more or less fused, elongate. Fruit a berry, ellipsoid, 7-10 x 4-5 mm, bluish-purple when ripe (Dassanayake and Fosberg, 1987).
Prevention and control
Cultural Control

One of the techniques being tested for controlling L. robustum subsp. walkeri is the encouragement of indigenous species such as Dombeya sp., Monimia sp. and Weinmannia tinctoria as understorey components through silvicultural tending and release (Figier and Souleres, 1991).

Mechanical Control

Mechanical control is difficult since the plant can regenerate vigorously after treatment. In La Réunion a 100 m² plot under native scrub forest which was cut had around 15,000 individuals per ha after two years and 100% of cut individuals had coppiced (Macdonald et al., 1991). Young plants may be hand pulled but older shrubs need to be dug out.

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

Integrated Control

The most common approach to controlling privet is to cut the tree down and apply a stump treatment chemical to the cut surface. However, as L. robustum subsp. walkeri resprouts vigorously, it is likely to require follow up spray treatments to control subsequent growth.
Reduced biodiversity and homogenization of the attractive natural environment could lead to a reduction in eco-tourism potential which is a major driving force for the local economy. Cutting and stump treatment of 1 ha of L. robustum subsp. walkeri invasion was estimated at 3243FF (approximately UK£330 or US$500 per ha) for the year 1998-99 (Lecouer, 1997). In 1990-91, 400 ha were treated (Lavergne, 2000), with an estimated cost of almost 1.3 million FF (approximately UK£130,000 or US$200,000) per year for only a limited control effort.
Summary of invasiveness
L. robustum subsp. walkeri is one of the most serious invasive species in the Indian Ocean islands of Mauritius and La Réunion where it is having a direct negative impact on forest biodiversity. Seeds in both islands are thought to be dispersed almost entirely by another introduced species, the red-whiskered bulbul, Pycnotus jocosus. It is already one of the dominant species in remnant Mauritian native forests and is expected to become the same in La Réunion if control efforts are unsuccessful.
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