Black wattle trees are originally from Australia and was brought into South Africa for tannic production for the leather industry.

The use of integrated pest management (IPM) concerning invasive plants is a much more effective option than the exclusive use of pesticides. An IPM plan incorporates other sustainable options and substitution principles as the use of pesticides should be the last resort in the control of invasive plant species. IPM has been used in South Africa for many generations thanks to traditional knowledge handed down within farming communities. This invaluable information should be considered when developing IPM plans.

Prominent invasive trees

This article discusses three alien invasive tree species and their various forms of IPM. The invasive potential of all three species is similar, although they invade different areas of the country. However, they all lead to large-scale ecosystem degradation in veld, fynbos and perennial crops, as well as along watercourses and roadsides.

Black wattle (Acacia mearnsii), gum species (Eucalyptus genus) and pine species (Pinus genus) are all invasive species that were brought from Australia for several reasons. The black wattle was initially brought in to produce tannic acid for the leather industryin the early 19th century; it is significant in the timber industry as it supplies pulp and firewood.

There are currently 14 Eucalyptus species in South Africa, of which six are listed in the National Environmental Management: Biodiversity Act, 2004 (Act 10 of 2004) or the NEMBA, and three have become naturalised. These trees are mainly planted as windbreaks and ornamentals, and some are even used for pulp and glue in the timber industry.

There are currently eleven invasive pine species with eight listed under the NEMBA.All the pine species were introduced for the commercial timber industry, and the first pines were introduced to the Franschhoek area in 1825.Pine trees can transform the landscape, reduce the ecosystem’s carrying capacity and increase the risk of wildfires.

Integrated pest management

Table 1 indicates a particularly good IPM for black wattle. This plan would make an IPM assistance programme more feasible than a stand-alone herbicide assistance programme. Integrating biocontrol and herbicide assistance programmes into a single IPM programme would be more beneficial as these two distinct programmes complement each other over the long term.

Table 1: Black wattle integrated pest management. (Source: DEFF NRM Pesticide Policy, 2019)

Control method Treatment method Active agent/ biocontrol agent Application rate Cost/ha (DEFF) Cost (producer cost**) Toxicity classifications (from the European Chemicals Agency)
Biocontrol Seed feeder Seed-feeding weevil (Melanterius maculatus) 200 adults   Free N/A
Flower bud galler Flower bud-galling wasp (Dasineura rubiformis) 400 galls   Free N/A
Biopesticide Cut stump application Acacia wood rot fungus (Cylindrobasidium leave) (Stumpout) 1 sachet of 400ml oil = 100 trees   Cost of 400ml cooking oil N/A
Herbicide Foliar Clopyralid 90 + Triclopyr (as amine salt) 270 g/ℓ SL (Confront) 0,5% (0,9ℓ/ha) R250/ha R200/ha Eye Dam. 1 -H318
Fluroxypyr 200 g/ℓ EC (Voloxypyr) 0,13% (0,375ℓ/ha) R150/ha R200/ha Aquatic Chronic 3 – H412
Fluroxypyr (pyridyloxy compound) 320g/ℓ + Triclopyr (Pyridyloxy compound) 160g/ℓ (Impala 480 EC) 0,33% 0,99ℓ/ha R2 115/ha R200/ha Aquatic Chronic 3 – H412  
Cut stump Picloram (as potassium salt) 50g/kg + Triclopyr (as triethylamine salt) 50g/kg (Kaput 100 Gel) 0,7g/30mm stump Assuming 1 000 trees/kg* see label (2,5kg/kg) R450/ha R200/ha  

*These costs refer to the pesticide only based on RT12 2015-2018 prices.

**These costs include labour, personal protection equipment (PPE) and tools, and average prices, depending on existing stock.

Toxicity classifications

The toxicity classifications can be used to decide which herbicide is safer to use in specific conditions. For example, Voloxypyr and Impala are toxic to aquatic environments with long-lasting effects. Confront is a good alternative if the site is near a watercourse, but make sure you wear personal protection equipment (PPE), as it can damage your eyes.

Kaput is a no-mix gel that is painted directly onto the cut stumps of an invasive tree in a 2mm layer. The gel is packaged in 1kg tubs. This is the easiest option for herbicide assistance, as there is no water or mixing needed and environmental exposure due to spilling and overspray is limited. The primary control methods for black wattle will be biocontrol as indicated in Figure 1 and biopesticides such as Stumpout with the integration of safer pesticides as indicated in Table 1 and Figure 1.

Biocontrol agents for black wattle. M. maculatus (seed-feeding weevil) on the left, damage caused by D. rubiformis, the flower bud-galling wasp in the middle and Stumpout being applied on the right.

Biocontrol agents

The flower bud-galling wasp is highly damaging. It has been established across the country and is spreading at a steady pace. Biocontrol agents are free of charge, and the Department of Environment, Forestry and Fisheries (DEFF) assists with the distribution of these agents. Provincial biodiversity officers from DEFF also train producers to disperse the agents themselves once established on their farms. Producers can manage and monitor releases using an easy application available from DEFF.

Unfortunately, Eucalyptus and Pinus species do not have biocontrol agents due to the commercial conflict of interest. Table 2 highlights the herbicides registered for the main invader species of gums and pines. Not all gums and pines have registered herbicides so it would be advisable to consult the DEFF Natural Resources Management (NRM) Pesticide Policy 2019 for the registered species and herbicides.

Table 2: Gum and pine control methods. (Source: DEFF NRM Pesticide Policy, 2019)

Control method Treatment method Active agent Application rate Cost/ha (DEFF costs) Cost/ha (producer cost*) Toxicity classifications (from the European Chemicals Agency)
  Saligna gum (Eucalyptus grandis) Lopping/pruning Clopyralid 90 + Triclopyr (as amine salt) 270 g/ℓ SL (Confront) 6% (12ℓ/ha) R3 000/ha R200/ha Eye Dam. 1 – H318
Cut stump Clopyralid 90 + Triclopyr (as amine salt) 270 g/ℓ SL (Confront) 6% (12ℓ/ha) R3 000/ha R200/ha Eye Dam. 1 – H318
River red gum (Eucalyptus camaldulensis)   Sugar gum (Eucalyptus cladocalyx)   Black ironbark (Eucalyptus sideroxylon)       Lopping/pruning Fluroxypyr 80 + Picloram 80 g/ℓ ME (Plenum) 4,5% (9ℓ/ha) R1 350/ha R200/ha Aquatic Chronic 3 – H412
  Black ironbark (Eucalyptus sideroxylon) Clopyralid 90 + Triclopyr (as amine salt) 270 g/ℓ SL (Confront) 12,5% (25ℓ/ha) R6 250/ha R200/ha Eye Dam. 1 – H318
    Cut stump Fluroxypyr 80 + Picloram 80 g/ℓ ME (Plenum) 4,5% (9ℓ/ha) R1 350/ha R200/ha Aquatic Chronic 3 – H412
Clopyralid 90 + Triclopyr (as amine salt) 270 g/ℓ SL (Confront) 12,5% (25ℓ/ha) R6 250/ha R200/ha Eye Dam. 1 – H318
Cluster pine (Pinus pinaster) Ring bark Glyphosate (as sodium salt) 700g/kg WG (Kilo Max) 7,7% (15,4ℓ/ha) R1 309/ha R200/ha Aquatic Chronic 2 – H411 Eye Dam. 1- H318

* These costs include labour, PPE and tools, and average prices, depending on existing stock.

Invasive species and water use

Gum species in watercourses or near bodies of water must be removed as they are water intensive. However, gum trees in the landscape and around houses are not listed. It is important to be cautious with herbicides near water bodies to protect aquatic life (see Table 2). The herbicides listed in Table 2 do not negatively affect bees. Plenum must not be confused with Plenum WG 50, the pymetrozine insecticide that harms bees. –Debbie Muir

For more information and references, send an email to the author at DMuir@environment.gov.za.