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  • Writer's pictureShelby Pietersen

Hemel en Aarde: Sirex Woodwasp Pine Survey Report

Sirex Woodwasp Background

The Sirex Woodwasp is an invasive species in South Africa (first identified in South Africa in 1994). They are native to Europe, Asia, and North Africa and they do not have any predators in South Africa. In short, distressed Pine Trees (Pinus Radiata, Pinus Pinia, Pinus Pinaster) release a terpene to which the Sirex Woodwasp is attracted.


The wasps lay their eggs beneath the bark and in order to feed the emerging larvae they inject a fungus (Amylostereum areolatum) into the tree which causes death to the tree within 1-2 years.


It is evident that the Sirex Woodwasp has taken hold in the Hemel and Aarde Valley and the greater Western Cape. According to FABI (The Forestry and Agricultural Biotechnology Institute), the only way that this issue can be completely eradicated is by introducing nematodes (Deladenus siricidicola) that feed on the fungus.


The nematodes attack the fungus, making it impossible for the larvae to feed - causing a decline in the Sirex Woodwasp population. Although the nematodes are our best defence, for a number of reasons it has not been as successful in South Africa as it has been in other countries.


Unfortunately, FABI only produces enough nematodes to innoculate approximately 10,000 pine trees per annum. This will not make a dent in the effort to contain the spread of the Sirex Woodwasp. This leaves the South African Forestry and Arboriculture industry in a conundrum. Currently, the only way forward with the management of this pest is to focus on prevention and clean-up.


How to identify Sirex Woodwasp infection:

  • Discolouration in the foliage

  • 3-5mm Emergence holes all over the bark

  • The tree will release lots of sap

  • The limbs and branches will begin to decay and break off

Our Proposal

As tree care professionals, it is our duty to inform and educate the people of the Greater Overberg about the Sirex Woodwasp and the threat the affected trees can pose to nearby humans and the devastation it can cause to our pine plantations and thus our forestry industry.

Here is what Overberg Arborists propose:

  • Identification of infested and at-risk pine trees via a survey (see below)

  • Immediate removal of the at-risk and infested pine trees

  • Property owners reforesting with alternative trees & maintaining the pine stands that are left.

Our aim is to educate and inform the people of Overberg and the Overstrand about the Sirex Woodwasp so that we can gain control over this invasive species before it becomes an unmanageable threat.


General Pine Survey Findings:

Date Conducted

17 October 2022

Tree Species being surveyed

Pinus Radiata, Pinus Pinea, Pinus Pinaster

Estimated hectares surveyed

Approximately 250ha of rural area in the Hemel en Aarde Valley

Percentage of affected trees

5.2% Out of 3998 trees, 208 trees were infected. Approximately 33% of the total affected trees are considered highly infected.

Percentage of at-risk trees:

25-30%

Conclusions

  • The risk varies from farm to farm depending on density and tree stress factors.

  • In dense stands of Pine trees, there is a higher Sirex Woodwasp presence.

  • We found a direct correlation between the farms affected by fire and the Sirex -affected tree count.

  • There are many large-frame pine trees that have been heavily damaged by the infestation.

  • Infected and decaying trees pose a large threat to humans, animals, and infrastructure.

  • Many of the dead trees were infected more than 2 years ago.


If the decaying trees are not managed in the high-risk areas, the outcome can be extremely detrimental to the surrounding environment, structures, and people.


We can conclude that removing infected and at-risk (fire-damaged, and densely spaced) pine trees, spacing trees properly, and continual maintenance of the healthy pine trees will have a drastic effect on decreasing the Sirex Woodwasp presence in the Hemel en Aarde Valley.


We would like to extend a thank you to all the farms that chose to participate in this survey. We appreciate your willingness and time.


Kindest regards

The Overberg Arborists team

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