The Sirex woodwasp (Sirex noctilio) made its devastating presence felt in the commercial pine plantations of countries worldwide from as far back as the first half of the 1900s. Destroying swathes of varieties of this commercially valuable tree genus in its path wherever it was found, the sirex eventually reached South Africa in 1994.
From its initial population in the Western Cape, over a period of 13 years this pest has spread eastwards through pine plantations along the country’s eastern coastal regions. Its currently known front now lies about 40km east of the N3 highway, in the Greytown region of KZN.
The sirex wasp has caused approximately R300 million worth of losses to the local timber growing and processing industry – through its habit of laying eggs in pine trees while at the same time infecting these trees with the wood decay fungus Amylostereum areolatum, and as a result of its larvae burrowing through the pine tree stems.
While it is an accepted fact that this wasp will, in the future, spread across the rest of South Africa, and northwards into the rest of Africa’s pine plantations, it is important that methods are devised to manage its spread and negative impacts.
Experts put their heads together
Speaking recently at the International Sirex Symposium in Gauteng, and later at the Sirex Workshop in KZN, Prof Colin Dyer, director of the Institute for Commercial Forestry Research (ICFR) based at the University of KZN, told the many gathered international and local sirex experts that ideas adopted from their countries to control this wasp were currently not working effectively in South Africa.
“What we need from the symposium and the workshop is information that can help us to modify our existing sirex control strategy, and this strategy must be effective both here and northwards into Africa,” Dyer explained.
Years of international research have found three main methods for effectively controlling sirex. These involve using selected nematode species to render female sirex wasps infertile and to feed on the fungus A. areolatum growing in infected pine trees; thinning pine plantations at crucial times in the trees’ growth cycle to reduce tree stress and render them resistant to sirex attacks; and introducing control insects that parasitise various stages of the sirex’s life cycle. These parasites attack either the egg or larval stages of the wasp.
The Kamona strain of the parasitic nematode Beddingia siricidicola is the weapon of choice for most countries fighting sirex, with reports of 90% parasitism rates of sirex eggs being achieved in some cases. The wasp species Ibalia leucospoides that parasitises sirex has been widely accepted as another good method to control this pest.
However, as effective as these methods have appeared to be in other countries, for yet unknown reasons they are having limited impact on sirex in South Africa. While the thinning of pine plantations is proven to increase trees’ resistance to sirex attacks by limiting the pines’ intra-specific competition, this method has not found widespread favour because of its impacts on decreased plantation yield and, ultimately, timber company profits.
Workshop delegates from countries such as Australia, the US, Brazil and Chile urged the local timber industry to not only fight existing sirex infestations, but to set strategies in motion to act ahead of the current sirex front in South Africa. Suggestions that emerged include timing the regular practice of hygiene-thinning pine plantations to minimise the potential for sirex infestation, implementing effective sirex monitoring methods in currently uninfected plantations, and finding more effective biocontrol agents to prevent sirex epidemics.
Highlighting important issues raised at the Sirex Workshop, Dr Andrew Morris who heads Sappi Forests Research, said, “It has been discovered here that on average less than 10% of sirex adults emerging from trees inoculated with the Beddingia siricidicola nematode have been infected by the nematode. However, infection of female sirex wasps emerging from the lower third of early inoculated trees can reach high levels, indicating that for some reason the nematodes are not spreading throughout the whole length of the tree after inoculation.”
Questions were raised as to whether the South African climate was negatively affecting the efficacy of sirex controls here, when they seemed to be working so well elsewhere. One fact that was pointed out was that in countries where winter rainfall was significantly higher than that of South Africa, these controls were more effective against the pest wasp.
Mike Wingfield, Mondi professor of Forest Protection and director of the Forestry and Agricultural Biotechnology Institute (FABI) at the University of Pretoria, believes that a fundamental barrier in South Africa to the efficacy of these proven sirex controls elsewhere is that the sirex already in this country represents a unique combination of biotypes of the wasp and its fungal symbiont on the pine species Pinus patula that has not previously been affected in other countries. This is making the wasps less susceptible to introduced nematodes.
South Africa-specific solutions
“It seems that we have developed a uniquely South African problem that requires a uniquely South African solution,” Wingfield told delegates. “At the outset, we assumed that the parasitic nematode that was used in other countries would be effective in South Africa. This overlooked the fact that the biotypes of the wasp and the fungus that it carries in South Africa are different to those found where the nematode parasite was developed in Australia. What we need is to find strains of the nematode and fungus that are suited to our South African conditions and to deploy these as actively as possible.”
While developing an effective strain of B. siricidicola for South Africa is important, workshop delegates from various countries felt it equally important to back up this sirex control method with parasitoid wasps that could play a supporting role to the nematode, independent of nematode population densities.
“The workshop has pointed out that we should continue with, and accelerate, the introduction of Ibalia leucospoides from the Cape to KZN. In addition, we should complement ibalia with other parasites of sirex currently used in Australia and South America. It might even be wise for us to extend our search for new species of these parasitoids into the northern hemisphere,” Morris added.
While some representatives of the South African forestry industry might have left the symposium and workshop frustrated that no “silver bullet” solution to our sirex problem had been found, they could rest assure that the overall feeling of the meetings was that the sirex wasp would eventually meet its match through the concerted efforts of the industry, the ICFR and the FABI.
The foreign sirex experts encouraged the local timber industry to, in the meantime, immediately begin setting up systems to monitor the spread of the wasp. These methods should include trap trees and artificial sirex traps placed strategically ahead of the wasp’s current front. Perhaps something valuable could be learned by seeing more precisely at what rate sirex is spreading, during what times of the year, and which pine plantations they prefer and why. Another concern is the uncontrolled movement of sirex-infected timber and wood products from one region to another. Delegates urged the local timber industry to tighten up on these timber movements and minimise the risk of sirex moving from infested areas to non-infested areas.
The need for government support
The workshop expressed concern that all existing sirex control methods in South Africa are only supported through funding by SA’s forestry industry. Despite this pest’s already massive, and possibly even greater, impact on an industry that contributes huge amounts of money to the South African economy, government support for the fight against sirex is surprisingly lacking.
“Government is quick to jump into controlling outbreaks of foot-and-mouth or classic swine fever in livestock. What makes these animal disease outbreaks any more important than the sirex epidemic? The products produced from pine plantations are just as necessary for the country’s economy as meat products are,” one workshop delegate told Farmer’s Weekly.