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CHAPTER TWO Factors influencing parasitism of Sirex noctilio (Hymenoptera: Siricidae) by the nematode Deladenus siricidicola (Nematoda: Neotylenchidae) in summer rainfall areas of South Africa
Control of the invasive wasp, Sirex noctilio Fabricius using the parasitic nematode Deladenus siricidicola Bedding is a well known example of a successful classical biological control program. Despite its wide-scale success, this control method has recently had poor success in the summer rainfall areas of South Africa. Data from previous studies showed variation in nematode parasitism from inoculated trees (inoculation success) between different tree sections and amongst inoculation times. They also pointed to moisture content of the wood or virulence of the nematode as the most likely underlying factors influencing variations in inoculation success. The results from our study showed that the highest levels of parasitism were obtained from early inoculations and from the bottom sections of trees, where moisture content of the wood was highest, supporting the hypothesis that moisture content influences parasitism. However, even when moisture content was adequate, average inoculation success remained below 25 % and was often 0 %, suggesting that there are other barriers to inoculation success. Different sources from which the nematodes were produced did not influence inoculation success, indicating that nematode virulence is most likely not the cause of the low success. Another interesting finding was that parasitized wasps were larger than unparasitized wasps. Background parasitism was present despite the poor success with past inoculations, but the data also suggest that the natural build-up of this population could be constrained by the same factors that influence inoculations.
Introduction
The woodwasp, Sirex noctilio Fabricius (Hymenoptera, Siricidae), is native to Eurasia (Spradbery and Kirk 1978), but during the course of the last century, it has been accidentally introduced into various southern hemisphere countries. These include New Zealand (about 1900), Australia (1952), Uruguay (1980), Argentina (1985), Brazil (1988), South Africa (1994) and Chile (2000) (Miller and Clarke 1935, Gilbert and Miller 1952, Tribe 1995, Iede et al. 1998, Klasmer et al. 1998, Maderni 1998, Ahumada 2002, Hurley et al. 2007a). Most recently, in 2005, an established population of S. noctilio was confirmed in the United States of America and Canada (Hoebeke et al. 2005, de Groot 2007). In these countries, S. noctilio has become a pest in commercial plantations and native forests, where, together with its fungal symbiont Amylostereum areolatum (Chaillet) Boiden, it infests and kills Pinus spp. (Talbot 1977).
Biological control is the strategy most commonly used to manage S. noctilio in pine plantations of the southern hemisphere. In particular, the parasitic nematode Deladenus (=Beddingia) siricidicola Bedding is considered the primary biological control agent for the pest (Bedding and Iede 2005). Deladenus siricidicola was first discovered in 1962 parasitizing S. noctilio on the North Island of New Zealand, where it was unintentionally introduced together with S. noctilio from Eurasia (Zondag 1969). During the course of the next decade, considerable efforts were made to screen for species and strains of Deladenus that resulted in high levels of parasitism and to develop effective methods to deliver them to trees (Zondag 1971, Bedding and Akhurst 1974, 1978, Zondag 1979; Bedding and Iede, 2005). This research resulted in the selection of a strain of D. siricidicola from Sopron, Hungary, referred to as the Sopron strain. Trees inoculated with this strain in Australia achieved parasitism levels of almost 100 % (Bedding and Akhurst 1974).
A loss of virulence in laboratory cultures of the Sopron strain, detected during the Green Triangle outbreak in south-eastern Australia (1987-1990), led to the collection and establishment of a new culture of D. siricidicola for laboratory breeding and release (Haugen and Underdown, 1993). These nematodes were collected from the Kamona forest in Tasmania, where the Sopron strain had been previously released. With the exception of New Zealand, this Kamona strain of D. siricidicola has been used throughout the southern hemisphere where S. noctilio has been introduced (Hurley et al. 2007a).
Although D. siricidicola has become well established in Australia, its success has been variable in South America and South Africa (Hurley et al. 2007a). In the summer rainfall area of South Africa in particular, nematode parasitism in inoculated trees (inoculation success) with the Kamona strain has been very poor. The first 2 years of inoculation in the province of KwaZulu-Natal in 2004 and 2005 resulted in parasitism below 5 % and 10 %, respectively. This was despite considerable efforts to streamline rearing, transport and inoculation methods for the 2005 inoculations. These disappointing results suggested strongly that the inoculation technique was not the main cause for the low levels of parasitism (Hurley et al. 2007a).
The reasons for the low level of success with D. siricidicola in South Africa are unknown. The 2004 and 2005 inoculations in KwaZulu-Natal showed a possible influence of the part of the tree inoculated and the time of inoculation on parasitism (Hurley et al. 2007b). This could potentially be related to differences in moisture content of the wood over time and within trees. For example, results from 2005 inoculations indicated that parasitism obtained from the bottom section of trees was higher than that from the middle and top sections. These results suggest that it would be more cost effective to inoculate the bottom section of standing trees, rather than the conventional method of felling trees to inoculate the entire boles of trees, as described in Bedding and Iede (2005).
Another factor that could have resulted in low levels of parasitism in South Africa could be a low level of viability of the nematodes used for the KwaZulu-Natal inoculations. For these inoculations, the nematodes were imported from Australia and reared in South Africa for 3 and 15 months for the 2004 and 2005 inoculations, respectively. Deladenus siricidicola has been known to lose its ability to convert to the parasitic form when reared in the laboratory for long periods (Haugen and Underdown 1993, Bedding and Iede 2005), and such a loss of conversion to this form could have occurred in South Africa.
Sirex noctilio is currently the most important pest of Pinus spp. in South Africa that seriously threatens the forestry industry. Losses due to S. noctilio in the summer rainfall area of South Africa have been estimated to be approximately ZAR300 million (approximately US $45 million) per year (Hurley et al. 2007a) and it is, therefore, crucial to achieve an effective control strategy for S. noctilio. The causes of the low inoculation success in KwaZulu-Natal need to be understood to determine whether these obstacles can be overcome, and thus whether the Kamona strain of D. siricidicola can be an effective biological control agent for S. noctilio in the area. The aim of our study was to build on preliminary data to better understand the influence of tree section, inoculation time, nematode source (potential influence of virulence) and moisture content on inoculation success in the summer rainfall region of South Africa. The feasibility of inoculating standing trees in this region was also considered. Furthermore, the influence of these factors on the size and numbers of emerging S. noctilio wasps, and how this could influence parasitism success, was investigated.
Materials and Methods
Sites
The influence of site on parasitism was not examined in this study. However, the experiment was established at two sites as a precaution against one site being lost to fire or other causes. Both sites were in the KwaZulu-Natal province, South Africa. Site 1 was a 60.2 ha Pinus patula Scheide et Deppe compartment, planted in January 1991, located near Underberg (29˚53 25 S 29˚23 50 E). The site was inoculated with D. siricidicola in 2004 (25 trees inoculated) and 2005 (88 trees inoculated). Inoculation success was 0 % in 2004 and 6.4 in 2005. Site 2 was a 74.5 ha patula compartment, planted in June 1989, located near Boston (29˚40 16 S 29˚58 12 E). There have been no previous inoculations with D. siricidicola at this site Inoculation with D. siricidicola Trees were inoculated during three periods. These were from 28 February to 1 March 2006 (Period 1), from 11 to 12 April 2006 (Period 2), and from 30 to 31 May 2006 (Period 3). Four nematode sources were used and these were all of the Kamona strain. The Australian source was obtained directly from the company that supplies the nematode used successfully for inoculations in Australia (thus considered as a control for high virulence). The FABI source was from the rearing cultures of the Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, South Africa. These were also the nematodes used for the 2004 and 2005 inoculations in KwaZulu-Natal. The KZN source was nematodes isolated from parasitized wasps that emerged from plantations in KwaZulu-Natal, South Africa, from October 2005 to January 2006. The Cape source was of nematodes isolated from parasitized wasps that emerged from plantations in the Western Cape, South Africa, from November 2005 to March 2006. The control trees were considered as the fifth ‘source’. These trees were not inoculated with any nematodes. All nematode sources were used at each inoculation period, except the Cape source that was not used for inoculation Period 1, because sufficient numbers of this source of nematodes were not available at that time.
Twelve trees were used for every combination of site, inoculation period and nematode source (Table 1). The trees were inoculated following the standard procedures described by Bedding and Iede (2005). Trees are felled and de-branched and a specifically designed hammer is used to make inoculation holes of approximately 10 mm deep and 30 cm apart. These holes are made in 2 rows down the length of the tree where the tree diameter is greater than 15 cm and 1 row where the diameter is less than 15 cm. Nematodes suspended in a polyacrylamide gel are squeezed into each inoculation hole, with approximately 2000 nematodes per hole. The control trees were prepared in the same manner to all other trees, but were not inoculated with nematodes. At Site 1, an additional 12 trees per combination of site, inoculation period and nematode source were used. These trees were not felled prior to inoculation and only a 1.5 m section of the tree, from breast height (approximately 1.5 m from ground level) upwards, was inoculated. This treatment was included to compare parasitism in felled versus standing trees. Thus, a total of 336 felled trees and 168 standing trees were inoculated (Table 1).
Moisture measurements of trees
The moisture content of the inoculated and control trees was measured with a Bes Bollmann moisture probe (model H D1 3.10, Gottmadingen, Germany). The probe was inserted so that the tips penetrated approximately 45 mm into the wood. The probe tips were 8 long, and thus the measurements were taken at a depth between 442 mm and 450 mm. These measurements were taken over the bark. For each measurement, the probe was inserted three times within a 10 cm area and the mean of the three moisture values was considered as the moisture reading for that point. For the felled trees, moisture measurements were taken from the mid-point of the bottom, middle and top section of each tree. The bottom, middle and top sections of the trees were defined as the first, second and last third of the tree, respectively. This was after the tree had been felled and the top excised where the stem diameter was 5 cm, as described by Bedding and Iede (2005). For the standing trees, moisture measurements were taken only from the bottom section of the stems. These measurements were taken at breast height, approximately 1.5 m above ground level.
Moisture measurements were taken from the time the trees were inoculated until just prior to the samples being collected. For trees inoculated in the first inoculation period, moisture measurements were taken after 0, 41, 86, 125, 161 and 185 days (28.02.2006, 10.04.2006, 25.05.2006, 03.07.2006, 08.08.2006 and 01.09.2006, respectively), and these time points were referred to as MTIME 1 to 6, respectively. Trees that were inoculated in the second inoculation period did not have moisture measurements taken for MTIME 1, and trees inoculated in the third inoculation period did not have moisture measurements taken for MTIME 1 and 2.
To ensure that measurements taken with the moisture probe were accurate and meaningful, they were compared to the conventional oven-dry method of measuring moisture content. Thus, 62 discs of approximately 10 cm thickness were collected from trees at Site 2 on 5 September 2006. Directly after taking moisture readings from these discs with the moisture probe, the discs were weighed, placed in an oven overnight, and weighed again. The oven-dry measurements were calculated as percentage moisture content equal to oven dry weight over fresh weight x 100.
Acknowledgements
Preface
Chapter One: A comparison of control results for the alien invasive woodwasp, Sirex noctilio, in the southern hemisphere
Chapter Two: Factors influencing parasitism of Sirex noctilio (Hymenoptera: Siricidae) by the nematode Deladenus siricidicola (Nematoda: Neotylenchidae) in summer rainfall areas of South Africa
Chapter Three: The influence of Amylostereum areolatum diversity and competition on the fitness of the Sirex parasitic nematode Deladenus siricidicola
Chapter Four: Sequence data reflect the introduction history of the Sirex woodwasp parasitoid, Ibalia leucospoides (Ibaliidae, Hymenoptera)
Chapter Five: Perception and knowledge of the Sirex woodwasp, Sirex noctilio, and other forest pest threats in the South African forestry community
Chapter Six: The control of the Sirex woodwasp in diverse environments: The South African experience
Summary
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