NEMATODES FOR THE BIOLOGICAL CONTROL OF THE WOODWASP, SIREX NOCTILIO Robin A. Bedding CSIRO Entomology, PO Box 1700, ACT 2601, Australia ABSTRACT The tylenchid nematode Beddingia (Deladenus) been placed in a separate family of nematodes. siricidicola (Bedding) is by far the most important Poinar et al. [2002] have now placed both control agent of Sirex noctilio F., a major forms of Beddingia species in a new family, the pest of pine plantations. It sterilizes female Beddingiidae). sirex, is density dependent, can achieve nearly At about the time adult parasitized sirex emerge 100 percent parasitism and, as a result of its from infested trees, adult nematodes complicated biology can be readily manipulated have usually released most of the juveniles that are for sirex control. Bedding and Iede (2005) gave a within them into the insect’s blood cavity and the comprehensive account of this nematode and its juveniles have migrated to the insect’s reproductive use in Australia and South America. organs. In the male, this is a dead end but Throughout the 1960s and early 1970s, CSIRO and female sirex are effectively sterilized because, as various consultants conducted a comprehensive well as ovarial development being suppressed search, in hundreds of localities from Europe, to various degrees, each egg that is produced USA, Canada, India, Pakistan, Turkey, Morocco is fi lled with up to 200 juvenile nematodes. A and Japan, for coniferous trees infested with parasitized female sirex still oviposits readily, and any siricid species. Thousands of logs from these often in several different trees, but lays packets trees were then caged in quarantine, mainly at of nematodes instead of viable eggs. Since many Silwood Park, UK, where all emerging insects sirex often attack the same trees, larval progeny of were investigated for biological control potential. unparasitised sirex can eventually become infected As a result of these collections, seven species of with nematodes, but this is only made likely by the Beddingia were found parasitising 19 siricids many intervening free-living cycles which enable (associated with two fungal symbionts) and 12 the nematode to breed through the tree in vast parasitoids from 31 tree species and 29 countries numbers. It is only when juvenile nematodes (Bedding and Akhurst 1978). Only one species of reach the immediate microenvironment around nematode, B. siricidicola, was found to be suitable sirex larvae that they are stimulated by high CO2 for sirex control and this has now been released and low pH (Bedding 1993) to develop into the and established in Australia, New Zealand, Brazil, preparasitic kind of female rather than the fungal- Uruguay, Argentina and South Africa. feeding form. After mating the infective female uses its large anterior spear-like stylet to bore into B. siricidicola is extraordinary in having two sirex larvae and within a few weeks of penetrating separate life cycles associated with two a larva the nematode grows up to 1000-fold morphologically very different adult female types in volume. It now remains, often for several (Bedding1967, 1968, 1972). In the parasitic cycle, months, relatively unchanged, until the sirex larva 5 to 20 mm long, cylindrical, often green-colored pupates. Then the nematode’s reproductive system females release thousands of juvenile nematodes develops rapidly from a few cells to produce many into the body cavity of adult sirex wasps. In the thousands of juvenile nematodes in little more free-living cycle 1 to 2 mm long females feed on than a week, and these are released into the the symbiotic fungus, Amylostereum areolatum insect’s blood cavity at about the time the insect (Fr.) Boidin as it grows in the tree, and lay eggs emerges from the tree. in the tracheids (wood fi bers). (These two types of females are so morphologically different from The fungal-feeding life cycle is not only important each other that each on its own would have in enabling high levels of parasitism in sirex Proceedings—Interagency Research Forum on Gypsy Moth and other Invasive Species 21 populations, it can be used to manipulate the Important factors are: nematode for biological control purposes. Initially • Cutting the holes cleanly with a specially it was possible to maintain a library of hundreds designed hammer/punch so that nematodes can of isolates of the various nematode species and easily enter the tracheids strains on fungi growing on agar slopes which • Using gel so that the nematodes are suspended were kept under refrigeration for many years within the hole for adequate time while being evaluated as control agents. In addition, once the best species/strain had been • Introducing the nematodes when the wood is selected, it could be sterilely mass reared, for adequately moist liberation, in fl asks of autoclaved wheat/rice • Spacing of inoculation holes and number of inoculated with A. areolatum. nematodes introduced into each hole which Selecting the best nematode species for liberation has been carefully tested to provide maximum from the library of stored isolates was relatively parasitism (over 98%) and minimum competition simple; only two species fed on A. areolatum between fungal feeding nematodes and the (the symbiont of S. noctilio) and one of these fungal feeding sirex larvae so that the latter grow B. wilsoni was excluded because it parasitised to full size (Bedding and Akhurst 1973) rhysinne parasitoids. However, there still remained • Because parasitized female sirex rarely leave hundreds of isolates of B. siricidicola from 20 the compartment of liberation it is important to countries, 10 tree species and four sirex species. introduce nematodes to each forest compartment The best strain, from Sopron in Hungary, was and repeat inoculations, if required, each year selected because it totally sterilizes female S. until over 10 percent parasitism occurs in each noctilio, gives the highest levels of parasitism and compartment unlike strains that fulfi lled the fi rst two criteria, Where sirex killed trees exceed 1 percent of has little effect on the size of sirex emerging from the total, all naturally infested trees in every logs correctly inoculated with it. (The large size 5th row (20 percent) should be inoculated to of parasitized sirex is important because they fl y achieve adequate control. Examples of successful further, visit more trees, lay more nematode fi lled treatment of sirex populations that were out of eggs and live longer than smaller ones.) control because nematodes were not previously Once nematodes are well established in a sirex present are: population, the parasitized female sirex perform A) The Mt. Helen plantation in Tasmania in 1974 the main task of spreading them. However, where 10 percent of all mature trees had been initially (where less than 1 percent of trees are dying for each of the previous 6 years, all dead naturally killed by sirex) the nematodes have to trees in every 10th row were inoculated. In be introduced into new plantations by inoculating 1975 there was near 100 percent parasitism and ‘trap trees’ set up at easily accessible strategic in 1976 there were negligible sirex. points. A group of 5 to 10 preferably low quality trees near the roadside are selected for each B) The “Green Triangle” in South Australia/ compartment (50 acres or so) a few months Victoria in 1987 where there was up to 80 percent before the sirex fl ight season, and injected with tree death in some areas, 166,000 sirex-killed trees just enough weedicide, so that they die slowly (from every 5th row) were inoculated and by 1989 and are particularly attractive to sirex. Once sirex nearly 100 percent parasitism was followed by a populations are established in these trap trees, the population collapse; sirex has been rare in that trees are felled and inoculated with nematodes, region ever since ( Haugen and Underdown 1990) suspended in polyacrylamide gel, at a rate of 2000 C) In a 12,000-ha plantation in Encruzilhado per inoculation hole with holes about 1 cm deep do Sul in Brazil, where sirex infested about and 30 cm apart. 30 percent of trees in some compartments in 22 Proceedings—Interagency Research Forum on Gypsy Moth and other Invasive Species 1991, nematodes were released from 1990 to 1993, Literature Cited resulting in levels of parasitism of 45 percent in Bedding, R.A. (1967) Parasitic and free-living cycles 1991, 75 percent in 1992 and more than 90 percent in entomogenous nematodes of the genus in 1994. In 1995 it was diffi cult to fi nd any sirex- Deladenus. Nature 214, 174–175. infested trees in this area (Bedding and Iede 2005). Bedding, R.A. (1968) Deladenus wilsoni n.sp. and There has been one major problem with nematode D. siricidicola n.sp. (Neotylenchidae), control of sirex. Because B. siricidicola had been entomophagous nematodes parasitic siricid cultured in the free-living form for over 20 years woodwasps. Nematologica 14, 515–525. without intervention of the parasitic life cycle, this Bedding, R.A. (1972) Biology of Deladenus led to the selection of a strain that rarely formed siricidicola (Neotylenchidae) an the pre-parasitic infective stage (Bedding and entomophagous nematode parasitic in siricid Iede 2005). Unfortunately the situation did not woodwasps. Nematologica 18, 482–493. become apparent until this defective strain had Bedding, R.A. (1993) Biological control of been liberated for many years. However, in 1990 Sirex noctilio using the nematode Deladenus the original strain was re-isolated from where it had siricidicola. In: Bedding, R.A., Akhurst, R.J. and been liberated in Kamona, Tasmania in 1971 and Kaya, H.K. (eds) Nematodes and the stocks have been maintained since then in liquid Biological Control of Insect Pests. CSIRO nitrogen for re-establishment of the cultures used publications, East Melbourne, Australia, for all new introductions.