Australian Forestry 2004 Vol. 67, No. 2 pp. 101–113 101 Spatial analysis of an outbreak of Uraba lugens (Lepidoptera: Noctuidae) in the south- west of Western Australia: does logging, vegetation type or fire influence outbreaks? J.D. Farr1, D. Swain2 and F. Metcalf3 1Science Division, Department of Conservation and Land Management, Brain St, Manjimup, Western Australia 6258, Australia Email: [email protected] 2Forest Management Branch, Department of Conservation and Land Management, Brain St, Manjimup, WA 6258, Australia 3Fire Management Services, Department of Conservation and Land Management, 17 Dick Perry Av., Kensington, WA 6151, Australia Revised manuscript received 29 September 2003 Summary The aerial observations revealed that areas of severe defoliation existed as distinct localities in the southern jarrah forest, north- There was an outbreak of gumleaf skeletoniser (GLS, Uraba west of Manjimup and north of Walpole (Strelein 1988a; Abbott lugens Walker) in the southern jarrah forest in 1982–1988. Aerial 1992a,b). The extent of marked defoliation in successive outbreak and observational survey data were used in a Geographic years expanded from 90 000 ha in 1983 to 300 000 ha in 1985 Information System analysis of the possible impact of forest (Abbott 1987, 1990) such that the infestation was advancing on management practices on this insect. In addition, this allowed approximately a north-easterly front (Strelein 1988a; Abbott 1992b). investigation of the influence of vegetation type on the distribution of GLS. A total area of 6.7 million ha was interrogated, including Jarrah is an important Australian hardwood and its exploitation 89 900 ha of land infested with GLS. Neither logging nor has played a major role in the development of WA since the prescribed burning induced increases in GLS populations. settlement of Perth by Europeans in 1829. It occurs in the south- However, a decrease in area infested with GLS was indicated for western corner of the state, extending from Gingin (80 km north prescribed burning up to 3 y prior to the outbreak. Interrogation of Perth) southwards to Albany, with the exception of some eastern of vegetation complexes indicated that the GLS outbreak was outlying populations (Abbott and Loneragan 1986). The climate initiated on marginal jarrah forest on poorly drained sites that are is mediterranean with hot dry summers and cool wet winters. The prone to inundation in winter and drought in summer. In addition, jarrah forest is divided into two main regions, northern and southern, bivoltine GLS populations were discovered on the fringes of the demarcated by the Blackwood and Preston Rivers. southern jarrah forest during the observational surveys. This In the northern jarrah forest significant exploitation began in the supports the hypothesis that bivoltine GLS were more prevalent 1850s when a convict transportation scheme provided much- in the southern jarrah forest during the outbreak as opposed to needed labour for the developing colony (Mills 1989). In contrast, univoltine GLS during non-outbreak periods. although settlement of the southern jarrah forest began in the Keywords: insect pests; population dynamics; defoliation; outbreak; 1850s, the primary attraction was grazing land and minimal landforms; vegetation types; forest management; logging; controlled disturbance of the jarrah forest occurred (Berry 1987). Significant burning; gumleaf skeletoniser; Uraba lugens; jarrah; Eucalyptus disturbance of the southern jarrah forest began in 1911 with the marginata; marri; Corymbia calophylla; Western Australia extension of the railway south of the Blackwood River from Bridgetown to Jardee and the establishment of two sawmills Introduction (Jardee and Pemberton) to supply sleepers for the Australian transcontinental railway (Berry 1987; Mills 1989). Cutting in A severe outbreak of Uraba lugens Walker (gumleaf skeletoniser, jarrah forest was effectively uncontrolled until 1920, following GLS) occurred on jarrah (Eucalyptus marginata Donn ex Smith) the passing of the Forests Act in 1918 and the formation of the in the south-west of Western Australia (WA) during 1982–1988 Forests Department (Berry 1987; Mills 1989; Heberle 1997). (Strelein 1988a; Abbott 1990, 1992a). Prior to this, GLS was Thus, by 1920 parts of the northern jarrah forest had been exploited observed to attack isolated rural trees, but otherwise was not with uncontrolled cutting for over 70 y and parts of the southern considered a problem in WA. Although GLS is briefly mentioned jarrah forest for nearly ten. In the 1920s, dedication of State forest in a 1960 Forests Department questionnaire as a potential future was initiated, cutting was restricted to over-mature trees and a problem and by Jenkins and Curry (1971), the only previous record system of tree marking to control cutting was established (Havel of an outbreak was in 1947 between Calingiri and Cowaramup 1989; Heberle 1997). In addition, a fire management strategy was (Agriculture Western Australia, Insect Collection records; Western developed involving fire exclusion in young regrowth and low- Australian Department of Agriculture 1948). Nevertheless severe intensity fire in older regrowth and virgin mature forest (McCaw defoliation by GLS in January 1983 led to a scorched appearance and Burrows 1989). Prior to European settlement, the Aboriginal of jarrah crowns, enabling the detection of high population areas practice of frequent burning of the forest maintained its open ‘park- from the air. This event was regarded as unprecedented in jarrah like’ structure (Hallam 1975; Ward et al. 2001). Graziers and karri forests and caused considerable consternation. perpetuated the burning practices used by Aboriginal people, 102 Analysis of an outbreak of Uraba in WA particularly in the southern forests and associated coastal woodlands. But, with the expansion of European settlement and the ever-increasing accumulation of logging debris, fuel loads in the jarrah forest dramatically increased and wildfires were intense WA and devastating (McCaw and Burrows 1989; Burrows et al. 1995). From 1920 to the mid-1950s control burning was mainly restricted ANDREW to strategic strips around forest compartments. In 1954 a new fire BARLEE YANMAH LOCALITY EASTER GORDON management policy was developed involving broadscale fuel Manjimup reduction burning in spring, and this practice was expanded and IFFLEY refined over the ensuing years (McCaw and Burrows 1989) such that fire became an integral part of forest management in the jarrah GIBLETT CAREY forest. SUTTON GLS is a well-known defoliator of eucalypts and related species Pemberton throughout Australia (Brimblecombe 1962; Campbell 1962; Harris 1972, 1974; Harris et al. 1977; Elliot and Bashford 1979; Strelein 1988a) and more recently New Zealand (Bain 2001). WELD JOHNSTON Outbreaks in eastern Australia have been associated with drought MOSSOP WYE DEEP and flood cycles (Campbell 1962; Harris 1972; Harris et al. 1977). N LONDON Other studies suggest a decreased ability of the host plant to protect FRANKLAND SOHO PINGERUP COLLIS THAMES itself, due to environmental conditions such as drought, may induce TRENT GLS population increase (Cobbinah et al. 1982; Farr 1985). In Severe GLS defoliation GLS 1983 defoliation 1983 eastern Australia, outbreaks are favoured where there are trees growing close together with overlapping crowns, abundant low SCALE 0 20 Kilometres undamaged foliage, dense young regeneration and abundant litter Walpole on the forest floor (Campbell 1962; Harris 1975). Therefore, forest management practices such as logging and prescribed burning Figure 1. Gumleaf skeletoniser infestation in the southern jarrah forest may influence GLS populations. In the southern jarrah forests of in 1983, determined from aerial surveillance. (From Strelein 1988a). WA, however, there are subtle differences in the biology and behaviour of GLS. Pupation is not predominantly in the forest floor; larvae feed higher in the canopy than do eastern Australian height-class and vegetation complex (Mattiske and Havel 1998) populations; and GLS is predominantly univoltine compared with — were then cross tabulated with the GLS infestation theme and the bivoltine eastern coastal bio-morph on which most of the areas where overlap occurred were summed. Because logging outbreak studies have been made (Farr 2002). history in FMIS is categorised in sequences of 10 y, interrogation of logging history was limited to this time span. The forest species This paper therefore examines the spatial distribution of GLS both height class theme describes the potential height of native tree species during and following the outbreak in south-western WA, and based on the codominant height of a stand at maturity. These data investigates the extent to which logging, fire or vegetation type were interpreted from 1950s and 1960s aerial photographs (scale influenced the population. 1:15 840) and mapped to produce a 1:25 000 scale Aerial Photography Interpretation (API) map series (Bradshaw et al. 1997). Materials and methods Vegetation complexes are based on Mattiske and Havel (1998) and cross-referenced with soils and landforms classified by Logging and vegetation interrogation Churchward et al. (1988) and Churchward (1992). Climatic zones follow Mattiske (1997): hyper-humid refers to annual rainfall Aerial survey maps of the GLS outbreak for 1983–1986 inclusive >1100 mm and total summer evaporation of <450 mm; per-humid (Abbott 1987; Strelein 1988a) were digitised and recorded