25 Jo Sasse, Stephen Elms and Peter Kube Australian Forestry 2009 Vol. 72 No. 1 pp. 25–31 Genetic resistance in Pinus radiata to defoliation by the pine aphid Essigella californica Jo Sasse 1,2 , Stephen Elms 3 and Peter Kube 4 1 Sassafras Group, 2 O’Farrell St, Yarraville, Victoria 3013, Australia 2 Email: josasse@optusnet.com.au 3 HVP Plantations, PO Box 385, Churchill, Victoria 3842, Australia 4 CSIRO Marine and Atmospheric Research, GPO Box 1538, Hobart, Tasmania 7001, Australia Revised manuscript received 13 November 2008 Summary The Monterey pine aphid, Essigella californica, has been associated with extensive defoliation and growth losses in radiata pine plantations in southeastern Australia since it was irst detected in 1998. HVP Plantations (HVPP) observed variation in the level of defoliation between clones in clonal seed orchards and archives, and initiated a program of assessment of progeny trials. Between 2001 and 2005, defoliation was assessed in a provenance trial, a subset of clones within a seed orchard, and 18 progeny trials (some repeatedly). Needle retention in upper crowns was scored on a scale of 1–10, rather than needle loss, because this ensured higher scores equated to the desirable state of the assessed trait. There were signiicant differences in the extent of retained foliage between populations, and between sub populations within populations. The inland northern (Coastways Ranch) subpopulation of Año Nuevo population had the highest level of retained foliage in the upper crown (mean score of 8.3), and the northern (Pico Creek, Haarst Ranch) subpopulation of the Cambrian population had the least retained foliage (5.0). Estimates of heritability from progeny trials ranged from 0 to 0.9, and averaged about 0.5 in trials where there was a signiicant family (maternal) effect. Crosssite analysis using a family model resulted in an overall heritability of 0.4. Genetic correlations between assessments across two trials in 2004 and those in 2001 and 2003 were high, suggesting that defoliation due to aphids is consistent across years and can be considered to be the same trait. In addition to the observed differences between provenances and families, there were signiicant differences between clones, and average levels of retained foliage among clones ranged from 1.9 to 9.4. HVPP has used this information to develop an aphidresistant breed of radiata pine which has been deployed on an increasing scale since 2005 into the most susceptible areas of its Victorian estate. Keywords: defoliation; genetic variation; clones; families; provenance; resistance; heritability; radiata pine; Monterey pine aphid; Essigella californica; Victoria Introduction The Monterey pine aphid, Essigella californica, was irst detected in plantations of radiata pine in southeastern Australia in 1998 (Carver and Kent 2000). It causes yellowing and premature drop of needles, typically in the upper third to quarter of the crown of older trees (20–25 y), but has also been observed in young trees (4–6 y, Carver and Kent 2000; 1–5 y, Smith et al. 2008). In Victoria and South Australia, damage is greatest during late summer and early autumn and needles are shed a few months later (May and Carlyle 2003). Defoliation has been associated with signiicant growth losses. High correlations between defoliation and growth loss have been reported, with a 10% loss of foliage estimated to reduce growth by about 5% (Hopmans et al. 2008) to 8% (May and Carlyle 2003). The extent of defoliation has been observed to vary with location, nutritional status (Smith et al. 1999; Hopmans et al. 2008), stand age and condition (i.e. defoliation increases with age, Smith et al. 2008; and thinning, May 2004), and from tree to tree (Smith et al. 1999). In Victoria, defoliation has been regularly monitored from 2002 as part of HVP Plantations’ (HVPP) routine health surveillance program. Defoliation due to aphids has been consistently highest in the northeast. Some plots lost 50–70% of the upper crown over successive years, and average upper crown defoliation for all stands above 11 y old has generally been low to moderate (11–30%). Plantations south of the Great Dividing Range had much less damage in 2002–2003, but have experienced damage comparable to that in the northeast during the period 2004–2007 (Smith et al. 2008). Defoliation of individual trees has been observed to vary between 0 and 100% within a locality and this, together with observations of a range of consistent degrees of defoliation among ramets within clones in clonal seed orchards and archives on the HVPP estate, suggests the presence of a genetic component to resistance to defoliation. Genetic variation in the extent of defoliation of conifers by aphids has been observed at varied levels in a number of species. Simpson and Ades (1990) reported variation in susceptibility of provenances of P. radiata and P. muricata to the woolly aphid Pineus pini and the needle aphid Eulachnus thunbergia. The green spruce aphid (Elatobium abietinum) causes defoliation and consequently growth losses and mortality in Sitka spruce in the British Isles (Leibhold and Csóka 2001); resistance to defoliation is variable, heritable and stable (Harding et al. 2003), enabling effective breeding for increased resistance. Observations in 1999 of more than 240 clones in three of HVPP’s clonal seed orchards conirmed that there was variation between