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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 first 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 significant differences in the extent of retained foliage between populations, and between subpopulations 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 significant 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 significant 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.