Timing and frequency are the critical factors affecting the impact of defoliation on long term growth of plantation eucalypts

Insect defoliation of plantation Eucalyptus sp. is a ubiquitous problem, not only in their native Australia but also in many other countries where Australian defoliators have invaded the introduced eucalypt plantations. Although eucalypts are very resilient to defoliation, their growth suffers and reduces the economic benefits of the resource. An artificial defoliation trial investigated the critical factors relating to patterns of insect defoliation that affect the long-term growth of the plantation eucalypt, E. nitens, in Tasmania, Australia. Current season’s adult-phase foliage was removed manually from two- to three-year-old trees to test four factors: severity (50% or 100% of current adult foliage, equivalent to 11% or 25% of total foliage); disbudding following defoliation (with or without disbudding), timing of defoliation (early or late in the summer) and frequency (for one or two consecutive years). Growth parameters of the trees were measured annually for four years after the initial defoliation, and again thirteen years later, before harvest. The most significant factors affecting the growth of the trees were timing and frequency of defoliation; severity of defoliation and disbudding did not have significant effects over the long term. Trees that received either light or heavy defoliation late in the season for two consecutive years were at least 17% smaller in diameter and MAI in diameter was reduced by at least 21% compared to untreated trees over one rotation. This means they would need to be grown for three to four more years to reach the same stand volume as undefoliated trees at harvest. This would have serious cost implications for plantation managers. To prevent these economic losses, an integrated pest management system should focus on protecting eucalypts from defoliation of 50% or more of current season’s adult foliage late in the summer, and in particular, preventing defoliation from occurring in concurrent years. In addition, the continual decline in growth rates of defoliated trees relative to undefoliated trees beyond the initial four years of measurement also suggests that defoliation impacts predicted by models based on short term studies may need to be treated conservatively.