The constraints of selecting for insect resistance in plantation trees
Abstract:• High productivity in plantations of exotic tree species is achieved by management for fast growth in the absence of the full complex of co-evolved insect herbivores. In the case of Eucalyptus, silvicultural selection for desirable wood traits is concomitant with a trade-off against defence and a reduction of chemical and genetic diversity. These factors, combined with accidental introductions, rapid insect evolution and the emergence of new pests, increase the likelihood that future plantations will need insect pest management to maintain productivity.
• Forestry researchers have suggested that selecting for resistant genotypes may be beneficial in insect control. There are, however, significant differences between long-lived trees and annual crops that make this approach unlikely to be successful. This is illustrated using several examples of research into resistance to insect herbivores in trees.
• Selection for resistance to insects in trees requires an assessment of trial plantations for heritable variation in insect damage and then a determination of the effect of variation in resistance on insect population parameters. Identifying rare resistant genotypes using markers is difficult because many factors interact to produce a resistant phenotype, and phytophagous insects have less intimate relationships with their host than pathogens, resulting in weak associations with genetic loci.
• If resistant genotypes are identified, their widespread deployment in plantations might not provide satisfactory management of insect pests when the use of extensive monocultures is continued. In this paper, experiments are suggested that would explore the effectiveness of polycultures or chemotype mixtures with respect to ameliorating the damage of insects on plantation productivity. In addition, mitigating the effects of some insects on plantation productivity by maintaining vigour of fast-growing eucalypts should be considered.