The pressure to attain sustainable primary production necessitates that novel alternatives to issues such as pest management are researched and developed. Given this context we tracked (using Malaise traps) the changes in diversity and abundance of native wasp parasitoids (belonging to the superfamily Ichneumonoidea, comprising the families Ichneumonidae and Braconidae) in three field trials comprising mixed plantings of Eucalyptus and/or Acacia. Our aim was to examine the influences of time since planting (determines architectural complexity), tree species composition and presence/absence of understorey vegetation (i.e. vegetational complexity) on populations of Ichneumonoidea. In the trial referred to as "GES1" the diversity of wasps increased from 10 to 37 morphospecies in the 3 years following planting; over this time some of the trees were estimated to have undergone an approximate 2.7-fold increase in height. In the plantings comprising Eucalyptus and/or Acacia in the proportions 10:90, 38:62 or 100:0, respectively ("GES2"), there was a tendency for Ichneumonoidea to be more abundant in arboreta in which the composition of Eucalyptus and Acacia was more closely matched (i.e. 38:62 eucalypts to acacias) compared to arboreta in which trees of one or other of the two genera dominated (i.e. 10:90 and 100:0 eucalypts to acacia). We also report a tendency for Ichneumonoidea to be less abundant in the halves of the third planting ("GES3") where understorey vegetation had been killed with herbicide. Our results were in accordance with the general ecological principle that greater habitat complexity favours greater species diversity. We suggest that increasing the vegetational complexity of commercial Eucalyptus plantations in ways that favour desired species of Ichneumonoidea could be a means of enhancing the biological control of incipient populations of pest insects. © 2006 Elsevier B.V. All rights reserved.
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