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Trap tree trial proved the concept

Jane Elek
Forestry Tasmania

For a second year, the concept of attract-and-kill as an alternative to aerial spraying of insecticides for managing leaf beetles in eucalypt plantations was tested in the field. Eucalyptus delegatensis or E. regnans trap trees within another six plantations aged 3 or 4 years were treated in mid-October- early November, 2010, to ensure there was not a repetition of the previous season’s defoliation by the early emergence of chrysomelids. Trap trees were infused with diluted systemic insecticide, imidacloprid (SilvaShield®), to create lethal trap trees. The effectiveness of the treatments at creating toxic foliage, the attractiveness of the trap trees and the effectiveness of the lethal trap trees for protecting the surrounding E. nitens plantations were assessed for six months after treatment.

Populations of the target leaf beetle, Paropsisterna bimaculata, varied 50-fold among the six sites, and even more widely throughout the season. It was the dominant species in five of the six sites, with the newly described species, P. selmani , dominating in the sixth site. There were significant populations of P. agricola in the three northern sites. The lethal trap trees did not target either of the latter two species. Sticky traps showed that the lethal trap trees were effective at attracting P. bimaculata adult beetles into the centre of trap plots. Bucket traps under the treated trees caught significant numbers of the target adult beetles and very few non-target insects, showing that the treatment remained effective for at least three months.

Estimates of adult chrysomelid populations in the E. nitens plantations were made by (i) using sticky traps that trapped beetles over a fortnight and (ii) visual monitoring.  Estimates made using sticky traps were more closely related to the defoliation levels of the E. nitens in subsequent months than were the instantaneous population estimates made by visually monitoring the beetles on the trees; the latter were much more influenced by the weather on the day.

The lethal trap trees reduced defoliation levels of the E. nitens 5% below the control plots within a 50 m radius of the trap trees, but then from 50 m to at least 100 m beyond the trap trees, defoliation increased to levels that were up to 10% higher than in the control plot. These results suggest that the toxic foliage of the lethal trap trees was able to kill enough chrysomelids to protect the plantation trees within 50 m of the block, but for a radius of over 100 m they were acting as attractants only.

The lethal trap tree trial has shown that the attract-and-kill concept shows promise for protecting plantation trees for the leaf beetle season. Although the current method of lethal trap trees only protected trees within a radius of 50 m from the attract-and-kill trees, there is scope for improving the effectiveness of the attract-and-kill concept. For example, deployment around the edge of the plantation rather than the centre or increasing the differential attractiveness to the pest, relative to E. nitens could be more effective.

The next step would be to develop an artificial attractant infochemical that is more attractive than the trap trees that can be used as a lure in attract-and-kill traps. Then the most effective deployment strategy for the traps would need to be determined.