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Summary
The aim of this study was to determine the usefulness of trunk and scaffold limb sprays of the insect growth regulator diflubenzuron on reducing horse chestnut leaf miner (HCLM) severity on the white flowering horse chestnut tree Aesculus hippocastanum L. One bark and scaffold limb spray applied to coincide with the emergence of the first adult moth generation reduced leaf mining severity and increased larvae/pupae mortality at week 4 after treatment, but this effect faded by the end of the growing season. Two and three repeat bark and scaffold limb diflubenzuron sprays significantly reduced leaf mining severity and increased larvae/pupae mortality rates at the end of the growing season compared to non-diflubenzuron treated controls but did not significantly differ between themselves indicating little beneficial influence of a third diflubenzuron spray. There was no significant influence of diflubenzuron sprays applied in 2007 on leaf mine number and larvae pupae survival the following year (September 2008). This suggests that the treatment would have to be re-applied on an annual basis. Consequently, the cost benefit of such trunk and scaffold limb treatments needs to be carefully considered, especially as systemic insecticides exist that provide longer term control with less frequency of application.