Assessment of the causality of Essigella-ascribed defoliation of mid-rotation radiata pine and its national impact in terms of cost of lost wood production
This study estimated the national cost of defoliation in terms of lost wood production including predicting future defoliation and identifying potentially susceptible stands using climatic or nutritional information. It showed Essigella (Essigella californica) causes defoliation of the mid and upper crowns of radiata pines, There was no defoliation of sprayed trees compared with 14% defoliation of unsprayed trees. A 50% reduction in aphid numbers should result in a 30-40% reduction in defoliation.
Biological control is likely to be effective in reducing defoliation, provided a suitable agent can be found. Spraying plantations with Actara can reduce aphid numbers by 95% for at least 4 weeks. Relationships between aphid population size and climatic conditions and between defoliation and stand age could be used to develop a risk management tool to predict where and when defoliation will be most severe and allow susceptible stands to be targeted for protective or remedial measures.