The risk of severe damage faced by a transplanted seedling in a typical re-forested management unit depends upon 1) the recruitment to the site of weevils from elsewhere, 2) the level of recruitment at the site while breeding habitats remain favourable, 3) the number of recruited adults, young or old, which are attracted to the seedling over a period of about three years, and 4) the ability of the seedling to discourage weevil feeding. A very large number of factors are known, and expected, to affect each of these phases. Some will have a rather specific influence, for example the amount and availability of living bark tissue on stumps of felled trees will strongly influence their attraction to breeding weevils and their ability to support larval development. Others will have diverse influences, for example the length of time following felling at a site will affect stump quality (breeding environment) and also the density and height of regenerated vegetation (weevil access to target seedlings). Most factors will fall into the latter category and affect weevils or seedlings in many ways.
This level of complexity provides a daunting task for the development of practical risk-rating systems, but such systems are clearly a necessary component of the integrated management of Hylobius since they will enable plant protection or pest suppression measures to be accurately targeted.
We report on the potential of different types of risk model and discuss the level of resolution which may be required for management purposes. Using data collected in Northern Ireland, we have described the level of damage sustained by samples of trees at more than 80 sites by a linear response model (GLIM) in which four terms appear to explain more than 50% of the variation. Potentially interesting relationships between weevils, seedling trees and site factors are highlighted and the future development of this approach, together with the need for greater predictive accuracy, is discussed.
Key words: Hylobius, risk assessment, IPM.
Correspondence: Keith Day, Environmental Research, University of Ulster, Coleraine BT52 1SA, Northern Ireland, UK
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E-mail: kr.day@ulst.ac.uk