Modelling Complex Insect Invasions: European House Borer as a case study

Abstract

<p>The European House Borer (EHB) is a serious insect pest of untreated dry softwood. The adult beetle lays its eggs into cracks and holes in the deadwood, with larvae subsequently hatching from the eggs and causing damage by feeding on the timber. Serious structural damage can occur when found in timber in buildings. Traditional optimisation models of invasions are of limited applicability for EHB because this pest not only infests the dead parts of pine trees in forests, it also infests buildings. Pine forests are the main habitat of EHB but the damage occurs mostly in residential areas. This means that knowledge of the extent of the invasion (infested area) does not provide enough information to describe the state of the system for management purposes. Another complication is that control of the invasion involves “packages” of actions. These actions cannot be easily related to reductions in area invaded as continuous variables.</p> <p>A three-state variable model was developed to represent the problem. A number of parameters need to be estimated to represent the spread and management of the invasion, but the data needed for direct estimation are not available. We developed a numerical model that derives time trajectories of forest area, houses at risk and number of infested houses for any combination of parameter values and control packages. The four available control options within each package are building restrictions within restricted movement zones; early harvest of softwood plantations; forest hygiene activities; and fixed-cost activities associated with the EHB management programme. A full factorial design was used to test the effects of all possible combinations of control options. All the control packages result in a gradual reduction of the infestation. Under the base assumptions any form of control is preferred to no control. The present value of total cost is around $7 billion under no control and $800 million under full control.</p> The tool developed in this research may be used by biosecurity agencies to estimate plausible parameters sets based on their experience and considering the population dynamics of the EHB. The model can be used in an iterative approach to guide further data collection and should be applicable to other pests with similar spread and impact characteristics.