PREDICTING THE EFFECT OF A NOVEL VERTEBRATE BIOCONTROL AGENT - A MODEL FOR VIRAL-VECTORED IMMUNOCONTRACEPTION OF NEW-ZEALAND POSSUMS

1. A model is described for predicting the outcome of biological contr ol of New Zealand possums, using viral-vectored immunocontraception ba sed on a sexually transmitted herpes-type vector. 2. The model shows t hat success is possible in ecological terms, and identifies the probab le circumstances under which it is achievable. These provide targets w hich a genetically modified virus must meet, aiding in the quest for s uitable vectors and appropriate genetic modifications to them. 3. In p articular, the female reproductive system rather than the male's shoul d be targeted, contact rate (i.e. number of potentially infectious con tacts) per possum carrying the virus (i.e. infected, not infectious) m ust be in the order of one or more per year and at least 75% of female s carrying the virus must be sterile at mating. Achieving this inciden ce is critical, with only slightly lower values exerting a disproporti onately lower effect on possum densities.4. Spatial aggregation of the viral vector and the existence of a recovered and immune class of pos sums would both reduce substantially the impact of the control agent, but the presence of even limited vector-induced mortality would dramat ically enhance it. 5. Immunocontraception is likely to confer a select ive advantage on the engineered virus by allowing multiple matings for an affected female. This will raise the contact rate and prevalence, and allow the vector to compete successfully with any existing wild st rains. 6. Given an appropriate mechanism of action on the reproductive system, viral-vectored immunocontraception of this kind would, if suc cessful, offer a uniquely acceptable control for a vertebrate pest suc h as the possum, being humane, species-specific, cost-effective and en vironmentally benign. 7. Even if immunocontraception caused only limit ed suppression of an otherwise uncontrolled population, it could contr ibute to successful integrated control by greatly reducing the need fo r conventional poisoning operations. Such integrated control also redu ces possum densities more rapidly than would occur with immuno-contrac eption alone.