Retrospective modelling indicates minimal impact of non-target parasitism by Pteromalus puparum on red admiral butterfly (Bassaris gonerilla) abundance

Abstract

There is anecdotal evidence that populations of the New Zealand endemic red admiral butterfly Bassaris gonerilla (F.) have declined since the early 1900s. This decline has been associated with the introduction of the generalist pupal parasitoids Pteromalus puparum (L.) and Echthromorpha intricatoria (F.). The former was deliberately introduced for the biological control of the cabbage white butterfly (Pieris rapae (L.)); the latter is an adventitious arrival from Australia. A discrete-time model for B. gonerilla population dynamics was constructed; it had two summer generations per year and a partial overwintering generation. The model suggested that the impact of non-target parasitism by P. puparum has been minimal, with estimated suppression levels of 5% for B. gonerilla populations on the Banks Peninsula. In contrast, parasitism by E. intricatoria was estimated to have caused 30% suppression of B. gonerilla abundance. The model suggested that the presence of an overwintering larval generation of B. gonerilla provides a temporal refuge from the high levels of E. intricatoria parasitism. An important assumption of the model was that parasitism rates are independent of B. gonerilla density. This assumption appears valid for P. puparum parasitism, but may not be so for E. intricatoria; therefore the estimated suppression levels due to this adventive parasitoid should be viewed with some caution.