How New Zealand pasture ecosystems provide  unique opportunities for biocontrol and ecological experimentation

Stephen Goldson *1

1 Better Border Biosecurity, c/o AgResearch, Private Bag 4749, Christchurch 8140, New Zealand

This contribution discusses the biological control impact of three beaconed parasitoid wasps (Microctonus aethiopoides Loan (Moroccan strain), Microctonus aethiopoides Loan (Irish strain) and Microctonus hyperodae Loan) on the broad-acre exotic forage pests, the lucerne weevil (Sitona discoideus Gyllenhal), the clover root weevil (Sitona lepidus Gyllenhal) and the Argentine stem weevil (Listronotus bonariensis (Kuschel)) respectively.  Observed repeated success of these biocontrol releases was unexpected, given that the historical frequency of efficacious releases has been c. one in ten.  The mechanisms for this result are considered.  It is noted that the exotic invasive weevil species themselves reached spectacular population densities compared to those found in their centres of origin. These high population levels are usually attributed to the lack of biodiversity in New Zealand forage production systems, including few natural enemies and an abundance of unfilled niches.  As an extension of this, it is therefore proposed that the parasitoid wasp species, via an abundance of hosts and again, a lack of biotic resistance, have similarly reached very high levels of parasitism again far beyond those noted in their original ecosystems. These simplified New Zealand ecosystems, where there is little plant diversity and very simplified host-parasitoid interactions, offer great opportunity for the advancement of understanding of parasitoid-host interactions.  Such consideration includes complex density-dependent effects in the presence of sometime maladapted relict behaviours.  In some circumstances has been possible to manipulate weevil population densities to determine the presence of damage thresholds, thereby demonstrating useful biological pest suppression.  Insights thus gained have the ability to develop ecological theory applicable to more complex ecosystems elsewhere.

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