Great white pest no longer a problem for Nelson

AgResearch scientists, via the Better Border Biosecurity research collaboration with additional funding from MPI, have been assisting the Department of Conservation and other industry partners in the response to an unwanted invader of the Nelson area.

​In May 2010, a species of butterfly never before recorded in New Zealand was discovered in a home garden in Nelson.

The arrival of great white butterfly (Pieris brassicae) raised concern because of its potential threat to a number of rare and endangered native Lepidium spp. and commercial and home brassica crops including forage brassicas for livestock feed.

The Department of Conservation (DOC) began an eradication programme in December 2012 with assistance from the Ministry of Primary Industries (MPI), Better Border Biosecurity and industry stakeholders. With the aim of eradicating the pest, DOC developed a two-pronged approach – eliminating outliers to limit further spread, while also suppressing the larger population found in residential gardens near the port.

AgResearch became involved in the project in mid-2012 when an eradication attempt was being considered and MPI and DOC asked the Biocontrol and Biosecurity team to provide technical advice, modelling and data analysis expertise.

“It has been an exciting, challenging and absorbing project from the start,” says AgResearch Senior Scientist Craig Phillips.

“The eradication attempt began with a fairly unsophisticated strategy, but with AgResearch assistance DOC continually refined it.

“Attempting to eradicate any pest is a lofty goal, requiring total commitment, excellent management, smart planning, effective techniques and intensive monitoring. Our team’s main focus has been on providing DOC with the knowledge it needed to target its butterfly control efforts at the right places at the right times.”

Monitoring the progress of the eradication programme was assisted by AgResearch’s work to analyse DOC’s inspection records to determine how the butterfly’s distribution changed and to track population trends through time. They also provided advice and modelling to the eradication team about how to prioritise their activities, and developed a phenology model for great white butterfly to predict its seasonality in Nelson.

AgResearch genetic expertise also played a role. The genetic diversity of the Nelson population was examined to help DOC track the movements of individual female butterflies in Nelson.

“Genetic information about butterfly movements in Nelson told us more about how far females were flying while laying eggs, which helped DOC refine its strategy for responding to detections of eggs and caterpillars,” says Craig.

The genetic analysis also gave insights about where New Zealand’s population came from (European origins), enabled rapid identification of specimens and provided the capability to determine – had any other populations been found in New Zealand – if they originated from Nelson or elsewhere.

The use of two tiny parasitoid wasps already present in New Zealand for biological control of small white butterfly was also vital to the eradication.

It was fortuitous that these parasitoids also attack great white butterfly caterpillars and pupae. AgResearch and Plant and Food Research provided DOC with thousands of individuals of both species for release in Nelson.

AgResearch then worked with the parasitoid that kills great white butterfly pupae. The team developed methods to mass rear it at Lincoln, and couriered them to DOC who released them in Nelson locations where there was a high probability great white butterfly pupae were present.

These various research projects were funded from a range of sources including AgResearch core funding via Better Border Biosecurity, MPI and the TR Ellett Agricultural Research Trust.

A key to the success of the programme has been the involvement of the Nelson community.

“The program relied on them notifying DOC when they discovered the pest, and DOC did a great job of fostering and maintaining support from Nelson residents,” says Craig.

“Engaging the community increased the number of people looking for great white butterfly from just 15-40 DOC staff to thousands, and made it much more difficult for the pest to go unnoticed.”

The official announcement has yet to be made by MPI that this pest has been eradicated, but all the signs are looking good according to Craig.

“The last sighting of the pest was in December 2014, he says.

“It was a lonely male butterfly looking for a mate. All the current evidence says the pest has been eradicated. The project is finalising the data analysis and having final discussions with MPI before that is officially signed off.”

DOC discontinued the eradication programme in June 2016, but is still relying on Nelson residents to report any possible sightings of great white butterfly.