Introduction

The great white butterfly population in Nelson Tasman has been knocked down to a very low level but efforts continue to fully eradicate the pest, says the Department of Conservation.

The great white butterfly population in Nelson Tasman has been knocked down to a very low level but efforts continue to fully eradicate the pest, says the Department of Conservation.

A great white butterfly grey-white pupa.
A great white butterfly grey-white pupa

Just 12 detections of great white butterfly adults or young were recorded last month compared to 227 in November last year, even with a bigger search effort this year. There have been no detections for two weeks since a male great white butterfly was found on 28 November.

DOC Great White Butterfly Project Manager Michael Shepherd said, significantly, no eggs were found in November and the last find of caterpillars was on 17 November indicating butterfly breeding had been suppressed.

“It’s exciting progress to have achieved knockdown of the pest great white butterfly population. We couldn’t have got this far without the enormous support of the public and we thank the Nelson Tasman community for it.

“Some of the butterfly population would have pupated with some butterflies expected to emerge from pupae this month and others in February or March so our work continues to find and eradicate the butterfly. 

“We still need residents’ help in looking for pupae, eggs and caterpillars and reporting finds of these to the Ministry for Primary Industries Exotic Pest and Diseases hotline 0800 80 99 66 so DOC rangers can respond.”

Knockdown was the first of three phases of the programme to eradicate the pest butterfly that is only in Nelson Tasman and poses a threat to brassica crops and native cresses.

“Our eradication programme is now entering the ‘mop-up’ phase with the focus on finding and eliminating individuals and any small population flare ups that may occur,” said Michael Shepherd.

“The third and last phase of the programme is ‘surveillance’. This phase is reached when it is believed the pest has been eradicated but this needs to be confirmed with ongoing garden searches.” 

With summer holidays approaching, DOC is reminding people going away to check caravans, motor homes, campervans, trailers and boats for great white butterfly pupae as caterpillars can crawl into these to pupate. Any pupae found should be reported to the MPI hotline and DOC rangers will then respond.

The best way to check is to start at the top of the vehicle and work around it, particularly checking overhangs like windowsills and awning tracks, and then underneath around wheel arches.

The great white butterfly pupae lie flat on the surface (they do not hang) and are often hidden away such as inside rims. They are 25 mm long, distinctively dotted with yellow and black spots and have blunt-spiked ridges. The grey-white form is more commonly found on vehicles than the pale green form. 

Background

For achievement of the ‘knockdown’ phase of the great white butterfly population to be confirmed, the following criteria had to be met:

  • no evidence of the butterfly spreading beyond Nelson Tasman
  • the detection rate (properties infested divided by properties searched) across the search area being less than 0.01 percent (the overall spring detection rate was 0.005%)
  • no eggs or caterpillars found in at least half of the area searched.

Related link

Great white butterfly

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