Executive summary of 2015-2016 report
Pieris brassicae, great white butterfly, is a Northern Hemisphere species that was first found in New Zealand in Nelson in May 2010. It is a threat to New Zealand native cresses as well as to forage and vegetable brassicas, thus the Department of Conservation (DOC) launched an eradication attempt on 19 November 2012.
To find Pieris brassicae in Nelson during 2015/16, DOC staff either searched sites (mainly residential properties), or responded to reports from the public. Nelson residents remained supportive of the programme and DOC gained access to all of the > 30,000 sites within the operational zone.
In 2015/16, DOC conducted 70,106 site inspections and found no P. brassicae. Detection rates (number of infested sites divided by number of searched sites) steadily declined from 0.048 in 2012/13, to 0.019 in 2013/14, 0.002 in 2014/15, and zero in 2015/16.
The distribution of P. brassicae also declined: the maximum distance from the centre of Nelson (–41.267, 173.278) that P. brassicae was detected was 24.1 km in 2012/13, 14.2 km in 2013/14, and 8.1 km in 2014/15.
The operational area covers 9,742 ha and, for operational purposes, DOC divided it into 46 management blocks. The number of blocks in which P. brassicae was detected declined from 38 in 2012/13, to 34 in 2013/14, 24 in 2014/15 and zero in 2015/16.
The last P. brassicae detected was an adult male captured near central Nelson on 16 December 2014. Since then:
- Sufficient time has elapsed for P. brassicae to complete six generations.
- DOC has conducted 106,545 general surveillance inspections of 29,445 different locations (mean of 3.6 inspections per site) situated 0–20 km from central Nelson. Search emphasis was on sites where P. brassicae host plants occur.
- DOC has continued to encourage the public to report possible sightings, and responded to all 41 reports received.
- DOC and Plant and Food Research have conducted 55 searches of native cress populations and brassica crops at 12 sites in Nelson-Tasman.
- Modelling indicated that there is a low relative probability of P. brassicae presence across all Nelson management blocks.
Thus, DOC is now confident that P. brassicae has been eradicated and the eradication programme was discontinued on 4 June 2016.