New Zealand Science Teacher

Land of the great white butterfly

11/03/2015

JAINE CRONIN writes about how the hunt for an insect pest is bringing the Nelson community together, in the latest in our 'Science in the community' series.

An observant pensioner first spotted great white butterfly (Pieris brassicae) caterpillars in her Nelson garden in 2010.These caterpillars, like their small white ‘cabbage’ cousins, love brassicas.

Great white butterflies are a bigger threat than cabbage butterflies because they lay large clusters of eggs and their caterpillars mob-feed on host plants, stripping them quickly. They are of particular threat to endangered and nationally critical cresses, some of which grow very close to the Nelson infestation zone.

New Zealand has 79 native cress species (nearly three per cent of New Zealand’s indigenous flora), of which 55 are currently threatened or at risk of extinction. Concerned about the impact of great white butterflies on endangered cresses, the Department of Conservation (DoC) took responsibility for the incursion response, called the Great White Butterfly Eradication Programme, in September 2012.  

Two examples of nationally critical plants that are particularly vulnerable, due to their small populations and proximity to the infestation zone, are coastal peppercress and chalk cress. Coastal peppercress (Lepidium banksii), related to Cook’s scurvy grass, with less than 150 known individuals, is confined to several sites in Tasman Bay. Chalk cress (Pachycladon fasciarium) is rarer than kākāpō, with only 37 known individuals. This cress is confined to the limestone bluffs of the Chalk Range, South Marlborough. 

Left: Great White Butterfly caterpillars on Honesty (Lunaria) plant. Photo: Richard Toft.

The eradication programme is a world-first attempt to eradicate a butterfly and is progressing well with an encouraging decline in great white butterfly detections. The Nelson Tasman community has been very supportive. Rangers spend time inspecting properties by searching brassica plants and nasturtium for eggs and caterpillars, butterfly catching and reducing host plants that are too hard to search.

In 2014, the department ran a Great White Butterfly School Hunt during spring. The focus was for schools to search for great white butterflies, with prizes being offered for the best presentation about their hunting experiences and what they found. Eight schools, from Atawhai to Richmond, registered for the hunt.In September 2013, the department ran a one-off bounty hunt, offering $10 for every great white butterfly caught. The hunt was timed to coincide with the spring breeding surge and the school holidays. The hunt was well received by children and adults alike, bringing in 134 butterflies and alerting DoC to new infestation locations within Nelson. 

Schools provided data about their catch efforts and a presentation. Presentations were judged and the final prizes went to Room 2, Richmond Primary School (first prize) for a comprehensive and hilarious video, Te Pouahi 16, Nelson Central School (second prize) for information posters, and Room 11, Victory Primary school for a large mural mapping their hunt sites. The students of Victory School were also responsible for the only adult great white butterfly (female) caught during the hunt period, although families continued to bring in great white butterfly eggs, caterpillars and adults after the hunt competition period finished.

In spring 2014, the department also hosted a Family Fun Day to thank the public for their support. The day was well attended, with fun family activities including a treasure hunt, dress up competition, pony rides, building butterfly traps, caterpillar races, bouncy castles and mini jeep rides.

(Left: Figure 1. Monthly comparisons.)

Comparing monthly detections between years gives a good indication of the declining population. In November 2013, the team made 227 great white butterfly detections, compared with twelve in November 2014. There were similar results for the month of December, where 220 detections in December 2013 dropped to just one in December 2014. 

The orange and purple graphs indicate the detections per month in 2013 and 2014 respectively. The dips in both graphs demonstrate when the majority of the great white butterfly population pupates over winter and summer. Both graphs indicate a rise in March and continued vigilance is needed to help find whatever remains of the pest this March. (Fig 1. Monthly comparisons)

When comparing seasons, it is clear that detections are trending downwards, including the dips and rises for each season. The small increase in summer 2014, compared with summer 2013, is due to an increase in the number of rangers searching for the pest.

(Figure 2. Seasonal comparisons.)

- Jaine Cronin is a communications and advocacy ranger at the Department of Conservation in Nelson.