A savvy 79-year-old Nelsonian has sparked a biosecurity alert that could have a serious effect on New Zealand's green crops.
Rae Herd, a member of the Monarch Butterfly NZ Trust, raised the alarm after her 74-year-old brother, Neil, told her he'd found some unusual caterpillars on the nasturtiums in his Cambria St garden.
Miss Herd popped around from her home in nearby Tasman St and gathered up eight of the brightly striped specimens.
She contacted Landcare Research, which suggested that Nelson lepidopterist John Dugdale take a look. His first-up identification brought about "all sorts of interesting phone calls back and forth", Miss Herd said.
The caterpillars were taken away by a MAF inspector from Port Nelson, and her brother was visited by officials from Auckland and Christchurch.
MAF Biosecurity response manager Bruce Philip said the find had been confirmed as the caterpillars of great white cabbage butterfly, Pieris brassicae, a significant pest on brassica crops such as cabbage, broccoli and swedes in Europe and India. There could be a breeding population in Nelson.
It had possibly come into New Zealand as a pupa in the chrysalis stage.
"Potentially, it could have pupated on somebody's furniture or something and they've shifted their personal effects to New Zealand, or it's got on to a container or something like that."
The other potential pathway was on imported fresh brassica produce, perhaps as eggs on leaves. The butterfly is not found in Australia, the largest source of imported brassicas, and there was doubt about existence in China, where increasing amounts of brassicas were sourced for New Zealand, Mr Philip said.
"We've got the very closely related white butterfly here, the very common one, so it's going to have a very similar host range and if this one established here, whether it would do a lot more damage than we're already getting with the white butterfly, we don't know.
"Whether the current management practices that are used for the white butterfly would also control the the new one, we don't know that either.
"All we can really say is that it's known to be a significant pest overseas and could potentially be a significant pest here."
The biosecurity response team was looking at the options for how to deal with the incursion, Mr Philip said.
"It's only been found in this one garden, but we don't know if it's very localised still. Because of the time of year, there's not going to be caterpillars out there; there's not going to be adult butterflies flying around."
The adult butterflies were likely to emerge in October. They are similar to the common white butterfly, but larger.
The caterpillars are black and white, with yellow stripes, grow to about 45mm, and tend to feed in groups. Chrysalises are likely to be found close to host plants on vertical structures such as fences and walls.
Mr Philip said Miss Herd had been "spot on the mark".
"For us it's a huge boon to have people like her with an interest in the subject and the knowledge to do something about it."
Miss Herd said she was pleased to have taken action. `You never know when you're going to get something coming in off the boats."
Her brother said the caterpillars were all within a very small area on one plant – about half a square metre. "We're in a straight line from the wharf," he said.
One suggestion had been that two butterflies flew from there and became romantically entwined in his garden "on a nice sunny day".
If you think you have seen the caterpillars or chrysalises, please phone MAF Biosecurity NZ on 0800 80 99 66.
- © Fairfax NZ News