Butterflies are appealing insects. We actively encourage them to visit our gardens, but white butterflies (Pieris rapae rapae) are an exception. We consider them unwelcome dinner guests because of the damage they do to broccoli, cabbage and other brassica vegetable plants. If we can look past their destructive tendencies, however, we find a hardy and fascinating butterfly different to any other we have in New Zealand.
An accidental introduction
The white butterfly is native to European and Asian countries. It has spread throughout much of the world aided by worldwide transportation and the common use of their food crops.
Its introduction to New Zealand is thought to have occurred in 1929. Most likely, it was transported in imported coolstore vegetables. White butterfly pupae are able to enter diapause – a period during which their growth is suspended in response to adverse environmental conditions. Pupae in diapause could have survived for many months on vegetables transported in refrigerated ships.
Establishment in New Zealand
A small number of butterfly species are recognised as occasional visitors to New Zealand. However, the white butterfly is one visitor that came and conquered!
Within 2 years after the first recorded sighting of a white butterfly in Napier, it had spread up to 200km, moving to wherever brassica crops were planted. White butterflies crossed mountain ranges and dense forests on the wing and were helped to the South Island via vegetable shipments. By 1935, the white butterfly was well established throughout the country. The lack of natural enemies allowed the species’ numbers to explode. Parasitic wasps were introduced to eat the larvae and, along with predation by birds, ladybird beetles and the occasional spider, these now keep the numbers under control.
Identifying males and females
Pieris rapae rapae is the only species of white butterfly in New Zealand so it is easily identified. The butterfly is white or creamy white with greyish/black wing tips. You can tell the males from the females by looking at their forewings. Males have a single black spot, and females have two black spots. Female wing colouration is also slightly yellower than in males.
It is interesting to note that the white butterfly’s physical characteristics are seasonal. The early spring generation of butterflies – those that overwintered in diapause – are smaller and have a reduced amount of black colouring on the wing tips. Summer generations have larger wings and darker pigmentation. These seasonal characteristics are not found in our native butterflies and are a direct result of the white butterfly’s European heritage.
White butterfly life cycle
Except for diapause, white butterflies follow a similar life cycle to other butterflies in New Zealand.
The female uses the sense organs on her feet to help locate host plants. She lays a single, bullet-shaped egg on the underside of a leaf. The egg’s whitish colour deepens to yellow as it matures. During warm weather (19°C+), the larva grows to maturity in just over 3 weeks, reaching a length of 30mm. The larva may travel away from the food plant to pupate. It spins a silk pad to anchor itself and then spins a silk strand around its middle for additional support. During the summer period, it takes about 11 days for metamorphosis to occur. Temperature has an effect on development rates. Warmer areas of the country – from Auckland north – produce an average of 5 generations of white butterflies annually. Wellington and Christchurch have 3 generations, while Invercargill has 2.
The shorter days of autumn trigger a change to the life cycle. Instead of metamorphosis, the pupa goes into diapause and remains in this state for 3–5 months. Native New Zealand butterflies usually overwinter in their larval stage, while monarchs and admirals overwinter as butterflies.
An educational pest
While horticulturalists and home gardeners consider the white butterfly a pest, it is nevertheless a useful educational tool - when swan plant food sources are in short supply, you can pick up a cabbage at the supermarket and grow white butterfly larvae instead of monarchs!
In this activity students find out more about the white butterfly life cycle and compare it to a monarch butterfly.
The giant white butterfly was first spotted in Nelson in 2010. It is destructive to native and cultivated brassica plants. However, it was successfully eradicated by 2016. Read about this world-first achievement in the Department of Conservation media release.
The University of Notre Dame has a citizen science project involving the white butterfly (Pieris rapae). Researchers will investigate genetics and diversity, how phenotype (colour, shape and size) has changed as butterflies adapted to new countries and if nitrogen fertilisers affect the wing colour. They ask citizen scientists to ship specimens to Notre Dame, Indiana in the US. (New Zealand residents do not need a permit to ship the Pieris rapae butterfly). Visit the Pieris Project website for further information.