Margaret Campbell, of Oamaru, asks :-
Many Monarch butterflies hatched in the Oamaru Gardens. Where have they gone?
George Gibbs, an entomologist at Victoria University of Wellington, responded.
The straightforward answer is that they normally do not persist south of Marlborough or Nelson in the South Island. This means that although individuals can roam further south during warm summers, and they may breed a summer generation or two, the climate is marginal for them and they are unlikely to overwinter there. Thus in Canterbury and Otago the monarch does not normally succeed as a viable breeding population. I am suggesting that in Oamaru perhaps you do not see monarchs every year. When the spring-summer is favourable (or when people bring caterpillars in from further north!) you will see them around the gardens but normally they will perish at the end of the season. This is what I mean by marginal for them. Some favourable years they do well, but then several years may pass without seeing any.
There are some problems with this simple answer. First, in North America where they originated from, they are famous for their long distance migrations from Canada to Mexico. Migrations have not been recorded in New Zealand but may eventually develop so that butterflies bred in the south will fly north for the winter. Either way, they will disappear from Oamaru gardens in autumn. Second, the assisted movement of monarch butterflies by people will mess up their natural behaviour. I am aware that some enthusiasts in Dunedin breed up summer generations for release. These efforts will give you summer monarchs but are unlikely to establish a permanent population.
Monarch butterflies survive for several months as adults. Thus the individuals you see in April may have been caterpillars back in December and freshly hatched butterflies in autumn will survive through until October-November (in northern regions where they can overwinter). The caterpillars do not survive cold winter weather and adults are unable to lay eggs between about April and October.