Masses of Caper White Butterflies in a fly through of gardens, parks and roadways was reported from the Fassifern, Ipswich and Brisbane towards the end of last week.

​A migration of Caper White Butterflies is a natural phenomenon which happens on an irregular basis. Sometimes it’s an annual occurrence; other times it can be decades apart.

​In Sydney in January 1935 it was described as an “invasion”; over New England in November 1886 it was described as a “snow storm”; on the south coast of South Australia in December 1999, it was described as a “plague”; and in other times, in other places it’s been described as a “blitz”, a “blizzard” and an “unusual visitation”.

​These descriptions, and many others, are contained in newspaper reports from Queensland through to Victoria and South Australia from as early as the 1860s and all relate to the mass movement of the Caper White Butterfly.

​Some reports estimated the numbers in tens of thousands, others in millions; one description from Coraki, New South Wales, in December 1931, described “dense clouds of butterflies flying north in a continuous stream for five hours”.

​The trigger for the migration is thought to be the need to find a suitable food source for the Caper White larvae, Caterpillars. This follows an ‘eruption’ of the butterflies in their home territory in western areas due to weather conditions. The local food sources, plants of the Caper family, are depleted and the adults take wing to find new food.

​The mystery is not in the cause of the migration but rather in the routes that are followed as ‘streams’ of butterflies merge as they fly in from western areas towards the coast, often to places where there are no Caper bushes.

​Some believe the butterflies carry a genetic memory of these flight paths from generation to generation and any divergence is slow in evolving. Others suggest that as the butterflies usually fly inland rather than towards the coast, those which are reported from coastal areas have been blown off course.

​It’s estimated that the migration can cover as many as 3,000 kilometres. Some are blown off course and there are reports of tens of thousands being washed back to shore after exhaustion causes them to falter over the ocean.

​In early years of settlement and even as recent as fifty years ago, it was thought the appearance of the Caper White Butterflies would precede mass depredation on vegetable crops. However, if the Caper Whites land on garden plants or crops, it is to rest before continuing their flight - unless of course they find a Caper bush or vine on which to lay their eggs.

​The most complete newspaper description we could find of one of the migrations was in a report in the Fassifern Guardian from the Maroon district in November 1921.

​“They came from the south and were all day, for two days, going in a northerly direction.

​“They flew past like an endless army, all heading the one way and never diverting from their course and between 3.00pm and 4.30pm they were thickest and were massed in thousands and thousands.”

​The correspondent went on to describe the butterflies as … “all the same variety - a bright white with black lines on their wings and some with little yellow colouring near the base of the wings.”

​“These travellers did not settle on the flowers of which there were plenty or on the lucerne plants which were blue with flowers.

​“They showed no sign of halting, but kept on going, flying at a rapid rate as though their very life depended on getting to a certain place in a given time.”