The annual migration of the Brown-veined White Butterfly in South Africa takes place during midsummer every year. 
The sky is full of butterflies flying steadily in a North-easterly direction, as the annual migration of the Brown-veined White Butterfly hit especially the Central, Northern, and North-eastern parts of South Africa. Depending on climatic conditions like rain, drought etc. their numbers differ each year.  
Following them and preying on them, are hundreds of insect-eating birds, as well as many dragonflies (Anisoptera) out for a quick attack on a slow moving and low-flying butterfly. Also noticed were some butterflies caught in spider webs. 
The Brown-veined White also called Pioneer White or African Caper White (Belenois aurota) is of the Family Pieridae, with a wingspan of 45 millimetres, is known as South Africa’s most common butterfly and occurs in most areas in South Africa. Every year in midsummer (December or January) they gather in their millions, when they migrate in a north-easterly direction. 
It originates in the arid regions of the Karoo and Kalahari. These populations owe their strength to the main food plant of the caterpillars, the Shepherd’s Tree (Boscia albitrunca). The core populations are maintained by the females laying eggs on the Shepherd’s trees before they move off to migrate. The mass of white butterflies probably plays an important role in pollination but this is still poorly understood. In fact there is much that we still do not know about this widespread butterfly.
After crossing the provinces of the Northern Cape, parts of the Free State and North West Province on their journey northeast, they fly especially in the province of Gauteng as well as in parts of Mpumalanga province and also parts of Limpopo Province. Some years I have seen the sky white with the Butterflies as they flew past for about 3 days in uncountable numbers. This butterfly favours amongst others, the Shepherd’s Tree (Boscia Albitrunca) on which they breed. 
These butterflies start flying, and laying eggs, from a specific area in the South West – Kalahari (Northern Cape region). As they fly in a north-easterly direction, more and more join the flight. They also pause to lay eggs along the way. 
Little research has been done with regards to where exactly these butterflies fly to, but they have been noticed flying above the ocean near the Mozambique coast, where at the end of their journey, they most probably fall into the sea. They need not fly back to sustain the population, as eggs have already been deposited on the way. 
In reality the flight is an emigration and not really a migration, as they only fly one way and then die at the end of their journey.
Photography: Johan van der Walt