African Caper White butterflies on the move now

African Caper White butterflies on the move now

The butterfly is called African Caper White in English, well known in Africa and south Asia for their mass movements.  While some species of butterfly do migrate (such as Vanessa cardui, which actually migrates from Africa to Europe and back), Belenois aurota populations seem to “erupt”.  That is, when populations reach a particularly high level and there is not enough food, the population, en masse, heads off to new foraging grounds.

Dr Colin Beale, (currently based in Tanzania studying climate impacts on savannah ecosystems) noted on his blog that, “There is a phenomenal butterfly migration happening in the Arusha area - from Ilboru I'm counting around 100-150 white butterflies heading west …every minute.”  He noted that the butterflies involved seem to be the African Caper White. He also expects that there could easily be some African Common Whites (Belenois creona) and African Migrants (Catopsilla fiorella). 

He tells us on his blog that these Belenois, as caterpillars, feed on plants of the Capparaceae family, which is why they are called Caper butterflies.  Common food plants in that family in Tanzania are Maerua and Boscia species – of which there are many on the Maasai Steppe.

He explains that movement like this happens every year to a certain degree, but in varying numbers.  Comparing this eruption to historic records he is saying using words such as “quite extraordinary” and “phenomenal”. In response to the excitement of this current movement, he has searched the academic literature and could only find single site records of the species’ movements.  

In order to record the event over time and space, he is organizing, through social media, multiple reports to try to understand this spectacular natural phenomenon.   He has invited “anyone in the region and seeing butterflies”. Note where you are, and that you saw movement of these butterflies, at what day and time. You could also add the direction, Dr Beale explains: Step one; Select a 10 or 20 metro section. Step two; Count how many butterflies pass through that space during one minute.” 

Then send all this information to  http://safari-ecology.blogspot.com/2012/02/mapping-butterfly-eruption.html. There is an interactive map at the site including icons showing the predominant direction in each site. By  February 8 three foci of activity had appeared on the map. 

The main and most spectacular reports were from along the Pare Mountains, heading westwards over Moshi and Arusha, carrying on west over Manyara; by the time the butterflies reach Eyasi they are heading South; there the trail was lost. Some movement was also reported in Kenya from Nairobi to Mt Kenya and along the Coast of the Indian Ocean. 

I sent a report: “While staying at Bagamoyo from  February 4 to 5, 2012, I saw along the coast, strong southward movement of white butterflies, at a rate of about 100 per minute. In Dar es Salaam, in my garden about 500 metros from the coast I noted a southward movement of white butterflies at about six per minute on February 6 at 7am.” 

On the map my observations have connected up with observations sent in by somebody from a place called “Tent with a View” near Saadani Game Reserve. Therefore a route has been noted from Saadani to Dar es Salaam along the coast.  Dr Beale did some calculations of how many butterflies might be involved. 

He calculated that they might be moving at 6-10 kilometers an hour, as is usual with similar species.  From the map that everyone has been contributing to, the movement front is at least 200 kilometers wide. If the butterflies are moving at a conservative density of 50 per minute for 5 days now, across the whole front, then each minute about 500,000 butterflies move across the whole 200- kilometre front. 

They've been moving for about seven hours each day, five days now which gives us about 1,050,000,000 butterflies on the move. More than one billion individuals - and they're still moving. He adds: Out of interest, I weighed a few butterflies and they come to about 0.2g each, which gives us about 315 tons of butterfly on the move. Or put it another way, with a wingspan of 5cm, put them all together and you can easily go around the earth!


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