Already in the 18th century, the Francke Foundations experimented with silkworm breeding. Prussian King Frederick II hoped to avoid dependence on expensive imports from China by producing his own silk. With a royal order and a donation of 330 mulberry trees, silkworm breeding began in 1744 and raw silk was actually produced. After initial successes, however, disease damage and adverse climatic conditions led to a slump in yields, so that silk production was completely discontinued in 1805. 213 years later, in 2018, Cornelia Jäger succeeded in hatching silkworms again. Within the framework of the environmental educational offers in the Botanical Garden, the children can experience the growth and transformation from egg to butterfly.
After hatching, the small caterpillars are very hungry. Several times a day they are supplied with fresh mulberry leaves so that they can grow and pupate after about a month. Fortunately there are still four mulberry trees on the grounds of the Francke Foundations. Because the hungry caterpillars devour vast quantities. Thanks to regular feeding and uniform climatic conditions, the caterpillars grow rapidly. After four to five weeks, the silkworms are big enough. They begin to spin their silk thread and pupate in their cocoon. The children have built racks to help the caterpillars pupate.
The cocoon consists of a single silk thread up to 900 meters long. The construction of the cocoon takes about 3 to 4 days. Then the caterpillar turns into a butterfly. Some of the silkworms did not manage to build a cocoon. In the wild they would probably be too obvious prey for predators. In the protection of our Botanical Garden, however, they have begun with the pupation process. The school garden children could observe how the caterpillar shrinks and discolours more and more.
The children had to wait three weeks before they could marvel at the miracle of metamorphosis: The first butterflies dare to come into the daylight and slip out of their cocoons. They fight their way out through a tiny hole in the cocoon, which they created with a corrosive intestinal fluid. As soon as the head has made it through the hole, everything goes very quickly and the silkworm is back as a butterfly. Immediately after hatching, the moths still seem quite dazed. They will not fly at all. They wait for their fellow moths to start mating. The mating of the silkworm moth takes several hours. Can the females be distinguished from the males? The children of the school garden have their own theory: »The females are thicker«, they conclude. This seems logical, especially if the females are about to lay their eggs. A single silkworm female lays between 400 and 500 eggs.
Shortly after laying the eggs, the butterflies die. The eggs can be stored for the next mulberry season, which begins in the Botanical Garden as soon as the trees sprout fresh leaves again.
The history of silk production in Prussia
At the beginning of the 18th century, the Huguenots brought the art of silk making from China to Prussia. Silkworm breeding was soon regarded as a magic formula to combat the Prussian foreign trade deficit and to restructure the state budget. The Prussian king therefore ordered mulberry trees to be planted all over the country to nest up myriads of hungry caterpillars, from whose cocoons valuable silk threads would then be extracted. On 17 February 1744, Frederick II (1712–1786) also ordered the Halle Orphanage to set up a mulberry tree plantation for silkworm breeding »for whose arrangement and dignified cultivation the orphans were to be used and constantly led [...]«. Under the direction of Gotthilf August Francke (1696–1769), a tree nursery with over 4,600 larger and 15,000 young mulberry trees was built in the following two years on the basis of 330 donated plants at the site where the skyscrapers in Voßstraße stand today. The silkworms can only be fed with their leaves. In the archive there are many records about this current topic of the 18th century up to drawings and historical technical literature. Three years later, the production of raw silk began here. In 1805, silk production had to be discontinued throughout Prussia due to the adverse climatic conditions. Silkworms are sensitive and actually at home in warmer regions.