Table of Contents
What are Silkworms?
The silkworm (Bombyx mori) (Lepidoptera: Bombycidae) is one of most economically important insect species in human history for its production of silk (sericulture). This moth species was domesticated over 5,000 years ago in Ancient China, with none currently existing in the wild. White silkworms were selectively bred for their white silk, leading to their characteristic color seen today. The silkworm is also used as a model insect in many scientific studies (like Drosophila melanogaster and Manduca sexta).
Silkworm Life Expectancy
These species are holometabolous, meaning that they go through a complete metamorphosis—egg, larva, pupa, and adult. The average lifespan of a silkworm, from caterpillar to moth, is about seven weeks (40–50 days), with the caterpillar pupating in about four to six weeks after hatching from an egg. The silkworm has five instars, or growth stages, which are separated by four molts. The instar duration is as follows: 3–4 days for the first instar, 2–3 days in the second instar, 3–4 days in the third instar, 5–6 days in the fourth instar, and 6–8 days in the fifth instar. After the fifth instar, the larvae will begin to pupate by creating silk cocoons.
How to Keep your Silkworms
Temps and Humidity
Storage of your silkworms is best carried out in large plastic containers lined with cardboard in rooms where temperature, relative humidity, and photoperiod can be controlled. For optimal growth, it is best to store silkworms at temperatures of 77°F (25°C), with a relative humidity of at about 80%, and a photoperiod of 16 hours light to 8 h dark. Lower temperatures will inhibit physiological functions, thus delaying molting and preventing pupation. Therefore, temperatures of 68°F (20°C) can be safe for the insect and can possibly delay growth. Remember that insects are cold-blooded animals, meaning that they rely on the environment to control their body temperature. Thus, if storage temperatures fall below 68°F (20°C), physiological processes are slowed, especially in early instars. This will lead to worms that are too weak to survive and increase their susceptibility to various diseases. On the other hand, temperatures that are too great (greater than 86°F [30°C]), will directly affect the health of silkworms, causing heat stress and death. Humidity levels of about 80% are suggested as they may help with prolonging the larval stage, because silkworms require drier environments to pupate. Be sure to keep your container at a constant temperature and humidity.
Because caterpillars are voracious eaters, they will also excrete a comparable amount of waste (frass). It is recommended to clean the container after every instar (day estimate listed above); however, in the later instars (fourth and fifth), the container might need two cleanings. If you grow plants, be sure to save the silkworm’s droppings, as they can be used as a biofertilizer. Research indicates that the chitin present in the droppings can induce disease resistance in plants.
When rearing your silkworms, you will find that they constantly eat—only stopping to molt. Caterpillars eat enough to reach a weight threshold, which spurs a hormonal cascade, causing the caterpillar to shed its head capsule (the only hardened portion on the caterpillar’s body). The silkworm is a highly specific feeder, preferring to eat leaves of the mulberry tree. Although not the optimal choice, other fresh leaf tissue can be used in a pinch. For example, beetroot leaves or lettuce leaves can be used when your fresh mulberry leaves are out. Additionally, you can create an artificial diet for the silkworms using equal parts of hydrated mulberry leaf powder and soybean powder.
Depending upon your goals, you may want to use them as feed during their third instar or wait until they become fat during their fifth instar. Regardless, it is best to keep the caterpillars fed at all times, removing dried/moldy leaf material or food pellets as necessary. During the first few stages, the silkworms will likely eat very little; you can expect larvae to weigh about 0.3 g at the third instar. During the later instars, the silkworms will eat double their mass in leaf material—weighing up to 3.5 g for fifth instars.
Please be advised that stressing the insects through food shortages to prolong the larval stage is not recommended, as this could lead to death or cause silkworms to enter the pupal stage earlier.
It must be stated that while suggestions can be given for the care of silkworms, one must remember that these are living creatures and each one will act differently with specific stimuli.
It is recommended to keep your silkworms fed at all times and clean their container after every molt. It is also recommended to keep your silkworms at temperatures of 6875F (2025C), with humidity levels of about 80%. Although, to prolong the larval stage of your insects, you might find success at keeping the temperature around 68°F (20°C).
Colin Bonser, Entomology PhD
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