PLANT OF THE WEEK

Dr. T. Ombrello - UCC Biology Department

 

THE MULBERRY TREE

AND ITS SILKWORM CONNECTION

 

Common names:      

There are several species of Mulberry trees in the United States, most of which are very similar in appearance.  Their common names often reflect their fruit color or area of origin. 

Examples:  White Mulberry (and a variety of this species, the Russian Mulberry), Black Mulberry, Red or American Mulberry

 

Scientific Names:     

Morus alba  -  White Mulberry

Morus alba tatarica  -  Russian Mulberry

Morus nigra  -  Black Mulberry

Morus rubra  -  Red or American Mulberry  

Explanation of scientific names:

Morus     -  the classical name for the Mulberry

alba         -  white

tatarica   -  for Tatary.  Historically this was a vast region of Central and Western Siberia and Southern Russia, invaded and inhabited by Tatar tribes.

nigra       -  black

rubra      -  red

Mulberry trees are not held in high regard among landscapers and homeowners in our area.  Small and short-lived, with a tendency to drop a multitude of soft, ripe fruits that stain everything beneath them, they are rarely chosen as landscape subjects.  The taste of the fruits is not appreciated by most people, but the fruits are occasionally made into jams and jellies or fermented to make something a bit stronger.  Wildlife ranging from birds to deer, however, find the fruit most attractive and they frequent the trees in the summer to consume the bountiful crop.  These animals eventually disperse the seeds far and wide, with the resulting Mulberry seedlings becoming deep rooted and difficult to remove weeds in cultivated areas.

A Weeping White Mulberry

growing on the Union County College campus in Cranford, NJ.

Where did all these Mulberry species come from?  

Some, like the Red Mulberry,  are native Americans.  Most species, however, were brought here to support a silk industry.  There is an intimate connection between the Mulberry and the silkworm.  This insect, the foundation of an industry that dates back over 4000 years to China, eats only one thing  -  the Mulberry and especially the White Mulberry.  The silkworm’s or Mulberry moth’s scientific name, Bombyx mori reflects this close relationship with mori being derived from Morus.

Silkworm larvae feasting on Mulberry leaves. Adult Silkworm with recently laid eggs.
   
Silkworm cocoons.

Note the hole in one of them, through which an adult emerged.

The silkworm has a relatively short life cycle, and if kept at 75oF would proceed as follows:

Eggs hatch and yield 1/8 inch-long larva that immediately start eating Mulberry leaves.  They must be fed constantly since they will not seek food and rarely wander away.  Over the next 2 months, they will reach a full size of 2 inches, having molted (shed their “skin” and growing) four times in the process.  They then spend 5 days spinning a cocoon.  The cocoon is composed of silk that is made in 2 silk glands within the body.  The single threads from each gland are formed into a double thread by the silkworm’s mouthparts and spun around the body, totally encasing it.  The continuous thread is hundreds of feet in length.  Within 5 days of completing the cocoon the larva changes into a pupa and after another 20 – 24 days emerges as a moth.  The moth escapes from the cocoon by emitting a liquid that digests away the silk enabling the moth to crawl out.  The adult moths have no mouth parts and while they can flutter their wings, they cannot fly.  After mating the male dies.  Soon after laying her 300 eggs, the female also dies.  The  domestication of this species for such a long period of time has made it very dependent on humans for survival.

In commercial silk operations, the silkworms are killed shortly after the cocoons are formed, since emerging adults would ruin the silk for spinning.  Traditionally, the thread is spun off cocoons soaked in hot water.  Eight to 12 cocoons spun off together make a sizeable thread.  The thread is then dyed and woven into cloth.  The remainder of the cocoon, made up of entangled silk not capable of being spun, can be used for a lower quality thread.

Why go to so much trouble for cloth?  The answer lies in the quality of what is produced.  Besides its luxurious texture, silk is 3 times stronger than flax, almost waterproof, not easily soiled, not readily burned, and quite durable in clothing.

Limited silkworm raising began in America in the late 1700’s, but it has a longer and more extensive history in China, Japan, Italy, France, and Spain.  Several notable scientists have had a hand in the development of the modern silk industry.  Luther Burbank, probably the most successful plant breeder of all time, developed a white Mulberry in the early 1900’s with twice as much foliage for Japanese silk growers.  Louis Pasteur worked out several techniques for raising disease-free silkworms after diseases ravaged the industry in Europe, and in 1870 published Les Maladies des Vers a Soie.

The mulberry-silkworm connection is a fascinating case of one species’ complete dependence on another for its existence and an interesting example of mankind’s exploitation of the arrangement.