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Summary of full report
Silk Production in Australia
by J.G. Dingle, E. Hassan, M. Gupta, D. George, L. Anota, and H. Begum
November 2005
RIRDC Publication No 05/145 RIRDC Project No UQ-96A
Despite the development of synthetic fibres to replace silk from the late 19th century onward, silk has maintained its appeal and value. It has many unique physical advantages and the lustre and drape of silk fabrics make silk the most attractive and highest priced natural fibre. The worldwide demand for silk is increasing but production is decreasing and an opportunity exists for Australia to use technical expertise to develop our own specialist silk industry.
Australian environmental conditions are very suitable for the development of sericulture, which includes both silkworm rearing and mulberry plant cultivation for silkworm food. Silkworms (Bombyx mori) and mulberry plants (Morus spp.) are widespread in Australia but little development work has been undertaken on these resources until the present. The first step in development is to identify the most productive strains of silkworm and mulberry plants. The second step is to try to decrease the risk of disease affecting production. The third step is to try to increase production. In the case of mulberry production this latter aim is multifaceted and involves finding the best conditions under which to grow mulberry plants and the most efficient and economical way to harvest them. This project examined these aspects as a basis for establishing an Australian sericulture industry.
Several attempts to establish a silk industry in Australia were made in the mid to late 1800s, but they failed due to lack of funds, poor mulberry stock, unsuitable land, disease and lack of knowledge.
The Japanese economic recovery after World War II was greatly aided by developing a silk industry second only to China. However, the Japanese silk industry in rapidly declining nowadays as industrial employment offers higher incomes. In many countries sericulture is a sideline industry that offers a means for people to increase their incomes especially where prices for rural products are low. It is possible for this to occur in Australia, but it is also possible that a highly sophisticated sericultural industry would be viable.
China produces about 75% of the world’s raw silk, which is valued at $1.6 billion. India produces about 15% and Korea and Turkmenistan produce just under 5% each. About 30 other countries produce commercial silk. However, it is difficult to compete with China in the international silk market and it is suggested that Australia should aim at satisfying some of its own silk needs first and then produce silk to supply niche markets overseas. In 2001 Australia imported $40 million worth of silk based products and over the last 10 years over $500 million worth of silk products have been imported.
There are 400-500 species of silk-producing moths in the world, but only 8 or 9 species are cultivated commercially and the domesticated mulberry silk moth (Bombyx mori) produces 99% of the world’s silk. Several countries have state sponsored sericultural research institutes and approximately 2000 strains of mulberry silkworm have been developed. Strains differ mainly in their preferred environmental temperature and in the number of generations per year. Temperate strains usually have 1-2 generations per year (uni- and bi-voltine), produce finer stronger silk and are mainly fed on Morus alba mulberry leaves. Tropical strains usually have 4-5 generations per year (multivoltine), produce coarser and weaker silk and are usually fed on Morus nigra mulberry leaves.
Within these varieties strains also differ in production in terms of eggs produced, growth rate of larvae, size of cocoon and length of thread in the cocoon.
Other factors that influence silk quantity and quality include the growing conditions of the mulberry plants and silkworms. Among these factors are photoperiodicity, number of moults per generation, population density, temperature, humidity and mounting materials. Twenty-one characters of silkworm can influence silk yield. Silkworm eggs can be stored at low temperature and can be artificially hatched.
Silkworms are susceptible to viral, bacterial and fungal diseases. Good hygiene and disinfection can keep growing areas clean but do not prevent the entry of infectious agents. Breeding for resistance is one way of decreasing the effects of infection. An additional hazard for any insect culture is the widespread use of a biological agent, the bacterium Bacillus thuringiensis (Bt) to control plant pests.
This agent will also kill susceptible B. mori. It is possible to increase the silkworm’s natural resistance to this bacterium by exposing to the micro-organism and breeding from the survivors.
In addition to direct selection of silkworm varieties for higher production and disease resistance, it is possible to obtain a boost in these features by crossing strains to produce hybrid vigour.
Sericulture
All the methods used to hatch and
rear silkworms and measure silk production are described.
Methods of measuring disease resistance and the methods used for selecting and breeding for increased disease resistance to Bt are described.
Comparison of two local Australian silkworm populations (QBill and QBite) with a population imported from Indonesia (Insab), showed that the two Australian populations had similar increases in body weight and length over the larval period, but the Insab population was superior in both characters. All three populations spent similar periods in the first four instar stages. However, Insab took two days longer to reach maturity in the fifth stage. Insab produced a heavier cocoon and a better shell ratio (cocoon shell weight: pupa weight) and greater number of eggs with a higher hatchability than QBill or QBite.
The QBite population had the highest resistance to Bt. However, susceptible individuals in the Insab population took longer to die. Both characteristics should be incorporated into a breeding program for the Australian silkworm industry. The third generation Bt tolerant Insab population (30-43% larval mortality) was crossed both ways with the Bt susceptible QBill population (60-85% larval mortality). The Insab-QBill (male x female) hybrid had the higher tolerance to Bt (35% larval mortality) and the QBill-Insab hybrid population had the lowest tolerance (87.5% mortality). These mortality percentages were similar to those of the male parent and the conclusion was drawn that the Bt tolerance gene(s) was on the male sex chromosome.
The variety of mechanisms involved in entering and terminating diapause (seasonal rest) enables several different procedures to be used to artificially terminate diapause. We developed three methods. (i) Cold (HCl) acid treatment was successful if eggs were refrigerated for 30 days, but the hatching period was prolonged. (ii) Hot acid treatment was successful if eggs were treated within 24 hours or refrigerated for 90 days. The hatching period of these eggs was much shorter. (iii) With no acid treatment, eggs did not hatch after 30 or 60 days refrigeration but they hatched naturally after 90 days refrigeration. All three silkworm populations produced over 97% hatch rates.
Even in tropical areas mulberry plants have seasonal growth and the leaves become tough and dry in autumn and winter. Two techniques were developed to overcome this seasonal unavailability of fresh mulberry leaves: (i) Refrigeration of fresh mulberry leaves in sealed plastic bags and (ii) Artificial diets. The artificial diets contained about 30% mulberry leaves to which were added bean powder, yeast, sucrose, cellulose, agar and water. Blocks of this diet were stored in less space than mulberry leaves and could be kept refrigerated until needed. Both refrigerated leaves and the artificial diet were used to successfully maintain larval development during autumn and winter.
Moriculture
Mulberry (Morus sp.) is a fast
growing, deciduous, deep-rooted perennial tree that grows throughout temperate,
sub-tropical and tropical regions. There are at least 24 species and 100
varieties that vary in habitat, yield and nutrient content. It is estimated
that one tonne of mulberry leaf is required to feed silkworms emerging
from one ounce (28.3 grams) of eggs, which will yield 25-30kg of cocoons.
One hectare of fertile land can yield 15-40 tonnes of mulberry leaves per
year; plants in temperate regions yielding half that of plants in tropical
regions. However, the higher cellulose content of tropical mulberries makes
the leaf less acceptable and less nutritious. Yield and nutritional value
are affected by soil type and plant density and they can be increased by
fertilisation and irrigation. Twenty kilograms of raw silk per hectare
can be produced from rain fed unfertilised mulberries using inferior silkworms;
whereas up to 120 kg of raw silk per hectare can be produced using the
best variety of mulberry with good cultivation techniques and a good silkworm
breed.
Harvesting is usually by picking leaves by hand or chopping branches using hand tools or machines.
All these factors were examined in this project and efficient cultivation and harvesting methods were developed.
Seven varieties of M. nigra (LV1-4, 6-8) and one variety of M. alba (LV5) were grown. LV2, 5 and 6 had faster growth, greater leaf mass, larger stem diameters, longer internodal lengths and fewer branches. Significant variation among varieties was apparent enabling selection for characters most suitable for various Australian conditions.
A cutting machine was designed, constructed and tested. After initial testing and modification, the finished model, which is fully described and illustrated in the text, was connected to a John Deere tractor (JD 1750) with hydraulic flow rating of 47L/min at 19 MPa maximum pressure. The field test was carried out on a single row of mulberry plants 60 m long. The harvester was adjusted to an initial cutting height of 1.6m using a lifting mast and the tractor speed was 1.6km/hr. Reeling in, cutting, transferring and collection in the bin of stems cut at two heights were satisfactory.
Implications and recommendations
This report described techniques and
methods suited to the selection and development of suitable silkworm and
mulberry varieties, and the cultivation and harvest methods that enable
a sericulture industry to be commenced in Australia. It is recommended
that this information be provided to potential silkworm farmers and that
the silkworm and mulberry varieties and harvesting equipment developed
be made available to this new rural industry. It is also recommended that
the breeding and development centre established at the University of Queensland,
Gatton, be funded to support the new sericultural industry.
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