Some information on the silk industry was adapted from the article by J. Kubota in the 3rd edition of this Encyclopaedia.
More than a million species of insects exist in the world, and the global mass of insects exceeds the total mass of all other terrestrial animals. Insects such as crickets, grasshoppers, locusts, termites, beetle larvae, wasps, bees and moth caterpillars are among about 500 species that form part of the regular diet of people around the world. Usually humans hunt or gather insects for food rather than intentionally rearing and harvesting them.
In addition to food, humans use insects as sources of pollination, biological controls of pests and fibre. Different uses depend on the four stages of the insect’s life cycle, which consist of egg, larva, pupa and adult. Examples of commercial uses of insects include beekeeping (nearly 1 billion tonnes of honey produced annually and pollination of fruit and seed crops), insect rearing (more than 500 species in culture, including those used for insect biological control), shellac production (36,000 tonnes annually) and silk production (180,000 tonnes annually).
Beekeepers raise the honey-bee in apiaries, a collection of hives that house bee colonies. The honey-bee is a source of flower pollination, honey and wax. Bees are important pollinators, making more than 46,430 foraging trips per bee for each kilogram of honey that they produce. During each foraging trip, the honey-bee will visit 500 flowers within a 25-minute period. The honey-bee’s source of honey is flower nectar. The bee uses the enzyme invertase to convert sucrose in the nectar into glucose and fructose and, with water evaporation, honey is produced. In addition, bumble-bees and cutter bees are grown for pollinating, respectively, tomato plants and alfalfa.
The honey-bee colony collects around a single queen bee, and they will colonize in boxes—artificial hives. Beekeepers establish an infant colony of about 10,000 bees in the bottom box of the hive, called a brood chamber. Each chamber contains ten panels with cells that are used for either storing honey or laying eggs. The queen lays about 1,500 eggs per day. The beekeeper then adds a food chamber super (a box placed on top of the brood box), which becomes the storage chamber for honey, on which the bees will survive through the winter. The colony continues to multiply, becoming mature at about 60,000 bees. The beekeeper adds a queen excluder (a flat panel that the larger queen cannot enter) on top of the food super to prevent the queen from laying eggs in additional shallow supers that will be stacked on top of the excluder. These additional supers are designed for harvesting only honey without the eggs.
The beekeeper moves the hives to where flowers are budding. A honey-bee colony can forage over an area of 48 hectares, and 1 hectare can support about two hives. The honey is harvested during the summer from the shallow supers, which can be stacked seven high as the colony grows and the bees fill the panels with honey. The supers with honey-laden panels are transported to the honey “house” for extraction. A sharp, warm knife, called an uncapping knife, is used to remove the wax caps that the bees have placed over the honeycombs within the panels. The honey is then extracted from the panels with a centrifugal force machine. The honey is collected and bottled for sale (Vivian 1986).
At the end of the season, the beekeeper winterizes the hives, wrapping them in tar paper to protect the colonies from the winter wind and to absorb the solar heat. The beekeeper also provides the bees with medicated sugar syrup for their winter consumption. In the spring, the hives are opened to begin production as mature honeybee colonies. If the colony becomes crowded, the colony will create another queen through special feeding, and the old queen will swarm with about half of the colony to find another accommodation. The beekeeper may capture the swarm and treat it as an infant colony.
Beekeepers are exposed to two related hazards from honey-bee stings. One hazard is sting envenomation. The other is venom hypersensitivity reaction and possible anaphylactic shock. Males at 40 years of age and older are at highest risk of fatal reactions. About 2% of the general population is thought to be allergic to venom, but systemic reactions in beekeepers and their immediate family members are estimated at 8.9%. The reaction incidence varies inversely to the numbers of stings received. Anaphylactic reactions to bumble-bee venom are rare except among bumble bee keepers, and their risk is greater if they have been sensitized to honey bee venom.
If a honey-bee stings the beekeeper, the stinger should be removed, and the sting site should be washed. Ice or a paste of baking soda and water should be applied to the site of envenomation. The victim should be watched for signs of systemic reaction, which can be a medical emergency. For anaphylactic reactions, epinephrine is administered subcutaneously at the first sign of symptoms. To assure safe beekeeping, the beekeeper should use smoke at the beehive to neutralize the bees’ protective behaviour and should wear a protective hood and veil, thin gloves and log sleeves or coveralls. Bees are attracted to sweat for the moisture, so beekeepers should not wear watch bands or belts where sweat collects. In extracting the honey, the beekeeper should keep his or her thumb and fingers clear of the cutting motion of the uncapping knife.
Mass Insect Raising
More than 500 species of arthropods are reared in the laboratory, including ants, beetles, mites, flies, moths, spiders and ticks. An important use of these arthropods is as biological controls for other animal species. For example, 2,000 years ago, markets in China sold nests of weaver ants to place in citrus orchards to prey on crop pests. Today, more than 5,000 species of insects have been identified worldwide as possible biological controls for crop pests, and 300 are successfully used regularly in 60 countries. Disease vectors have also become targets for biological control. As an example, the carnivorous mosquito from Southeast Asia, Toxorhynchites spp., also called the “tox” mosquito, has a larva that feeds on the larvae of the tiger mosquito, Aedesspp., which transmits diseases such as dengue fever to humans (O’Toole 1995).
Mass rearing facilities have been developed to raise sterile insects as a non-chemical pest-suppression tool. One such facility in Egypt rears a billion fruit flies (about 7 tonnes) each week. This rearing industry has two major cycles. One is the feed conversion or larval incubation cycle, and the other is the propagation or egg-production cycle. The sterile insect technique was first used to eliminate the screw worm, which preyed on cattle. Sterilization is accomplished by irradiating the pupae just prior to adult emergence from the cocoon with either x rays or gamma rays. This technique takes mass quantities of reared, sterile insects and releases them into infested areas where the sterile males mate with the wild, fertile females. Breaking the insect’s life cycle has dramatically reduced the fertility rate of these pests. This technique is used on screw worms, gypsy moths, boll weevils and fruit flies (Kok, Lomaliza and Shivhara 1988).
A typical sterile insect facility has an airlock system to restrict unwanted insect entry and fertile insect escape. Rearing tasks include mopping and sweeping, egg stacking, tray washing, diet preparation, inoculation (placing eggs into agar), pupae dyeing, emergence tending, packing, quarantining, irradiating, screening and weighing. In the pupae room, vermiculite is mixed with water and placed in trays. The trays are stacked, and the vermiculite dust is swept with a broom. The pupae are separated from the vermiculite with a sieve. The insect pupae chosen for the sterile insect technique are transported in trays stacked on racks to the irradiation chamber in a different area or facility, where they are irradiated and rendered sterile (Froehlich 1995; Kiefer 1996).
Insect workers, including silkworm workers, may have an allergic reaction to arthropod allergens (scales, hairs, other body parts). Initial symptoms are itchy eyes and irritation of the nose followed by intermittent episodes of wheezing, coughing and breathlessness. Subsequent asthma attacks are triggered by re-exposure to the allergen.
Entomologists and workers in sterile fly facilities are exposed to a variety of potentially hazardous, flammable agents. These agents include: in entomology laboratories, isopropyl alcohol, ethyl alcohol and xylene; in the diet preparation room, isopropyl alcohol is used in water solution to sterilize walls and ceilings with a sprayer. Vermiculite dust poses respiratory concerns. Some vermiculites are contaminated with asbestos. Air-handling units in these facilities emit noise that may be damaging to employee hearing. Proper exhaust ventilation and personal respiratory protection can be used in facilities to control exposure to airborne allergens and dusts. Non-dusty working materials should be used. Air conditioning and frequent changes of filters may help reduce airborne levels of spines and hairs. X rays or gamma rays (ionizing radiation) can damage genetic material. Protection is needed against x rays or gamma rays and their sources in the irradiation facilities (Froehlich 1995; Kiefer 1996).
Vermiculture, the raising of worms, has a long history in some cultures. Worms, especially the meal worm (which is a larva rather than a true worm) from the darkling beetle, are raised by the billions as animal fodder for laboratory animals and pets. Worms are also used in composting operations (vermi-composting).
Sericulture is the term used for silkworm cocoon production, which includes silkworm feeding and cocoon formation. Cultivation of the silkworm and the silk moth caterpillar dates back to 3000 BC in China. Silkworm farmers have domesticated the silkworm moth; there are no remaining wild populations. Silkworms eat only white mulberry leaves. Fibre production thus has historically depended upon the leafing season of the mulberry tree. Artificial foods have been developed for the silkworm so that production can extend the year around. Silkworms are raised on trays sometimes mounted on racks. The worms take about 42 days of feeding at a constant temperature of 25 °C. Artificial heating may be required. Silk is a secretion from the silkworm’s mouth that solidifies upon contact with air. The silkworm secretes about 2 km of silk fibre to form a cocoon during the pupal stage (Johnson 1982). After the cocoon is formed, the silkworm farmer kills the pupa in a hot oven, and ships the cocoon to a factory. At the factory, silk is harvested from the cocoon and spun into thread and yarn.
Nine per cent of silkworm workers manifest asthma in response to silkworm moth scales, although most asthma in silkworm workers is attributed to inhalation of silkworm faeces. In addition, contact of the skin with silkworm caterpillar hairs may produce a primary irritant contact-dermatitis. Contact with raw silk may also produce allergic skin reactions. For silk moth production, hyposensitization therapy (for moth scales and faeces) provides improvement for 79.4% of recipients. Corticosteroids may reverse the effects of inhaled antigens. Skin lesions may respond to topical corticosteroid lotions and creams. Oral antihistamines relieve itching and burning. Carbon monoxide poisoning has been identified among some silkworm farmers in their homes, where they are maintaining warmth with charcoal fires as they raise the silkworms. Charcoal fires and kerosene heaters should be replaced with electric heaters to avoid carbon monoxide exposures.