Shellac Insects
The shellac insect is a type of scale insect, representing a category of insects with significant economic value. These insects have over 300 host plants and live on these hosts, from which they extract plant sap. The female insects secrete shellac—a pure, natural resin—through their glands. Shellac insects are primarily found in tropical and subtropical regions of Southeast Asia, South Asia, Oceania, Africa, and South America, including countries such as India, China, Pakistan, Bangladesh, Thailand, Myanmar, and Indonesia. In China, they are mainly distributed in the southern regions of Yunnan Province, typically producing two generations per year. In traditional Chinese shellac production, the generation from October of the current year to May of the following year is referred to as the winter generation of shellac, while the generation from May to October is known as the summer generation. The winter generation primarily provides seed lac for the summer generation, which is the main season for shellac production. Shellac insects move only once in their lifetime, from hatching from the egg to finding a suitable branch on the host plant to settle on. They insert their mouthparts into the host's epidermis and remain stationary for life. After undergoing complete metamorphosis, the male insects emerge with limited flying ability, either winged or wingless, and typically mate with nearby females before dying. Once the female insects are fixed on the host plant, their legs and thorax degenerate as they grow, and their bodies are enveloped by the secreted shellac. After mating, the females become pregnant and give birth to their offspring through ovoviviparity.
There are more than five species of shellac insects used for shellac production worldwide, including:
· For China, the Yunnan shellac insect (Kerria yunnanensis) is utilized.
· In India, the production species is the lac insect (Kerria lacca).
· Thailand employs the Chinese shellac insect (Kerria chinensis) for its shellac production.
· The Sind shellac insect (Kerria sindica) is used in Bangladesh.
· Pakistan's shellac production relies on the fig tree shellac insect (Kerria fici).
· In Myanmar, the shellac production species include the Pusana shellac insect (Kerria pusana) and the Nepal shellac insect (Kerria nepalensis).
Yunnan Shellac Insect
The Yunnan shellac insect is primarily found in the subtropical regions of Yunnan, China, and has been introduced to other areas including Guangxi, Guangdong, Fujian, Guizhou, Sichuan, Jiangxi, Hunan, and Hainan. This species is the main type used in China's shellac production and is closely related to the Chinese shellac insect, leading to significant taxonomic controversy.
Historically, this species was considered to be the same as the lac insect (Kerria lacca) (Liu Chongle, 1958). However, subsequent comparisons with specimens of Kerria lacca led to the conclusion that they were not the same species. Since then, this insect has been tentatively classified as the Chinese shellac insect. In 1990, Ou Bingrong and others named it the Yunnan shellac insect (a new species), and this classification has been supported by hybrid experiments, molecular markers, and chromosome studies (Chen Xiaoming et al., 1992, 1995; Chen Hang et al., 2006, 2007), affirming the Yunnan shellac insect as a distinct species.
Life Cycle of the Yunnan Shellac Insect
In the subtropical regions of Yunnan, at the Jingdong Experimental Station of the Institute of Resource Insects, the Yunnan shellac insect produces two generations per year. The summer generation spans from May to October, lasting approximately 150 days, while the winter generation extends from October to the following April, approximately 210 days.
· The first instar of female larvae lasts about 20 days, the second instar about 15 days, and the third instar also about 15 days.
· Male larvae in their first instar last about 20 days, the second instar about 18 days, with the pre-pupal and pupal stages lasting about 12 days, and the adult stage about 8 days.
· The first instar of female larvae lasts about 50 days, the second instar about 45 days, and the third instar about 30 days, with the adult stage lasting about 90 days.
· Male larvae in their first instar last about 50 days, the second instar about 60 days, with the pre-pupal and pupal stages lasting about 20 days, and the adult stage about 15 days.
The secretion quantity of shellac is higher in the summer generation than in the winter generation (Ou Bingrong et al., 1984). The growth and development cycle of the shellac insect is primarily influenced by climate, with temperature being the most significant factor. The life cycle of the Yunnan shellac insect varies by location and year, and development rates also differ on various host plants. The start times of each generation may advance or delay, with April to June and September to November serving as the transition periods between the two generations. Prolonged release times can lead to generational disarray.
Settling Density of the Yunnan Shellac Insect
After hatching, first instar larvae of the Yunnan shellac insect emerge from the mother and seek suitable branches to settle on. The settling density of these insects is between 180 to 230 individuals per square centimeter, which is the highest among all shellac insect species. This high density underscores the prolific nature of the Yunnan shellac insect and its potential for substantial shellac production under optimal conditions.
After dispersing onto tender branches, the Yunnan shellac insect larvae begin to settle within approximately one hour. If not disturbed by strong winds or heavy rain, the vast majority of larvae will have settled after about 20 hours, with only a very few taking up to 24 hours to begin settling. The Yunnan shellac insect exhibits significant selectivity in its choice of settling and feeding sites.
The Yunnan shellac insect prefers to settle on more tender branches, typically choosing 1-year-old branches for this purpose. Generally, the insect does not settle on branches of Pterocarpus macrocarpus (Siamese Rosewood) that exceed 10cm in diameter. On Millettia laurentii (Wenge), the preference is for branches not exceeding 8cm in diameter; on Dalbergia odorifera (Fragrant Rosewood), the limit is 5cm in diameter; and on Entada phaseoloides (St. Thomas Bean), the preference is for branches not exceeding 3m in length.
This specificity in habitat selection underscores the importance of understanding the ecological preferences and behaviors of the Yunnan shellac insect for effective cultivation and shellac production.
Overloading the branches with seed lac can lead to high settling densities of Yunnan shellac insect larvae, resulting in a significant portion of the larvae dying after settling. The primary reason for this phenomenon is that the larvae that settle later cannot find suitable positions and space for feeding, leading to starvation and death. The tolerance of different host plants to shellac insects varies, and generally, an infestation rate of about 60% on effective branches (branches suitable for the growth of shellac insects) is considered optimal.
This situation highlights the importance of managing the quantity of seed lac placed on host plants to ensure sustainable shellac production. It underscores the need for careful planning and execution of shellac cultivation practices to maintain healthy populations of shellac insects and to maximize the yield of shellac resin without compromising the well-being of the host plants or the insect population.
During the larval stage, Yunnan shellac insects secrete relatively small amounts of shellac, whereas adult females secrete significantly more. The shellac secreted by adult Yunnan shellac insects gradually forms a protective coating or shell, covering both the insects and the branches of the host plant. Both male and female larvae are capable of secreting shellac, but males secrete much less. By the pre-pupal, pupal, and adult stages, males cease shellac secretion entirely and die after mating. In contrast, adult females continue to secrete shellac after laying eggs, maintaining this process until their offspring disperse.
Female Yunnan shellac insects exhibit different shellac secretion characteristics at various developmental stages. The rate of shellac secretion during the larval stage is 3.85x10^-5 mg/day, in the early adult stage it increases to 1.79x10^-3 mg/day, during the first and second egg-laying phases it reaches 3.11x10^-3 mg/day, in the third and fourth egg-laying phases it is 1.46x10^-2 mg/day, and during the fifth and sixth egg-laying phases, the rate is 7.06x10^-3 mg/day. The peak of shellac secretion occurs during the third and fourth egg-laying phases.
The amount of shellac secreted by Yunnan shellac insects begins to increase significantly in the early adult stage, with the largest quantities being produced in the middle to later stages, reaching a peak during the middle phase of the adult stage (Chen Xiaoming et al., 1993). This pattern underscores the importance of adult female Yunnan shellac insects in the production of shellac, highlighting the need for careful management of the insect population to optimize shellac yield.
The secretion of shellac by the Yunnan shellac insect varies across different generations, ecological environments, and host plants. Notably, the winter generation of the Yunnan shellac insect produces less shellac than the summer generation. During the summer, the thickness of the shellac coating generally ranges from 0.4 to 0.8 cm, with individual secretion amounts between 14-16 mg.
Additionally, the Yunnan shellac insect exhibits a wax-secreting behavior. Wax secretion is most pronounced during the second and third instars of female larvae, with wax filaments reaching lengths of over 3 cm. This wax production plays a crucial role in the lifecycle and survival of the insect, contributing to the protective layer that encompasses the insect and its immediate environment on the host plant.
Sex Ratio
In the Yunnan shellac insect population, the proportion of females is higher than that of males. During the summer generation, females constitute approximately 75% to 80% of the population, while in the winter generation, the proportion ranges from 50% to 78%. The sex ratio in shellac insects is a complex issue, primarily influenced by the genetic characteristics of the species. Environmental factors may also contribute to variations in the sex ratio of shellac insects. This skewed sex ratio towards females is advantageous for shellac production, given that female insects are the ones responsible for secreting shellac. Understanding and managing the factors that influence the sex ratio can be crucial for optimizing shellac yield.
The Yunnan shellac insect is characterized by its high fecundity, with adult females having a substantial capacity for egg-laying. Typically, the number of eggs laid by a female during the winter generation ranges from 200 to 400 eggs, while in the summer generation, the egg-laying capacity increases to between 300 and 700 eggs per individual. This high reproductive capacity is a significant factor in the sustainability and productivity of shellac cultivation, as it ensures a continuous supply of shellac-producing insects. Effective management of the breeding and lifecycle of the Yunnan shellac insect can thus significantly impact shellac production volumes.
Principal Host Plants
The lac insect in Yunnan has over a hundred host plants, with more than ten commonly used in production. These include the pigeon pea, Dalbergia obtusifolia Prain (commonly referred to as the beefwood), Pterocarpus macrocarpus (Burmese padauk), Erythrina variegata (Indian coral tree), Eriolaena spectabilis, Pueraria wallichii (a species of kudzu), Ficus racemosa (cluster fig tree), and Ficus cunia. These plants are integral to the cultivation and production of shellac, providing the necessary environment and nutrients for the lac insects to produce the resinous substance.
Lac Insect Cultivation Techniques
The fundamental techniques for harvesting and pruning brood lac are essential for its cultivation. The cultivation of lac insects typically involves four key steps in the production of shellac: pruning for cultivation, releasing the insects, and retrieving the brood lac.
1、Harvesting Brood Lac At the stage when lac insects reach maturity, it is crucial to accurately predict and prepare for the harvesting of brood lac. Especially towards the end of the lac insects' growth and development phase, regular observation of the egg embryo development is necessary. This helps in predicting the dispersal period of the next generation of larvae. The methods employed include the "Scratch Method," "Sand Method," and "Yellow Spot Method."
1. The Scratch Method refers to the practice of harvesting brood lac when larvae begin to disperse and crawl on the surface of the brood lac, indicating it's time to collect.
2. The Sand Method is based on assessing the developmental progress of lac insect embryos to determine the appropriate time for brood lac harvesting. This method is generally used for brood lac that needs to be transported over long distances.
3. The Yellow Spot Method involves judging the right time for harvesting based on phenological signs on the lac surface. Typically, the appearance of lighter yellow spots on the surface of the lac indicates that the brood lac is nearing maturity and can soon be harvested.
2、Pruning of Brood Lac The harvested brood lac should be pruned into specific shapes and lengths to facilitate the cultivation of lac insects. Typically, the brood lac is pruned to lengths of 30 to 50 cm, with both ends shaped into a "horseshoe" or slanted cut. This shape and size are considered optimal for the subsequent steps in the cultivation process.
3、Inoculation The pruned brood lac is usually affixed to the branches designated for cultivation using ropes or similar binding methods. The selection of the binding site on the host plant should take into consideration factors such as the tree's shadow and height. The quantity of brood lac to be inoculated depends on the quality of the brood lac, the availability of suitable host plant branches, and the varying resistance to insects among different host plants. This step is critical for ensuring that the lac insects are effectively established on the host plants, thereby maximizing the production potential.
4、Retrieval of Brood Lac After the larvae from the inoculated brood lac have largely dispersed, it is necessary to retrieve the brood lac that was hung for cultivation. Subsequently, the raw lac is peeled off from these collected brood lacs. This step is crucial for completing the cycle of lac cultivation and for preparing the raw material that will be processed into shellac.
Shellac Insect Secretion Quantity
The measurement of shellac secretion quantity is a crucial indicator of the shellac insect's production value. In research on shellac insects, the individual secretion quantity is commonly used to assess the secretion status of these insects. The method for determining individual secretion quantity involves the following steps: After the female adults have matured, branches covered in shellac are collected, and the shellac coating is peeled off and divided into small pieces. Each piece of shellac is weighed using an electronic scale and then dissolved in 95% alcohol in separately numbered containers. Once the shellac is completely dissolved in the alcohol, the shellac insects are filtered out and counted. After the alcohol has evaporated and the shellac insects have naturally dried, their weight is measured using an electronic scale. The individual secretion quantity is calculated using the following formula:
Individual Secretion Quantity = [Weight of the Shellac Piece (including the insect body) - Weight of the Shellac Insects] / Number of Shellac Insects in the Piece
A random sampling method is typically employed for this measurement, with a general sample size of more than 50 individuals.
Shellac
Shellac, secreted by the shellac insect, is an important natural raw material for the chemical industry, possessing significant economic value. After processing, shellac is extensively used in various industries including chemicals, pharmaceuticals, food, electronics, and cosmetics.
Shellac is primarily a natural resin secreted by the shellac insect. The raw resin appears in shades of purple-red, golden-yellow, or yellow. The density of shellac flakes ranges from 1.43 to 1.207, with a molecular weight between 964 and 1100. Shellac exhibits commendable tensile strength, wear resistance, and hardness. It is a typical thermoplastic resin with a melting point ranging from 77 to 90°C.
Shellac resin has unique electrical properties: it has a low dielectric constant and an unusual characteristic of becoming non-conductive after being subjected to an electric arc, making it suitable as an insulating material. Shellac is soluble in various organic solvents but insoluble in water. However, by injecting its alcoholic solution into water and evaporating the alcohol under pressure, or by dialyzing its ammoniacal solution, a water-soluble form can be produced. Shellac can also be made into a water-soluble resin solution by heating it in a strong alkaline solution.
Composition of Raw Shellac
The primary component of raw shellac is shellac resin. Additionally, it contains shellac wax, shellac pigment, sugars, proteins, salts, and other extraneous impurities. The content of these substances varies depending on the production area, host tree species, harvesting season, and type of lac insect.
The various substances in raw shellac can be separated based on their solubility in different solvents.
Main Components of Shellac
Shellac resin is the primary constituent of shellac. It is generally considered to be a solid solution composed of lactones and esters of hydroxy carboxylic acids, with a highly complex structure. The structure is widely recognized as an elastic network formed by esters of hydroxy fatty acids and sesquiterpene acids, containing a mixture of low molecular weight fatty acid esters in its voids, some of which act as plasticizers. The average molecular weight of the resin is around 1000, and its molecular formula can be represented as C60H90O5. It is an acidic resin. The chemical composition of the resin is also quite complex. The main component, identified through the alkaline hydrolysis of the resin, is aleuritic acid. Additionally, shellolic acid, shellac acid, and butolic acid are present. Indian scientists have isolated and identified five new shellac acids from the hydrolysis products of the resin, namely butolic acid, shellolic acid, shellac acid, aleuritic acid, and shellolic aldehyde acid.
Shellac pigment is a metabolic product of the shellac insect. There are two types of pigments in shellac: one is water-soluble, known as shellac color acid (or shellac red pigment), which constitutes about 1.5% to 3% of the raw shellac, mainly found within the body of the shellac insect. During the shellac processing, it can almost be completely washed out with water. The other type is the water-insoluble red shellac pigment (also known as shellac yellow pigment), which is less abundant, about 0.1%. During the processing of shellac, because it is insoluble in water and dissolves in solvents such as ethanol along with the resin, this is why shellac resin products appear yellow, orange, or orange-brown.
Also known as red shellac dye or laccaic acid, it is the most significant substance in the aqueous extract. When washing raw shellac with water, it can almost be completely extracted. This red dye is acidic and is a type of anthraquinone derivative, composed of a mixture of five shellac color acid components: A, B, C, D, and E.
Shellac color acid is also soluble in methanol, pentanol, propanol, propylene glycol, acetic acid, and formic acid, but insoluble in ether, chloroform, and benzene. It dissolves slowly in ethanol and can also dissolve in alkaline solutions such as sodium bicarbonate, sodium carbonate, and sodium hydroxide. It is generally believed that shellac color acid may exist in shellac in the form of water-soluble sodium and potassium salts. It is highly unstable in alkaline solutions and prone to decomposition, which is why the refinement process of the pigment must be conducted in an acidic medium.
Shellac color acid forms colored precipitates with metal ions other than alkali metals and turns blue-purple upon reacting with proteins and iron ions. In acidic aqueous solutions, shellac color acid appears orange-red, while in alkaline solutions, it turns red-purple.
This is a water-insoluble pigment with the structure of 1,2,5,7-tetrahydroxy-4-methyl anthraquinone.
Erythrolaccin forms yellow needle-like crystals, insoluble in water but soluble in ethanol, ether, benzene, toluene, chloroform, and acetic acid, showing a red color. It turns distinctly purple when dissolved in alkalis. In alkaline aqueous solutions, it can be bleached by hypochlorites or decolorized with activated carbon. This characteristic has become the basis for the production of bleached shellac and decolorized shellac flakes both domestically and internationally.
3. Shellac Wax
Shellac wax, also a metabolic product of the shellac insect, constitutes about 4% to 7% of the raw shellac. As indicated in Table 13-1, the wax content in granular and flake shellac ranges from 4% to 5.5%, while in dewaxed shellac, it is less than 0.5%. Wax is a by-product of shellac processing that has found industrial applications.
Shellac wax is a hard wax characterized by a high melting point, significant hardness, excellent luster, strong solvent retention, and low permeability. It acts as a natural plasticizer for shellac resin, enhancing the adhesive properties of shellac film coatings.
Wax made from shellac, when sprayed or brushed on eggs, fruits, and vegetables, can reduce their natural evaporation of moisture and prevent bacterial invasion from the outside, thereby maintaining the freshness of eggs and fruits and extending their storage life.
4、Other Components
In addition to the resin, pigments, and wax mentioned above, shellac also contains a small amount of other components, such as sugars, proteins, salts, wood particles, insect remains, and a fragrant substance released during the melting of shellac. To refine the shellac resin as much as possible, all inorganic and organic impurities are to be removed during the shellac processing.
Physical Characteristics of Shellac
Shellac resin products are hard and brittle amorphous substances. Except for specially processed bleached and dewaxed decolorized flakes, resin products, due to containing varying amounts of shellac wax and pigments, often appear semi-transparent light yellow, dark orange, orange-brown, and almost opaque dark red. When heated or melted, they emit a distinctive aromatic odor.
1. Mechanical Properties
Shellac resin products exhibit good tensile strength, wear resistance, and hardness, as shown in Table 13-6. Therefore, incorporating shellac resin products into rubber and other synthetic resins as fillers can improve their mechanical and processing properties. This includes enhancing the strength and hardness of the products, improving their resistance to acids and oils, and reducing shrinkage.
Shellac resin products have strong adhesive properties, making them an ideal material for preparing various adhesives. They are widely used in military, electrical, heavy industry, and watchmaking industries.
2. Thermal Properties
Shellac resin products are typical thermoplastic resins with relatively low softening and melting points. This characteristic is exploited when shellac flakes are mixed into rubber rollers during milling; the heat from the rollers melts the shellac, allowing it to blend uniformly with the rubber, thereby enhancing the rubber's tear resistance, tensile strength, strength, and hardness.
Flowability and thermal life (thermal polymerization time or thermal curing time) are other important thermal properties of shellac resin. Resin products that have aged or polymerized generally have a shorter thermal life and reduced flowability. These changes directly affect the use of shellac products in the defense industry and the electrical insulation materials industry.
The thermal conductivity of shellac resin products is similar to that of paraffin, asbestos, and paper, indicating that the heat dispersion caused by conduction in these insulators in electrical applications is entirely comparable.
3. Electrical Properties
Shellac resin products are characterized by high dielectric strength and low dielectric constant. Particularly noteworthy is their non-conductivity after being subjected to an electric arc, a property known as "no electron trail." Coupled with their excellent adhesiveness and thermoplasticity, shellac is used in the electrical industry to manufacture insulating cardboard, laminated mica sheets, molded electrical insulating parts, insulating varnishes, and adhesives (soldering pastes).
4. Solubility
Shellac resin is soluble in a variety of organic solvents and certain alkaline solutions. While shellac resin is insoluble in water, it can be dissolved in water containing a sufficient amount of alkali upon heating, producing an aqueous solution of shellac resin. The most commonly used alkalis include sodium carbonate, borax, ammonia, triethanolamine, morpholine, and sodium hydroxide. Depending on the amount of alkali used, the solution can appear clear, semi-clear, or colloidal. Strong alkalis tend to cause saponification and hydrolysis of the resin. Due to the many excellent properties of shellac resin's alkaline aqueous solutions and their low processing costs, they are widely used as a type of water-based varnish, extensively applied in coatings for wood, paper, and leather, in the manufacture of inks, and as an ingredient in floor wax.
5. Viscosity of the Solution
Solutions of shellac resin dissolved in various solvents to form varnishes (primarily alcohol-based varnishes and water-based varnishes) are widely used in industry.