The tropical tasar culture is an important forest based agro-industry of producing vanya silk by rearing a wild silkworm,
The tasar food plant leaf quality in terms of nutrition can influence the health and growth of larvae, effective rate of rearing (ERR) and crop yields as it has correlation with the weights of cocoon, shell and silk ratio and can influence the crop economics. The leaf nutrient status of tasar food plant is fundamental not only for silk productivity, but also for its metamorphosis during life cycle and subsequent parental moth reproductive efficiency. The larval feeding status of any polyphagous commercial insect has impact on food storage and budgeting for biological activities so as to combat the adverse or to excel during favorable conditions (Reddy
The diseases in silkworm are the major constraints
in tasar culture, which adversely affect the economics of this culture by causing 35-40% crop loss. Among the diseases, pebrine is causing most devastating effect on the rearing of the tasar silkworm accounts for 20-25% yield loss (Sahay
Tasar silkworm is often infected with the intracellular parasite of the genus Nosema. Pebrine can be acquired from the mother moth (primary infection) or from the environment through food (secondary infection). Infected larvae show black pepper like spots on the integument. These infected hypodermal cells become enlarged and vacuolated and blackened due to the formation of melanin (Ganga, 2003). Larvae infected with Nosema sp. show extended development period, reduced size and larval weight in comparison to uninfected ones (Rath
Pebrine is a disease caused in tasar silkworms, by a parasitic microsporidian-
eggs laid by infected mother moths or exists in rearing facilities as spores or come from wild insects naturally infected (Fig. 2) with
This dreaded disease is also known as pepper disease or corpuscle. The name pebrine was given to the disease in 1860 by De Quatrefages because the black spots that appear on the diseased silkworms look like pepper grains. The disease was not properly understood until the researches of Louis Pasteur during the years 1865-70 brought out a method of diagnosis. Pasteur established that the disease invariably manifests itself in the mother moth though sometimes it may not be possible to detect it in the larval and cocoon / pupal stages (Fig. 3 and 4).
There are two stages in the life cycle of this organism, the spore stage and the vegetative stage. The mature spore is oval and measures 3-4 microns by 1.5-2 microns. The spore consists of spore membrane, which encloses the sporoplasm, in the form of girdle across the width of the spore, anterior and posterior vacuoles, two nuclei in the sporoplasm and polar capsule which encloses a spiral polar filament (30 times the length of the spore) which is projected through the small opening. The pathogen comes from infected eggs laid by infected mother moths. It may also exist in rearing facilities or
During the vegetative stage the
silkworm. The spores multiply in number; the digestive juices act on the spores, the polar filament is extruded and soon becomes detached from the spore. Soon these develop into globular planonts, which invade the haemocoel, intestine, gonads, malphigian tubules and silk glands by rapid proliferation. In the various body tissues of the silkworm, they gradually increase in size and become meronts / schizonts by draining the cytoplasmic contents of the host and finally becoming a sporont. The mode of infection and transmission of the spores are mainly through oral, cuticular and ovarial pathways (Kramer, 1976). In the ovarial pathway, the pathogen is incorporated into the egg within the female reproductive tract and the progeny from such females are also infected. The rate of disease transmission and degree of infection in the offspring is directly related to the intensity of infection in the mother moth (Griyaghey and Sengupta, 1989; Talukdar, 1995).
Some of the microsporidia show transovarial transmission and some are not (Fujiware, 1980; Fujiware, 1984; Ananthalakshmi
The silk protein fibroin is fibrous in nature, forming the main silk filament content, while sericin is a sticky coating substance between the layers of fibroin. Thus, the quality of cocoons depends both on sericin and fibrin which are controlled by atmospheric conditions (Shamitha and Rao, 2006). The sericin content as being the deciding factor in the quality of the cocoon and raw silk reeled was reported by Singhvi and Bose (1991). However, filament length and quality of the shell are based on the fibroin content. The present studies provides insight into the damaging effects of the Pebrine infection in the larval and cocoon stages resulting in poor quality of silk and identifies the need to designate proper measures to prevent the disease.
The trivoltine tasar silkworm,
Instar-wise Average Temperature (C), Average Relative Humidity (%) and Mortality due to Pebrine disease of Tasar silkworm, Antheraea mylitta (Daba TV)
The incidence of Pebrine disease in the rearing lot was identified by the symptoms in the early larval stages. The appearance of black spots on the body and unequal larval development are the most prominent symptoms of this disease. They exhibit poor appetite resulting into stunted growth and irregular moulting. The larvae detected with Pebrine infection are immediately removed and buried far away from the rearing site. The instar-wise larval mortality is presented in the results (Table 1).The larvae soon become pale and dull with wrinkled cuticle. As the larval stages advance, the appearance of black spots in the integument becomes more prominent, exhibiting melanosis.
The studies on silk gland were conducted by their collection from individual fifth instar larvae in a dissection tray containing normal saline. The dissected glands were weighed on Citizen Balance (Table 2) by removing any excess water. The infected silk glands (Fig. 6) along with posterior glands (Fig. 7) were spread on the slide and examined under microscope at 40X magnification for
Larval weight,Silk gland weight and % Decrease in Pebrine- free and Pebrine infected V-instar Antheraea mylitta (Daba TV)
last pair of legs of fifth instar larvae and smeared on a clean slide (Fig. 8) for examination under microscope at 40X magnification for
A severely infected larva may die before spinning or it may spin a flimsy cocoon or spits and wastes the silk. However, a good number of infected worms have been allowed to spin for studying the post cocoon
characters (Table 3). In the pupa, the abdominal area is soft and swollen and dark in colour. Black spots may be seen on the sides near the wing area. Sometimes the infected pupae do not show any symptoms. In order to detect the disease at pupal stage, the pupae were first washed with distilled water for two minutes, and then the lower half of the abdomen (gut) was placed in a
clean mortar. The tissue was crushed and the smear examined under microscope at 40X magnification for
The larval, cocoon and post-cocoon parameters of Pebrine free and infected worms were measured as follows:
The optimum temperature and relative humidity for the tasar silkworm rearing are 25-30℃ and 60-70%
Cocoon parameters of Pebrine free and Pebrine infected tasar silkworm, Antheraea mylitta (Daba TV)
respectively. In the present study, the temperature and relative humidity were recorded with the help of lab thermometer and hygrometer respectively. The average of lowest and highest values taken and noted them instar-wise.
The mortality was calculated by counting the number of worms dead and diseased due to microsporidian pathogen (Pebrine disease) during each instar by observing the symptoms and accordingly the percentage was calculated.
At the beginning of each instar, 10 healthy worms were selected at random from the rearing lot, which are feeding on the
The average silk gland weight dissected from five V fifth instar larvae selected at random feeding on the
Filament length was measured by using cocoon approvette. It has an axis around which four wooden sticks were arranged at equal distance with the circumference of 9/8 meters. It is rotated with the help of a handle at one end. And to the other hand of the axis is a bell by which number of rotations was known. The number of rotations multiplied by the circumference gives the filament length.
Filament length = 9/8met X No. revolutions
The reeled silk weight measured with the electronic digital balance in grams.
The reelability of cocoons for economic reeling is the ease with which the cocoons yield the bave in reeling, which is called the reelability of cocoons. This is calculated by the following formula:
The difference in thickness of the size of the bave from beginning to the end is so gradual and minute in tasar cocoon that it does not interfere with the quality of the size of ultimate raw silk reeled. Denier is obtained by the following formula:
The instar wise average temperature and its standard deviation of Tasar silkworm
The instar wise average Relative Humidity and its standard deviation of
The average larval weight and its standard deviation of Pebrine- free tasar silkworm,
The average Silk gland weight and its standard
deviation of Pebrine free tasar silkworm,
The post-cocoon characters of Pebrine free and Pebrine infected tasar silkworm,
The etiology of Pebrine (
A report on Pebrine incidence in Thailand (Somsri,
There are also several reports on Nosema bombycis, a highly virulent micro-organism, damaging the silkworm crops in Japan and other Asian countries. A detailed morphology and life cycle of
Earlier, tasar mortality due to Pebrine was reported by Jolly in 1968. Studies on larval mortality by instar in varied seasons by Mishra (1992) and Pebrine control measures by feeding of Benomyl, Carbestine, Bngrad in specified dosages from second instar was cited by Sinha (2005). The present study calls for a remedial measure to prevent this epizootic disease which causes alarming loss to tasar culture which has a direct relevance to tribal employment.
From the results it is observed that mortality due to Pebrine was apparent from 2nd instar and gradually increased in 3rd, 4th instars and reached its peak in the fifth instar, which may be due to the manifestation of virulent parasitic microsporidian, Nosema mylittensis, in the late instars. This is in corroboration with the earlier studies that the mortality of A.mylitta larvae accelerated from 3rd instar onwards, with increasing intensity of disease symptoms (Sen
It is found that there is a significant loss of larval body weight with a corresponding and drastic decrease in the silk gland weight of the Pebrine infected tasar silkworms. It is in good agreement with the view that Dabaecorace recorded highest concentration of silk protein in its silk glands compared to other ecoraces studied (Lokesh
The post-cocoon parameters have revealed that the weights of the cocoon, shell and filament have substantially decreased in the Pebrine-infected cocoons. The decrease in the weight and length of the silk filament is extreme. From the literature it is inferred that the quantitative as well as qualitative nutrition is highly essential for sericigenous insects to maintain important physiological status and lack of such nutrition in the tasar silkworm might have extended its larval span (Chapman, 1998; Behmer, 2006; Kumar
From the present studies it is evident that this dreadful disease which may be transmitted through the egg and ingestion of the contaminated food by the larvae / origination of the pathogen from infected eggs laid by infected mother moths / its pre-existence in rearing site as spores / wild insects naturally infected with
In the present investigation, it is revealed that the pupal weight in Pebrine infected larva has shown high reduction, which may lead to low fecundity. A recent study of
This study suggests that there is an urgent need to develop an efficient method to control the disease. Although medium temperature and relative humidity can stimulate the pathogen (Dasgupta, 1950), some studies have shown that high temperature and relative humidity can cause minimum incidence of Pebrine (Devaiah, 1975).
The origin of
The Pebrine infection is found to be one of the major constraints in tasar culture, which causes heavy loss to the crops. Hence, the prevention and management of this disease is a crucial factor for high yield of cocoons and quality silk. As there is a persistent demand for tasar silk in both domestic and international fronts, and on the other hand, the tasar production base is totally tribal community dependent, there is an immediate urge to prevent the disease incidence in the food plant and the silkworms by quick remedial measures and ensure quality production of the silk.