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Journal of the Lepidopterists' Society 46(2), 1992, 106-109

LARVAL MORTALITY OF INDIAN TASAR SILKWORM

(ANTHERAEA MYLITTA) (SATURNIIDAE)

DUE TO PEBRINE INFECTION

C. S. K. MlSHRA

Department of Zoology, College of Basic Science,

Orissa University of Agriculture and Technology,

Bhubaneswar 751003, Orissa, India

B. K. Nayak

State Sericultural Research Station, Baripada 757001, Orissa, India

AND

M. C. Dash

School of Life Sciences, Sambalpur University, Burla 768019, Orissa, India

ABSTRACT. Large scale mortality of tasar silkworm, Antheraea mylitta, occurs during commercial rearing seasons because of pebrine caused by Nosema sp., a micro-sporidian pathogen. This paper reports comparative mortality by instar in pebrine free and pebrine infected larvae during three rearing seasons—rain, autumn, and winter.

Additional key words: seasonal variation, Nosema, pathogen, Bombyx mori.

Pebrine is a common disease of the tasar silkworm caused by a pathogen Nosema sp., which results in heavy mortality to the tasar silkworm, Antheraea mylitta (Drury). Little literature is available on tasar mortality due to pebrine (Jolly 1968). Most studies on pebrine have focused on Bombyx mori L., examining effects of temperature and humidity on pebrine occurence (Dasgupta 1950), seasonal incidence of pebrine (Deviah & Krishnaswami 1975), and seasonal variation on larval mortality caused by pebrine (Noamani et al. 1971). Studies on drug effect and susceptibility of Malacosoma disstria Hiibner to Nosema distriae have been reported by Wilson (1984) and Chandra and Sahakundu (1983). However, there is no published information on the rate of mortality of the tasar silkworm due to pebrine. Hence, this investigation was conducted during 1988 to determine the susceptibility of tasar silkworm to pebrine disease in different larval stages and different seasons.

Materials and Methods

Mated females of A. mylitta were segregated into pebrine free (PF) and pebrine infected (PI) groups after microscopic examination of the moths in the grainage at the State Tasar Research Farm, Durgapur, Mayurbhanj District of Orissa, India. The grainage is a specially designed house for preservation of tasar seed cocoons and production of

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healthy tasar eggs at a commercial scale. Pebrine free and pebrine infected eggs were collected after oviposit ion. The eggs were allowed to hatch, and twenty replications of 500 hatchlings of both pebrine free and pebrine infected A. mylitta were reared on Termilalia tomentosa Wt. & Arn. (Combretaceae) in isolated rearing fields at the State Tasar Research Farm. The mortality of pebrine free and pebrine infected larvae at each instar was tabulated. The experiment was repeated during three commercial rearing seasons, i.e., July-August (Rain), September-October (Autumn), and November-December (Winter) of 1988. The data were analyzed statistically using Student's £-test (Snedecor & Cochran 1967). Climatological parameters, such as maximum and minimum temperature, relative humidity, and rainfall during experimental periods were recorded.

Results

Mortality of tasar silkworm is common in cultures due to outdoor rearing methods which subject them to adverse climatic conditions, predators, parasites, and diseases. Observations indicated that percentage mortality of pebrine infected larvae was greater than that of pebrine free larvae in all instars except the 5th and in all seasons. The probability values (P < 0.001) of the £-tests demonstrate a statistically significant difference in percentage mortality of pebrine free and pebrine infected larvae, except for 1st instars in the winter sample (Table 1). Mortality was consistently higher in pebrine infected larvae in instars 1-4 and in pebrine free larvae in instar 5. Within the pebrine free samples, percentage mortality was higher in the 1st and 5th instar than in the 2nd, 3rd, and 4th instar. Within the pebrine infected samples, percentage mortality was higher in the 3rd and 4th instar than in the 1st, 2nd and 5th instar.

Discussion

Sen et al. (1969) reported that mortality of A. mylitta larvae was accelerated from the 3rd instar onwards, with increasing intensity of disease symptoms. However, the present investigation indicated that the pathogen was most active and virulent during the 3rd and 4th instars inducing maximum mortality. This is similar to earlier observations by Wilson (1984) in M. distriae in which he concluded that 3rd instar larvae are more susceptible than 5th instar larvae.

Comparatively lower percentage mortality in pebrine infected larvae in the 5th instar may be due to elimination of larvae susceptible to the pathogen in the 3rd and 4th instars, a possible inactive phase of the pathogen in the 5th instar, or the development of immunity in the surviving larvae.

Table 1. Percentage mortality ± standard deviation of pebrine free (PF) and pebrin instars and during different seasons of 1988.

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Table 2. Mean values ± standard deviation of some environmental parameters during different rearing seasons of A. mylitta during 1988.

Parameters                                                 Rain                            Autumn                          Winter

Daily maximum temperature (°C)        35.17 ± 1.68        34.40 ± 1.39        29.70 ± 1.28

Daily minimum temperature (°C)        22.78 ± 1.23        20.33 ± 1.18         16.27 ± 1.14

Daily relative humidity (%)                  91.40 ± 1.78        90.00 ± 1.89        81.67 ± 1.58 Total rainfall (mm) 312.50 142.83                   20.83

The average temperature, relative humidity, and rainfall were all least during winter, coinciding with the lowest larval mortality. These climatic features were highest during Vain' and medium during 'autumn' (Table 2). The medium temperature and relative humidity regimes coincide with highest larval mortality, which is similar to observations by Dasgupta (1950) in B. mori. Medium temperature and relative humidity apparently stimulate the pathogen. However, Deviah and Krishnaswami (1975) found minimum incidence of pebrine in B. mori at higher temperature and relative humidity, and maximum incidence at low temperature and relative humidity. In contrast, Noamani et al. (1971) reported that temperature and relative humidity have no relation to incidence of pebrine. Although our data and results are consistent with the findings of some previous studies regarding the role of climatic factors on the mortality of pebrine infected A. mylitta, they are contrary to those reported in other studies. Hence, it is clear that this topic requires further investigation.

Literature Cited

Chandra, A. K. & A. K. Sahakundu. 1983. The effect of drug on pebrine infection

in Bombyx mori L. Indian J. Sericulture 21:67-69. Dasgupta, M. R. 1950. Monographs on cottage industries, No. 1, Diseases of silkworms.

Govt. Indian Press, Calcutta. 202 pp. Deviah, R. & S. Krishnaswami. 1975. Observations on the seasonal incidence of pebrine

disease on the silkworm Bombyx mori. Indian J. Sericulture 14:27-30. Jolly, M. S. 1968. Tasar Research Scientific Brochure No. 4:1-31. Central Tasar Research Station, Ranchi. Noamani, K. R., S. Krishnaswami & A. K. Sahakundu. 1971. Observations on the

seasonal incidence and intensity of pebrine disease of the silkworm B. mori under

West Bengal climatic conditions. Indian J. Sericulture 8:11-14. Sen, S. K., M. S. Jolly & T. R. Jammy. 1969. Diseases of tasar silkworm A. mylitta

(Saturniidae). Indian J. Sericulture 8:11-14. Snedecor, G. W. & W. G. Cochran. 1967. Statistical methods. Sixth ed. Iowa State

University Press, Ames, Iowa. 593 pp. Wilson, G. G. 1984. Pathogenicity of Nosema distriae, Pleistophora schubergi arid

Varimorpha necatrix (Microsporidia) to larvae of forest tent caterpillar, Malacosoma

disstria. Zeits. Parasit. 70:763-767.

Received for publication 13 June 1991; revised and accepted 23 February 1992.