Rearing of tropical tasar silkworm,
Drury is mainly conducted in outdoor on
W. & A. a nature grown primary host plant available in forest and also on raised primary host plant
Bedd. Temperature, relative humidity and rainfall are the main environmental factors for occurrence of pests (parasites and predators) of tasar silkworm during I, II and III crop rearing in the tropical tasar producing zones. The present study was aimed to study the influence of abiotic factors on prevalence of tasar silkworm pests. The study was conducted at different agro-climatic regions viz., Central Tasar Research &Training Institute, Ranchi, Jharkhand, Regional Extension Centre, Katghora, Chattisgarh and Regional Extension Centre, Hatgamaria during 2010-13 covering 3 seed crop and 6 commercial crops. Data on incidence of tropical tasar silkworm endo-parasitoids like Uzi Fly,
Walker and Ichneumon fly (Yellow Fly),
, Fabricius and Predators such as Stink bug (
Wolf), Reduviid bug (
Fabricius) and Wasp (
Linnaeus) was recorded Weekly. The meteorological data was collected daily. Data was collected from 4 different agro-climatic zones of tasar growing areas. Analysis of the data revealed a significant negative correlation between abiotic factors and incidence of ichneumon fly and uzi fly. Based on the 3 years data on prevalence of pests region-wise pest calendars and prediction models were developed.
The tasar culture is a cottage, agro-forestry and forestry based industry that provides sustainable livelihood to several rural communities and country to earn foreign exchange. The tasar silk industry generates cocoons through commercial rearings along with the collection from natural habitats by many Indian tribal families as tradition since immemorial.
is a sericigenous, polyphagous insect feeds primarily on
(Asan) besides variety of secondary and tertiary food plants
) available in tropical deciduous forests of West Bengal, Jharkhand, Bihar, Orissa, Chhattisgarh, Madhya Pradesh, Uttar Pradesh Maharashtra and Andhra Pradesh states of India. Earlier the tasar food plants were not extensively cultivated by the rearers. Natural tasar food plants were used for the silkworms rearing. But adaptation of scientific farm techniques led to the intensive cultivation of the food plants in huge acreage which resulted in monoculture which attracted more number of pests of both host plants and silkworm. It is estimated that in conventional rearing about 70-80% of the crop is lost due to these pests and finally the rearers hardly able to harvest 20-30% of the crop. The duration of the developmental stages of silkworm pests cannot be controlled completely in the nature, which further increases the chance of pest and predator attack, thus this peculiar situation in tasar silkworm rearing makes the pest and predator control more difficult.
Abiotic factors alter the survival, development and reproductive capacity of insects. Their activities are mostly dependent on the environmental temperature, prolonged periods of low or high temperatures or sudden change adversely affect the insect development. Similarly humidity and rainfall influence the population of insects (Prasad and Logiswan, 1997). These factors compelled the insects to adapt themselves to the changing climatic conditions or perish (
). Among the various physical factors, temperature, humidity and rainfall are considered to be the most important cause of insect population fluctuations.
The pests of tasar silkworm are broadly categoried into predators and parasitoids. Predators cause damage to tasar larvae during early instars and parasitoids during later stages of silkworm rearing. Due to these pests the production of tasar silk is effected considerably.
Keeping in view the above factors, present study was taken up to record the incidence of silkworm pests in different agroclimatic zones to know the influence of abiotic factors on pest prevalence and to prepare a pest calendar as well as to develop prediction model.
Materials and Methods
Data on prevalence of silkworm pests and predators were collected from 4 different agro-climatic zones of tasar growing areas
Research Extension Centre, Kathgora, Chattisgarh, Research Extension Centre, Hatgamaria, Jharkhand, Research Extension Centre, Bangriposi, Orissa and Central Tasar Research and Training Institute, Ranchi, Jharkhand along with the recording the data on abiotic factors like temperature (minimum & maximum), relative humidity (minimum & maximum) and rain fall at weekly interval for a period of 3 years (2010 − 2013). For collection of pest incidence data, 5 villages were selected in each centre consisting of 10 farmers/village. In each farmers plot 7 tasar food plants where silkworm rearing was being conducted were selected randomly for data collection. The predators of silkworm were collected by using country made sticky trap and insect catching net. The parasitoid uzi fly was observed based on its symptoms of attack. For the purpose 100 silkworms were randomly collected from each plant and were inspected for symptoms of uzi infestation. For ichneumon fly, after cocoon harvest 500 cocoon/farmers were randomly observed for symptoms of fly emergence. The percent pest incidence was calculated using the following formulae:
- Prevalence of Pests of silkworm:
The prevalence of major pests of silkworm and abiotic factors at CTR&TI, Ranchi, Jharkhand, REC, Katghora, REC, Hatghmaria and REC, Bangriposi are shown in
, respectively. Pest-wise occurrence at different locations and its relation with selected abiotic factors are summarized:
Prevalence of silkworm pests during rearing period in CTR&TI, Ranchi
Prevalence of silkworm pests during rearing period in REC, Hatgamaria.
Prevalence of silkworm pests during rearing period in REC, Katghora.
Prevalence of silkworm pests during rearing period in REC, Bangriposi.
- 1. Reduviid bug (Sycanus collarisFabricius)
The peak period of incidence of Reduviid bug was recorded in July and August months in most of the locations. A significant positive correlation between prevalence of Reduviid bug with maximum temperature (r = 0.853*), minimum temperature (r = 0.932**), maximum relative humidity (r =0.850*), and minimum relative humidity (r = 0.891**) was observed at REC, Hatgamaria. At CTRTI, Ranchi the observed correlation between Reduviid bug and maximum temperature (r = 0.817*) was significant and positive.
- 2. Stink bug (Eocanthecona furcellataWolf)
The peak period of incidence of Stink bug was recorded in August and November months in most of the locations. A significant positive correlation between prevalence of Stink bug with maximum temperature (r = 0.842*), minimum temperature (r = 0.808*) and minimum relative humidity (r = 0.794*) was found at REC, Katghora.
- 3. Wasp (Vespa orientalisLinnaeus)
The peak period of incidence of Wasp was recorded in the months of August and November in most of the locations. The correlation was significant and positive between prevalence of Wasp with maximum temperature (r = 0.880**), minimum temperature (r = 0.826*) and maximum relative humidity (r = 0.889**) at REC, Hatgmaria. However, at REC, Katghora the correlation between the pest and maximum temperature (r = -0.896**) was significant and negative.
- 4. Ichneumon fly (Xanthopimpla pedatorFabricius)
The peak period of Ichneumon fly incidence was observed in the months of November, December and January in most of the locations. The pest showed significant negative correlation with maximum temperature (r = -0.862*, r = -0.755*) and minimum temperature (r = -0.0.994**, r = -0.935**) at CTR&TI, and REC, Bangriposi, respectively. At REC, Hatgamaria the correlation was significant and negative with maximum temperature (r = -0.918**), minimum temperature (r = -0.977**), maximum relative humidity (r = -0.857*) and minimum relative humidity (r = -0.919**). Similarly, the pest exhibited significant negative correlation with minimum temperature (r = - 0.828*), maximum relative humidity (r = -0.852*), minimum relative humidity (r = -0.767*) and rain fall (r = -0.955**).
- 5. Uzi fly (Blepharipa zebinaWalker)
The correlation was significant and negative between uzi fly and the abiotic factors except rain fall at REC, Hatgamaria. The uzi fly showed significant negative correlation with maximum temperature (r = -0.958**) and minimum temperature (r = -0.936**) at CTR&TI, Ranchi and with minimum temperature (r = -0.898**) at REC, Bangriposi. At REC, Katghora the correlation between the pest and minimum temperature (r = -0.936**), minimum relative humidity (r = -0.921**) and rain fall (r= -0.814*) was significant and negative. Correlation coefficients are furnished in
Correlation coefficient between abiotic factors and silkworm pests
*- Significant at 5% and **- significant at 1%
- Region wise Pest calendar
The data collected from different locations for a period of three years were pooled and the region wise pest calendars were developed for silkworm pests (
Pest Calendar of Tasar Silkworm Pests in CTR&TI, Ranchi.
Pest Calendar of Tasar Silkworm Pests in REC, Hatgamaria
Pest Calendar of Tasar Silkworm Pests in REC, Kathgora.
Pest Calendar of Tasar Silkworm Pests in REC, Bangriposi. Note: In each row Dark colour indicates the peak prevalence of pest while Light colour indicates moderate occurrence and white colour indicates no occurrence of the pest.
- Prediction model
The data for biotic and abiotic factors (
) collected from four location namely CTR&TI Ranchi, REC Katghora, REC Hatgamaria and REC, Bangariposi was analyzed to develop the prediction models for silkworm pests using Multiple linear Regression, Y
( pests) = a + b
( Max.Temp.) + b
(Min.Temp.) + b
(Max. RH) + b
(Min. RH) + b
(Rain fall). Prediction models silkworm pests were worked out using Linear Regression Models (Y = Σ b
+ a ) with backward method (SPSS software). The difference was less between estimated and observed values for silkworm pests (
). The R
(R-Square) estimate in each model indicates total variation explained by predictive variables in getting the pest incidence. Location wise prediction models for silkworm pests are furnished in
Dependent and Independent variable
Dependent and Independent variable
Observed and Estimated value for Silkworm pests at CTR&TI, Ranchi, (a) Reduviid bug, (b0 Stink bug, (c) Wasp, (d) Ichneumon fly and (e) Uzi fly.
Observed and Estimated value for Silkworm pests at REC, Katghora, (a) Stink bug, (b) Wasp and (c) Ichneumon fly.
Observed and Estimated value for Silkworm pests at REC, Hatgamaria, (a) Reduviid bug, (b) Stink bug, (c) Wasp, (d) Ichneumon fly and (e) Uzi fly.
Observed and Estimated value for Silkworm pests at REC, Bangriposi, (a) Wasp, (b) Ichneumon fly and (c) Uzi fly.
Regionwise Prediction models for Tasar Silkworm Pests
Note: 1. *- significant at 5%, **-significant at 1% and NS-non-significant2. R2 = Coefficient of determination, F-test (Regression ANOVA) Explanatory variable: X1= Max. Temp. (℃), X2 = Min. Temp (℃), X3 = Max. RH (%), X4 = Min. RH (%) and X5 = Rainfall (mm) Response variable: Y1 = Reduviid bug (%), Y2 = Stink bug (%), Y3 = Wasp (%), Y4 = Ichneumon fly (%) and Y5 = Uzi fly (%)
- Major parasitoids of tasar silkworm
The Uzi fly,
Walker (Diptera: Tachinidae) and the Ichneumon fly,
Fabricius (Hymenoptera: Icheumonidae) are the major endo-parasitoids of tasar silkworm. It is widely distributed in tropical tasar region of India, China, Thailand, Bangladesh and Japan. It is also reported in North – Eastern India infesting Muga silkworm. Several species of lepidopteran caterpillars have been recorded as hosts of
The silkworm larvae infested by uzi fly in early instar die before they reach the spinning stage. If the infestation takes place in the late fifth instar, the mature maggot comes out by piercing the cocoon and thereby rendering the cocoon unfit for mass reeling. Infested silkworm can be identified by the presence of black scar on the part of the skin where the maggot penetrates in to the body of the host larvae. Sometimes, an egg shell is left behind in the centre of the black scar. At the initial stage of infestation minute creamy white oval eggs smaller than pinhead are observed on the skin of the larvae.
The ichneumon fly belongs to the order Hymenoptera, family ichneumonidae. The female fly has nearly 1 cm long prominent needle like ovipositor with two long stylets. The female fly lays eggs inside the pre-pupal body by inserting its ovipositor through freshly formed / flimsy cocoon shell. Only one egg is deposited in each host. The maggot after hatching consumes the entire pupal content except the skin and pupates. The adult fly emerges from the cocoon by piercing the cocoon which renders the cocoon unfit for reeling.
- Major predators of tasar silkworm
The major predators of silkworm are Stink bug, Reduviid bug, Praying mantis,Wasp and Red ant. The stink bug
Wolf (Hemiptera: Pentatomidae) is the most harmful predator and is responsible for about 15% of the crop loss particularly during early instars (chawki) of silkworms.
On an average single mated female lays 250–350 eggs in batches of 40–70 usually on the under surface of the leaves or on the twigs. Each egg mass consists of four to five rows of 8-15 eggs. The predator lives on the haemolymph of the host, which it sucks by piercing the larval body by its proboscis. It has been observed that single bug can kill 130-225 tasar silkworms of first to third instars throughout its life.
The Reduviid bug,
Fabricius (Hemiptera : Reduviidae) is also most harmful predator particularly during early instars of silkworm in some areas of Chattishgarh, India. The adult bug is black in colour. On an average single mated adult female bug lays about 300 eggs. The nymphs pass through five instars. The entire nymphal period is about 25 days. The nymphs and adults suck the haemolymph of silkworm. A single bug feeds and kills about 200 tasar larvae of first to third instars throughout its life.
Linnaeus (Hymenoptera : Vespidae) wasp feed on caterpillars, mantids, grasshoppers and other insects. But in a silkworm rearing field defenseless tasar larvae are the choice and they predate silkworm not only for self but also collect and deposit them to their comb cells where large numbers of young ones are developing.
(Hymenoptera : Formicidae) is one of the most damaging predators. It is harmful to all the insects, but the larvae of younger stages of silkworm are more easily affected. They generally attack in groups and carry the younger tasar larvae to their nests. They bite the larvae of advanced ages in an attempt to feed on their appendages including hairs and setae. This leads to swelling, paralysis and finally to death of the worms.
In the present study correlation analysis revealed that the abiotic factor showed significant and positive correlation between maximum temperature and reduviid bug at CTR&TI, Ranchi and REC, Hatgamaria. The results are in consonance with the reports of Joshi and
Khalid Zafar (2013)
, Pushpalatha (2008) and
Sanjay Kumar (2012)
and it differs from
Sardana and Das (2001a
who reported significant negative correlation of maximum temperature with the pests.
The minimum temperature showed significant positive correlation with reduviid bug, wasp and stink bug at REC, Hatgamaria and REC, Katghora, respectively. Significant positive correlation of minimum temperature with Jassid population and Jute semilooper was reported by
The maximum and minimum Relative Humidity revealed significant negative correlation with ichneumon fly and uzi fly at REC, Katghora and REC, Hatgamaria. The significant negative correlation between pest and RH is in agreement with the reports of Joshi and
Sanjay Kumar (2012)
Khalid Zafar (2013)
, Pushpalatha (2008) and Sardana and Das (2010a).
The influence of rain fall was significant and negative between ichneumon fly and uzi fly at REC, Katghora. Similar positive influence of rail fall on pest was reported by
. However, the correlation between the mango hopper and rain fall was reported significant and negative by Pushpalatha (2008).
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