MATERIALS AND METHODS

Location and agro-climatic condition: Raiganj [26°35’15” (N)-87°48’37” (W)] is the administrative head quarter of the district Uttar Dinajpur and is situated within the upper Gangetic plain. The climate of this zone is sub-tropical humid in nature. The average annual rain fall varies from 2100-3000 mm, the maximum rainfall occurs during the rainy months of June-September amounting to more than 80% of the total rain fall for last 10 years (2006-2016). The annual average day night temperature ranges between 19.7 and 29.9°C with the mercury soaring even as high as 33°C in April and cascading to a low of 3°C in January. The relative humidity at 8:30 h is 58 and 88% in March and July, respectively. The relative humidity in the afternoon at 17:30 h is 48 and 84% in March and November, respectively.

Jute plant: Corchorus olitorius are flowering plants belongs the family Malvaceae. The plants are tall, usually annual herbs which reaches a height of 2-4 m¹². These plants are mostly unbranched or with only a few side branches. The leaves are simple and arranged in alternate fashion. It produces small yellow flowers of 2-3 cm diameter and five petals. These flowers resulted into fruits which encapsulates many seeds. It thrives almost anywhere and can be grown year-around¹².

jute pest: Jute semilooper, Anomis sabulifera (Lepidoptera: Noctuidae) is a monophagus insect pest predominantly present in the upper Gangetic plains of West Bengal. It completes its lifecycle in an average 30 days. Adult is stoutly built with dark brown patches on the apex of forewing. Females lay eggs on both surfaces of top ten leaves. After hatching larvae loop their body as they crawl. Larvae vigorously feed on leaves specially confined up to top 9th leaf^13,14.

Experimental design: The jute seeds were sown in a row spacing of 25 cm in small plots of 4×4 m with a gap of 1 m between each plot. At completely grown condition, plant to plant distance was maintained at 6-8 cm apart after thinning. Fertilizer treatment and the necessary field management were done following national protocol with suitable modifications. The soil of the experimental field was sandy loam with pH value 6.8 and EC value 0.28 mmhs cm^–1. N, P₂O₅ and K₂O contents were 280, 26 and 265 kg ha^–1, respectively. However no insecticide was applied during the period of study. There were three replications for each of the experiment years.

Record on Agro-ecological parameters: Weekly noted pest incidence were correlated with the prevailing climatic factors such as maximum temperature (Tmax), minimum temperature (Tmin), temperature gradient (Tgr), maximum humidity (RHmax), minimum humidity (RHmin), humidity gradient (RHgr) and rainfall (Rfall). Further inter relationship of the climatic factors was also worked out and then tabulated in matrix pattern.

Record on A. sabaulifera incidence: Larval population of A. sabulifera was assessed by both (i) direct visual counting of larvae and (ii) by assessing the extent of infestation to plant and leaves of jute crop at 7 days interval in relation to Standard Meteorological Week (SMW) as delineated below:

Statistical analysis: The polled data were analyzed by one way ANOVA with 5% level of significance⁸ after square root transformation and mean was compared using INDOSTAT (DGW8) computer programmed. Linear regression and correlations analysis were calculated using KyPlot version 2.0 beta 15 (32 bit)⁶.

RESULTS AND DISCUSSION

Study on the incidence of jute semilooper, Anomis sabilifera in jute field was assessed by randomized block design during two consecutive kharif crop seasons (2015-2016) at Raiganj, Uttar Dinajpur, West Bengal. The results are delineated below.

Incidence of semilooper population: During kharif season, initially very low number of semilooper population was recorded up to 19 SMW. The population then gradually increased from about 20-23 SMW. The number then further improved gradually up to 28 SMW then steadily up to about 31 SMW. The appearance of peak population was restricted to about 27 SMW. Persistent high population was noted from 25-31 SMW. The population then subsumed at first slowly then abruptly. Incessantly low number was counted from 32 SMW. After 34 SMW very low detectable range of population was noted.

However, variation in semilooper incidence was noted in 2 years of observation. In 2015, semilooper infestation started at 22 SMW. The extent of plant and leaf injuries was at low level respectively. Damage gradually increases up to 27 SMW attaining peaks during 28 SMW with high plant infestation and leaf injury respectively. Semilooper remained active till harvest of the crop though the presence was sporadic.

In 2016 semilooper infestation was started quite earlier at 19 SMW 19 with a higher plant and leaf infestation respectively in comparison to previous year (2015). The rate of infestation gradually increased up to 29 SMW. Incidence of this pest reached the peak during 30 SMW with very colossal plant and leaf infestations and slowly declined thereafter but remained active till harvest of the crop.

In visual count, the number of individuals in general was 1 larvae/plant. Rarely in some occasions was 4 larvae/plant observed. In 2015 semilooper population was first visually traced at 23 SMW. The number was then gradually increased up to 27th SMW and attained a peak at 28 SMW and then the population slowly decreased with trace population un till crop harvested.

In 2016, visually semilooper population was first noted at 19 SMW. Though it was early infestation but numbers of pest was almost same as the year 2015. The number was then gradually increased up at 29 SMW with a peak at 30 SMW which showed very high pest incidence in comparison to previous year (2015). However, though in trace, the population remained active till harvest of the crop (Table 1).

Correlation of the semilooper population with climatic factors: In 2015, the incidence of semilooper showed negatively significant correlation with the Tmax (p<0.0001) and with Tmin (p<0.001), but incidence of semilooper showed positively significant correlation with RHmax (p<0.0001) and RHmin (p<0.05). Insignificant negative relations were noted between RHgr and pest in all the experimental years. However significant positive influence of RHavg (p<0.0001) was noted with the pest incidence in all the years. Showery Rain fall had a significant positive effect (p<0.001) on the pest dynamics. But heavy rainfall within a short period had imparted significant negative effect (p<0.0001) on semilooper incidence. Waines et al.¹⁵, Ahirwar et al.¹⁶ and Ali et al.¹⁷ reported in their literature that lepidopteran larva population decreased down due to heavy rainfall.

Table 1:	Incidence of jute semilooper, A. sabulifera on olitorius jute at Raiganj, Uttar Dinajpur, West Bengal
Data are expressed in Mean±SD

Table 2:	Correlation coefficient of incidence of the pests with the climatic factors indicating the level of significance in Raiganj
Significant at: *p<0.05, **<0.001, ***<0.0001, NS: Not significant

Ahmad and Ansari¹⁸ documented that 1 h of heavy rain resulted 95.30 and 42.70% decrease of first instar and last instar, respectively (Table 2).

In 2016, the incidence of A. sabulifera showed significant negative correlation with Tmax (p<0.0001), Tmin (p<0.0001) and RHgr (p<0.05). A negative impact of temperature on different pests also reported by Hafeez et al.¹⁹, Chakraborty²⁰ and Harish et al.²¹. Whereas data shows a significant positive correlation between pest and with the RHmax (p<0.0001), RHmin (p<0.0001), RHavg (p<0.001) and Rfall (p<0.001). Like previous year (2015) an insignificant negative correlation maintained with Tavr and RHgr, respectively (Fig. 1 and 2).

Present study showed that in 2015 the population was initiated at about 22 Standard Meteorological Weeks (SMW) and attained the maximum at about 28 SMW. The pest incidence then decreases slowly with time and it maintained a lofty abundance up to crop harvesting. In 2016, the population dynamics was quite different as early infestation and higher abundance was recorded with changing weather. Grossly abiotic conditions such as temperature, relative moisture and rainfall showed important influences in incidence of Anomis sabulifera.

The plant growth stage and agro-climatic conditions seems to be the main factors affecting the field distribution of the A. sabulifera⁹. As the growth stage of jute crop advances, it produces more leaves.

Fig. 1(a-i):	Correlation between incidence the incidence of A. sabulifera and climatic parameters in 2015 (a) Maximum temperature (Tmax), (b) Minimum temperature (Tmin), (c) Temperature gradient (Tgr), (d) Average temperature (Tavg), (e) Maximum RH (RHmax), (f) Minimum RH (RHmin), (g) RH gradient (RHgr), (h) Average RH (RHavg) and (i) Rainfall (Rfall)

Adults of A. sabulifera settle down, lay eggs and from that larvae develop; as a consequence the population increased²². Roul²³ and Rahman and Khan²⁴ reported that A. sabulifera was predominant species throughout the jute growing region of India and Bangladesh. Present study is in consonance with Sadat and Chakraborty²⁵. They had also observed that leaves of the top region of the jute plant are more affected by the pest. Rahman and Khan^9,10 from southern parts of West Bengal have noted that the incidence of that A. sabulifera was least in early vegetative stage of jute plant covering the calendar month April-May and had registered 3.58% damage. At maximum mature stage jute pant that coincides the calendar month of mid to late July the extent of loss was about16.56%. Present study corroborates with that study made by Rahman and Khan⁹. Pre monsoon rains followed by drought condition are congenial for the outbreak of semilooper and may lead up to 50% loss of crop as reported by Sadat and Chakraborty²⁶.

Fig. 2(a-i):	Correlation between incidence the incidence of A. sabulifera and climatic parameters in 2016: (a) Maximum temperature (Tmax), (b) Minimum temperature (Tmin), (c) Temperature gradient (Tgr), (d) Average temperature (Tavg), (e) Maximum RH (RHmax), (f) Minimum RH (RHmin), (g) RH gradient (RHgr), (h) Average RH (RHavg) and (i) Rainfall (Rfall)

This kind of findings was also observed in present study during the year 2016 where pest population achieved highest peak of 43.88% which was almost four times greater than its incidence (15.84) of the previous year (2015).

Waterhouse^27,28 in southeastern part of Asia and Rahman and Khan⁹ in southern part of West Bengal have observed that the semilooper population gradually increased during early crop growing stages, but had gained momentum and upsurge from early mature stage coupled with favorable agro-climatic conditions.

CONCLUSION

It was concluded that present study evicted a clear relation between jute semilooper and different parameters of weather. Abundance of pest population changes with changing climatic conditions throughout the jute growing season. The information about the seasonal incidence may be utilized for planning the appropriate time fitted insect pest management strategies for sustainable agriculture. Stipulated data in the present study may provide foundation to other researchers for developing their research model and understanding the pest-weather relation.

SIGNIFICANCE STATEMENTS

This study discover the influence of region based climatic condition on occurrence of jute semilooper pest which are major threat to jute cultivation that can be beneficial for construction of time fitted pest management to minimize pest menace and increase jute production to achieve sustainable development. This study will help the researcher to uncover the critical areas of pest weather relation and pest seasonality in the upper Gangatic plains that many researchers were not able to explore. Thus a new theory on jute pest management may be arrived at an earliest.

ACKNOWLEDGMENT

We acknowledge technical assistance from University of Gour Banga, Malda, India and we are thankful to the Additional Director of Agriculture Department and Mr.J. N. Laskar (Senior Meteorologist), North Bengal Region, India for providing meteorological data of Uttar Dinajpur district for the consecutive study period.