Population dynamics of natural enemies Stethorus punctillum on okra at Samastipur, Bihar

Introduction

Okra, Abelmoschus esculentus (L.) Moench (Family: Malvaceae), is one of the world's oldest, traditional cultivated vegetables. its cultivation occurs during both the summer and rainy seasons. It is commonly farmed as a summer crop in Northern India and is also prevalent as a winter crop in states like Gujarat, Andhra Pradesh, Karnataka, and Tamil Nadu. (Lal and Sinha, 2005) [7] . In India okra crop is cultivated in an area of 0.531 million hectares and its production is 6.47 million tons (Anonymous, 2020-21) [2] . The low yield of okra is attributed to the attack of different pests from sowing to harvesting. Recently, there has been a change in agricultural scenario and mites are becoming serious pests in most crops and okra is no exception. The spider mite, Tetranychus urticae Koch, poses a serious threat to okra crop particularly during spring, summer and post rainy seasons. This crop is infested mainly by six different mite pest species, viz., T. urticae, T. macfarlanei, T. ludeni, Brevipalpus phoenicis (Gij.), Polyphagotarsonemus latus (Bank) and Aceria lycopersici (Wolff.), (Prasad and Singh, 2011) [9] . Out of these mite species, T. urticae is responsible for causing the loss of foliage of the crop plant resulting in a reduction of the economic yield of fruits ranging from 20-45% depending upon cropping season and agro-climatic conditions. T. urticae is well adapted to various environmental conditions, causing loss of quality and yield or death of plants by sucking out the contents of leaf cells (Mondel and Ara, 2006, Kumaran et al., 2007) [8,6] . Due to high reproductive potential and extremely short life cycle, combined with frequent acaricide applications this mite has developed resistance to almost all conventional pesticides in vogue (Chiasson et al., 2004; Van Leeuwen et al., 2005) [4,10] . The Stethorus punctillum of family coccinillidae have great importance in biological control and have promising effect in regulating the phytophagous mites, if used judiciously as bio-control agents. Bio-agents are generally considered safer to the environment for controlling the pest (Gupta, 1985) [5] . Since the degree of incidence of Stethorus punctillum changes with the season, it is desirable to have a thorough understanding of the seasonal incidence of the mite, which will lead to the development of suitable management programs. Hence, an attempt was made to correlate the effect of weather factors on the incidence and population dynamics of the Stethorus punctillum in okra.

Materials and Methods

Field trials were conducted from the first fortnight of March 2023 to second fortnight of July during the summer, 2023 at the Vegetable Research plot of Dr Rajendra Prasad Central Agricultural University, Pusa, Bihar. Okra cultivar of the variety "Kashi Kranti" was sown on 3 rd February 2023. The plot selected had uniform topography with well-drained sandy loam soil. The latitude, longitude and altitude of the place are 85.67 0 E 25.98 0 N and 52.92 m respectively. The climate in this region is hot dry summers with mild winters. Other climatic factors such as rainfall, temperature and humidity in this region are highly favourable for the growth of vegetables and other crops in all seasons with the proliferation of mites infesting a variety of crops. A bulk-plot was raised to conduct research on the population fluctuation of Stethorus punctillum associated with okra. The crop was cultivated by following appropriate agricultural practices and managed without the application of any insecticide. The population of Stethorus punctillum was recorded at fortnight intervals. Five places were selected for each host and three leaves were plucked randomly from host plants for recording population. All the leaves were mixed, labeled, and brought to the laboratory of the Department of Entomology, Dr. Rajendra Prasad Central Agricultural University, Pusa, Bihar for counting the Stethorus punctillum. Care was taken to avoid disturbance to the natural population of mites on the leaf. All the mites were counted under a stereo binocular microscope. The whole population was recorded on per leaf and the mean population was recorded. The meteorological data were taken from the meteorological section, Dr Rajendra Prasad Central Agricultural University, Pusa, Bihar. After that, these abiotic parameters were correlated with the corresponding Stethorus punctillum population. For testing the significance of the simple correlation coefficient, the 't' test was applied. The multiple regression coefficients and multiple correlation coefficients (R) of the Stethorus punctillum population in relation to abiotic factors were also computed. The significance of all the multiple correlation coefficients (R) was tested for their significance by applying the 'F' test. The value of the coefficient of determination (R 2 ) was calculated to highlight the joint contribution of independent variables on Stethorus punctillum population. This findings are somehow similar with Adly (2023) [1] who noted Stethorus punctillum was introduced on two occasions, with a density of 0.5 individuals per square meter. There was a direct correlation between S. punctillum abundance and temperature, while relative humidity showed an inverse correlation. Across both seasons, the population of Tetranychus urticae dwindled gradually to complete absence on croton leaves within the controlled greenhouse.

Results and Discussion

In contrast the population increased on croton leaf in the control greenhouse. A simple correlation (r) was established between Stethorus punctillum population and weather parameters in okra (Table 2). It was observed that there was a positive and significant correlation with maximum temperature (r = 0.591*) and minimum temperature (r = 0.818**) and then it was observed that there is a negative and non-significant relation with maximum relative humidity (r = -0.137 NS ), positive and non significant with minimum relative humidity (r = 0.315 NS ) negative and non-significance with rainfall (r = 0.315 NS ). The regression equation for the data, with a population(Y) as the dependent variable and weather factors as the independent variables, was as follows a determined coefficient of determination and (R 2 ) and regression model (Table 2): X 1 = Maximum temperature, X 2 = Minimum temperature, X 3 = Maximum relative humidity, X 4 = Minimum relative humidity, X 5 = Rainfall. Based on the calculation, the coefficient value (R 2 ) derived as 88.10% which suggests that of the fluctuations in the Stethorus punctillum population were attributed to the impact of abiotic factors. In other words, R 2 value indicates that percent of variation in the Stethorus punctillum population were influenced by abiotic factors. Our present study is supported by Chandrakumar et al., 2008 [3] , They observed Coccinellid density was significantly positively correlated with wind speed velocity (0.580), but negatively correlated with maximum temperature and rainfall (-0.641 and 0.776, respectively). Relative humidity was found to be non-significant and unrelated to the coccinellid density.

Conclusion

The population of Stethorus punctillum was the highest during the 1 st fortnight of June 2023. There was a positive significant correlation of the Stethorus punctillum population with maximum temperature and minimum temperature.

Acknowledgement

AcknowledgementsThe authors are grateful thanks to the Head, Department of Entomology and Department of Horticulture, Dr. Rajendra Prasad Central Agricultural University, Pusa, Bihar for providing necessary facilities during the course of experimentation.

References

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