Investigations were carried out on the seasonal incidence of Spodoptera litura on groundnut at the Regional Research Station, Vriddhachalam, Tamil Nadu. Seven field experiments were conducted during Kharif 2001, Kharif 2002, Kharif 2003, Kharif 2004, Rabi 2001-02, Rabi 2002-03, and Rabi 2003-04 using groundnut variety VRI 2. The presence of Spodoptera was noticed two weeks after sowing and continued to exist up till harvesting of the crop. The peak incidence of pest occurs around 50-60 days after sowing. The correlation and regression studies showed that during Kharif season, the individual weather parameters have no significant correlation with the abundance of Spodoptera, but all weather parameters together can have influence up to 90% on the incidence of Spodoptera. During Rabi season also the effect of weather parameters individually on the abundance of Spodoptera was not significant. But, the R² value indicated that the influence of weather parameters on the incidence of Spodoptera was 98%. The forecasting model developed for Kharif season was γ = 12.309 + 2.37 X23, where, γ-Percent pest Incidence, X23 = Adjusted Minimum Temperature (2-week lag). The model developed for Rabi season was γ = 0.189 + 2.039 X12, where, γ-Percent pest Incidence, X12 = Adjusted Maximum Temperature (1-week lag). These models were validated with individual season data.

Groundnut (Arachis hypogaea Linn.) is one of the important oil seed crops grown in India occupying an area of 8.3 million hectares with an annual production of about 75.8 lakh tons (Hedge, 1999). The crop is cultivated in 45% of India's total oil seeds area and accounts for 40% of the total oil seeds production in the country. Although India ranks first in terms of area and production in the world, the average yields are low. One of the major causes for the low yield is the damage caused by insect pests besides the fact that a large area of land use for groundnut cultivation in India is unirrigated. Although many insect species live and feed on the groundnut crop, only a few defoliators cause significant damage that result in reduction of pod and haulm yield. Among the defoliators, Spodoptera litura (Fab.) causes substantial damage to groundnut plants (Prasad and Bhattacharya, 1975; and Tiwari et al., 1988). Growth and spread of this pest is triggered by many weather factors. The studies on seasonal incidence of pest and its relationship with weather factors would provide an opportunity to minimize the yield loss. Hence, the present study was undertaken to know the seasonal incidence and influence of weather factors on the extent of infection by this pest on groundnut.

Seven field experiments were conducted at the Regional Research Station, Vridhachalam, Tamil Nadu (Kharif 2001, Kharif 2002, Kharif 2003, Kharif 2004, Rabi 2001-2002, Rabi 2002-2003 and Rabi 2003-2004) using the groundnut variety VRI 2, to know the seasonal incidence and assess the influence of weather parameters on Spodoptera incidence in groundnut. The recommended agronomic practices were followed. The pest incidence was recorded in terms of percentage of leaves affected. The pest scoring was done at seven days interval starting from initiation of the pest, up to the harvest. The weather parameters such as maximum temperature, minimum temperature, morning relative humidity and evening relative humidity, were recorded daily during the crop period. The data on total rainfall also was recorded. The derived weather parameters like, Diurnal Variation (DV), Relative Temperature Disparity (RTD) and Growing Degree Days (GDD) were also calculated and were related to percentage of pest damage. For all the weather parameters, weekly average was worked out except rainfall for which a weekly total was worked out. The percent pest incidence was correlated with all the above weather parameters using the percent pest incidence as dependent variable (γ) and each of the weather parameters as independent variable (X) (Panse and Sukhatme, 1967). Based on step down regression analysis, the weather parameters were short-listed and significant weather parameters are selected. Using the weather weighted index or weather generated model and deviation model (statistical models) with Statistical Package for Social Sciences (SPSS) package, the forewarning models were developed. For calculating the weighted weather indices, the concerned weather parameters were multiplied by the correlation coefficient value.

Seasonal Incidence and Influence of Weather Factors on the Incidence of Spodoptera litura (Fab.) on Groundnut, Diurnal Variation (DV), Relative Temperature Disparity (RTD) and Growing Degree Days (GDD), Statistical Package for Social Sciences (SPSS), defoliators, pest incidence.