Rice (Oryza sativa) is an important cereal crop, and it is attacked by 800 species of insect out of which 20 (e.g. rice leaf folder, stem borer, plant hopper, grass hopper and gall midge etc.) cause economic damage and are considered as major pests (Jena et al., 2018). Among these, the rice leaf folder Cnaphalocrocis medinalis (Guenee) (Lepidoptera: Pyralidae) is a predominant foliage feeder and is one of the most devastating pests (Luo, 2010). It occurs in all growth stages of the crop. Larvae damage the crop by feeding on chlorophyll content of leaves after folding (Bisen et al., 2019). Its damage at reproductive stage can reduce yield up to 6.2%, and overall losses range from 63 to 80% (Satish et al., 2007; Teng et al., 1993). Growers rely heavily on pesticides to protect their rice crop. The weather parameters and the transplanting dates have an influence on the appearance of the pests. This experiment evaluates the seasonal incidence in relation to date of transplanting and the population dynamic is in relation to weather factors.

A field experiment was carried out at the agriculture research farm, Lovely Professional University, Jalandhar (31 24’ N, 75 69’ E, 4342 masl) during 2018 kharif season. Thirty days’ old seedlings of Pusa 1509 variety were transplanted, and randomized block design was followed in 5x 5 m plots replicated thrice. Recommended agronomic practices by the PAU, Ludhiana were followed. Four transplanting dates evaluated were: 20^th June/ 25^th standard meteorological week (SMW), 30^th June/ 26^th SMW, 10^th July/ 28^th SMW and 20^th July/ 29^th SMW, 2018. Incidence of C. medinalis was recorded at weekly intervals from randomly selected six hills from untreated plots. Weather data were obtained from the meteorological observatory, agriculture research farm, LPU, Phagwara (Punjab). The incidence data was subjected to angular transformation prior to statistical analysis. Pearson correlation was obtained to find out the relationship with weather factors. The statistical analysis was done using SPSS (version 22).

Incidence of C. medinalis was observed during 27^th SMW (0.32/ hill) which reached its peak during 28^th SMW (1.67/ hill) (Fig. 1); and with 29^th SMW it decreased till 36^th SMW (0.08/ hill) and then increased during 38^th SMW (0.82/ hill), while at maturity stage of crop (39^th and 40^th SMW) the incidence declined abruptly. The weather factors viz., temperature, humidity and rainfall showed a non-significant effect on the incidence; maximum (r = -0.214) and minimum (r = -0.154) temperature showed a negative correlation while relative humidity- RH (r = 0.420 and r = 0.375) and rainfall (r = 0.211) showed a positive one (Fig. 2). Similar results were obtained by Singh et al. (2017) and Zainab et al. (2017) showing a negative correlation with temperature and positive correlation with rainfall. Tiwari et al. (2021) observed a positive correlation with the maximum temperature but negative with the rainfall and evening RH. The present results agree with those of Singh et al. (2017) in terms of positive correlation of rainfall and RH. Yimjenjang and Mishra (2020) also observed that maximum (r= 0.527*, 0.590) and minimum temperature (r=0.849**, 0.853**), morning (r=0.244, 0.458) and evening RH (r=0.518*, 0.269) and wind velocity (r=0.356, 0.361) had significant positive relationship. Haider et al. (2020) obtained similar results with weather factors as having an insignificant relationship. Kakde and Patel (2015) observed non-significant positive relationship of maximum temperature, RH, rainfall and sunshine hours with the leaf damage by leaf folder; and non-significant negative correlation of minimum temperature was observed under SRI planting method.

About the dates of transplanting, least damage (0.03 -1.34% leaves/ hill) was observed in fourth transplanting date (20^th July/ 29^th SMW), followed by third date of transplanting (10^th July/ 28^th SMW); maximum damage was observed with the first date of transplanting (20^th June/ 25^th SMW, 0.35-9.58%) Rautaray et al. (2019) observed that the rice transplanted late (30^th August) had higher infestation. In the present study June transplanting was observed to be the best with least infestation. Rautaray et al. (2019) concluded that transplanting rice on first July would be a better option. As far as the crop stage is concerned, maximum damage (0.35-9.58%/ hill) was observed during vegetative phase of the crop, 30 days after transplanting (DAT) followed by 45 DAT (Table 1). Thus, the rice leaf folder incidence was more when crop was sown early. Similar observations had been reported by Ma and Lee (1996) and Chaudhari et al. (2018).

Figures in parentheses angular transformed values (p≥0.05)

References