Seasonal abundance and climatic correlates of fall armyworm (Spodoptera frugiperda J.E. Smith) in a maize ecosystem

Introduction

Maize is a key cereal crop for billions of people worldwide serving as feed, fuel, industrial raw material etc. Globally the area for maize crop is 193.7 million ha in around 170 countries with a production of 1147.7 million tonnes and productivity of 5.75 t/ha (Anonymous, 2020). Maize is referred as the "Queen of Cereals" due to its tremendous genetic yield potential. "It is being cultivated both in the tropical and subtropical climatic conditions. At present, the average yields of cereal grains are lower in India due to variety of factors, among which, the insect pests have been considered as one of the most important constraints. It is estimated that as many as 141 insect pests cause different degrees of damage to maize crop from sowing to harvesting" [1] . "Among these, stem borer (Chilo partellus), cob borer (Stenachroia elongella) and shoot fly (Atherigona soccata) were found to be as major pests. Stem borers are the major insect pests followed by defoliators but now, the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Noctuidae: Lepidoptera) a polyphagous and extreme pest of many important crops, including maize in India" [2] . The FAW invaded the Asian

Materials and Methods

The field experiment was conducted during Kharif,2024 maize variety Pusa Jawahar Hybrid maize 1 was sown in an area of 336 m 2 with plot size 4 × 3.5 m and spacing is 60 × 25 cm at Agricultural Research Farm, Institute of Agriculture science, B.H.U., Varanasi. Throughout the cropping season, the crop was maintained without any pesticidal application to allow natural infestation and observation of FAW population dynamics. Weekly observations were carried out starting from the third week after germination and continued up to harvest stage. In each plot ten plants were randomly selected and the number of FAW larvae present on each plant was recorded. Based on these the mean larval population per plant was calculated individually for each SMW to monitor pest progression over time.

The mean larval population of fall armyworm was calculated as

Mean Larval Population = Number of Larvae Ten Randomly selected plants

The number of plants infested with FAW larvae were observed for 10 plants from each plot, after the third week of germination and percent of infested plants (% IP) was calculated by using following formula The relationship between the FAW larval population and various weather parameters was analyzed by working out the correlation coefficients. The weather factors considered for this analysis included maximum temperature, minimum temperature, mean temperature, morning relative humidity, evening relative humidity, evaporation, bright sunshine hours (BSS) and rainfall. This analysis was undertaken to understand the influence of climatic conditions on the fluctuation and intensity of FAW infestation during the crop season. (Kumar et al., 2020) [8] .

The data obtained on the population variables, damage ratings were transformed and statistically analyzed.

Results

The observations on population dynamics of mean larval population of fall armyworm, Spodopetra frugiperda present per 10 randomly selected plants and percent leaf damage by selecting fresh damaged leaves on top 5 leaves by selecting 10 random plants have been summarized in Table 1 and Table 2 respectively.

In the present study, the incidence of FAW was observed starting from the first week of August (32 nd Standard Meteorological Week) and continued until the last week of October (44 th SMW). During this period, the larval population ranged from 0.97 to 4.08 larvae per plant.

Influence of weather factors on mean larval population of S. frugiperda on Kharif maize

The correlation analysis between weather parameters and the larval population of FAW has been depicted in table 3. Among all the parameters wind speed showed the highest positive correlation which is r= 0.60 with FAW larval population. This result indicate that increased in wind speed may accelerate the migratory movement of FAW from one plant to another probably supporting in their spread over cropping area. Evaporation exhibited a moderately high positive correlation which is r = 0.49, suggesting that less humid atmospheric conditions can possibly support the activity of FAW. In a similar manner minimum temperature (r = 0.40) and mean temperature (r = 0.38) were positively correlated, indicating that overall warmer conditions might be supportive for FAW survival and population increase. Bright sunshine hours also showed a positive correlation (r = 0.31), suggesting that increased in sunlight might be contributing factor for FAW population indirectly by affecting crop microclimate and insect behavior. Evening RH (r = 0.26) and mean RH (r = 0.20) had weak positive correlations, while morning RH was slightly negatively correlated (r =-0.14) suggesting that early day moisture may not in favor of FAW. The correlation of larval population with rainfall (r = 0.08) and maximum temperature (r = 0.09) indicated a very weak positive relationship, suggesting that these factors had minimal influence on FAW population buildup during the study period.

Discussion

The present study revealed that the larval population of S. frugiperda in maize was initiated from early August and peaked during September (37 th -38 th SMW), coinciding with favorable weather conditions of warm temperature, moderate rainfall, and higher evaporation. A gradual decline in larval population and infestation was observed from October onwards, likely due to less favorable climatic conditions and crop maturity. Similar trends have been reported by earlier workers, where peak activity of FAW was noticed during the vegetative to tasseling stage of maize under warm and humid conditions. The correlation analysis indicated that wind speed, evaporation, and temperature played a crucial role in pest buildup, whereas rainfall and relative humidity had little influence. These results suggest that September is the most critical period for FAW management in Kharif maize, and timely interventions during this phase could significantly reduce crop losses.

Conclusion

The study clearly demonstrated that fall armyworm infestation in maize begins in early August and peaks during September, with the highest larval population and leaf damage recorded between the 37th and 38th SMW. Weather factors, particularly wind speed, evaporation, and temperature, were positively correlated with population buildup, indicating their major role in pest proliferation. In contrast, rainfall and relative humidity showed minimal influence. A consistent decline in both larval population and crop infestation was observed from October onward, coinciding with crop maturity and less favorable conditions. These findings emphasize September as the critical period for FAW management, underscoring the importance of timely control measures to minimize crop losses.

References

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