Population dynamics of fall armyworm, Spodoptera frugiperda in maize

Introduction

Maize, scientifically known as Zea mays subsp. mays, traces its roots to Southern Mexico, where indigenous communities first domesticated it around 10,000 years ago. Among cereals, maize stands unparalleled in its vast potential, boasting remarkable biological and grain yield within a short growth cycle. Its distinction as the "Queen of cereals" arises from its extraordinary photosynthesis mechanism, attributed to the C4 mechanism, ensuring a significant place in the realm of agricultural importance. India produced 33.62 million tonnes in an area of 10.04 million hectares in 2021-22, whereas in Kharif 2022-23, maize production was 23.10 million tonnes in an area of 9.68 million hectares (Anon., 2022) [5] . In Gujarat, important districts growing maize are Dahod, Panchmahal, Vadodara, Kheda, Banaskatha, Bharuch, Anand and Dang. During 2020-21 in Gujarat, the kharif maize area, production and yield are 2867.05 ha., 4360.36 million tonnes, 1520.85 kg/ha yield respectively (Anon., 2020-21) [4] . The maize plant is attacked by 140 species of insect pests causing varying degrees of damage. The fall armyworm is an arthropod invasive pest and larvae are the most damaging stage. Its scientific name is derived from Latin, which is frugiperda for last fruit, named because of the ability of species to destroy crops (Anon., 2017a) [3] . The incidence of this pest was also reported on other crops in India viz., pearl millet and sorghum in Andhra Pradesh (Venkateswarlu et al., 2018) [18] , sugarcane in Tamil Nadu (Srikanth et al., 2018) [17] , paddy in Bihar (Ali et al., 2018) [2] , sugarcane and sorghum in Maharashtra (Chormule et al., 2019) [6] , sorghum in Karnataka (Sharanabasappa et al., 2019) [16] . Further, it was documented in Chhattisgarh (Deole and Paul., 2018) [8] , Telangana (Dayashanakar, 2018) [7] , Rajasthan (Ramesh Babu et al., 2019) [14] , Odisha (Sahu, 2019) [15] , Mizoram (Aizawl, 2019) [1] , Manipur (Leivon, 2019) [13] and Assam (Karmakar, 2020) [10] .

Materials and Methods

A study on the seasonal occurrence of S. frugiperda infestation in maize was conducted at the Instructional farm, Junagadh Agricultural University during the 2022, Kharif season. The experimental crop, adhering to recommended agronomical practices, was cultivated in a 20 m x 20 m plot with a spacing of 60 cm x 20 cm between rows and plants, respectively. The plot was subdivided into 1 m x 1 m quadrates. Throughout the season, the crop remained untreated with pesticides. Weekly observations on the number of S. frugiperda larvae were recorded on five randomly selected plants from each quadrate, starting one week after germination until crop harvest. The mean population of S. frugiperda larvae per plant was calculated separately. Additionally, percentages of cob damage and plant damage were determined.

Results and Discussion

Based on larval population

The fall armyworm, S. frugiperda, larval activity was observed from the first week of August 31 st Standard Meteorological Week (SMW) to the second week of November (46 th SMW), with a population range of 0.98 to 5.74 larvae per plant. The data, presented in Table 2 and Fig. 1, clearly showed the initiation of S. frugiperda population at the 31 st SMW, starting at 0.98 larvae per plant. Throughout the crop period, the larval population exhibited fluctuations. The larval population of fall armyworm consistently increased each week, reaching its peak of 5.74 larvae per plant during the 38 th SMW, corresponding to the third week of September. Subsequently, a decline in pest population began in the fourth week of September, with an average of 4.89 larvae per plant at the 39 th SMW. This downward trend continued in the following weeks, reaching its minimum of 0.21 larvae per plant during the 46 th SMW, aligning with the second week of November. Similar observations recorded by Harsh et al. (2020) [9] made similar observations in 2019, noting fluctuations in S. frugiperda larval population. In the 37 th SMW, a peak of 7.66 larva/10 damaged plants was recorded in the third week of September, followed by a decline to 4.33 larva/10 plants in the 40 th SMW (first week of October).

From the 1st week of August to the 2 nd week of November (31 st to 46 th SMW), the percentage of plants damaged by fall armyworm varied between 11.33% and 83.33%. The damage coincided with the larval population, reaching its peak of 83.33% during the 33 rd week of September 2022 (38 th SMW). The fluctuation persisted throughout the crop period. In their 2020 study, Harsh et al. examined the percentage of plants damaged by fall armyworm, S. frugiperda, coinciding with larval population from the 1 st week of August (31 st SMW) to the 1 st week of October (40 th SMW). The damage ranged from 10% to 81.66%, reaching its peak of 81.66% during the 3 rd week of September 2019 (37 th SMW). Meanwhile, Kuate et al. (2019) [11] found a positive and significant correlation between the incidence of S. frugiperda and the severity of damage.

The data on per cent of cob damage/quadrate are presented in Table 2. The data indicated that the per cent cob damage by fall armyworm commenced from the 2 nd week of September, 37 th SMW and continued till the 2 nd week of November (46 th SMW) which ranged from 44.61 to 77.27 per cent. Harsh et al. (2020) [9] observed that the per cent cob damage by fall armyworm started from the 4 th week of September, 38 th SMW and continued till the 1 st week of October (40 th SMW) which ranged from 46.66 to 60.33 per cent.

Correlation between Weather Parameters and Larval Population of Fall armyworm, S. frugiperda infesting maize

The results of the analysis on the correlation between the larval population of fall armyworm and weather parameters (Table 3) revealed that the larval population of fall armyworm had a highly significant positive correlation with mean temperature (r = 0.635**) while, it showed a nonsignificant positive correlation with bright sunshine hours (r = 0.137), maximum temperature (r = 0.025), minimum temperature (r = 0.461), morning relative humidity (r = 0.220), evening relative humidity (r = 0.128) and evaporation (r = 0.203). However, it exhibited a nonsignificant negative correlation with wind speed (r = -0.051) and rainfall (r = -0.152). This indicated that the larval population increased with the increase in the maximum, minimum and mean temperature while decreasing with an increase in rainfall and wind speed. While shifting the literature, according to Kumar et al. (2020) [12] larval population had a significant positive correlation with the maximum temperatures (r = 0.7205) and a significant negative correlation with relative humidity (r = -0.6739) and rainfall (r = -0.8293) during Kharif season which is closely in agreement with the present finding.

Conclusion

The S. frugiperda population exhibited a consistent weekly increase, reaching its zenith at 5.74 larvae per plant during the 38 th standard meteorological week, aligned with the third week of September. Following this peak, a decline ensued in the fourth week of September, with an average of 4.89 larvae per plant during the 39 th standard meteorological week. Examination of weather parameters revealed an inverse relationship between S. frugiperda population and rainfall. The population of S. frugiperda increased with the increase in mean temperature. Consequently, both rainfall was determined to be adversely correlated with the incidence of S. frugiperda in maize. In contrast, the consideration of temperature as a parameter indicated a positive correlation, signifying that the population of the maize fall armyworm, S. frugiperda, increased with rising temperatures.

References

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