Biology and morphometrics of fall armyworm, Spodoptera frugiperda (J.E. Smith) on sorghum

Introduction

Sorghum (Sorghum bicolor (L.) Moench) is an essential crop of semi-arid tropics. It is the fifth major cereal crop after rice, wheat, maize and barley in world (Sharma et al., 2003) [15] . Sorghum is a staple food for more than 600 million people who live in the semi-arid tropics and is grown in more than half of the world's semi-arid regions (Mohammed et al., 2015). Sorghum grain is highly nutritious, containing carbohydrates (70-80%), protein (11-13%), fat (2-5%), fibre (1-3%) and ash (1-2%). Being gluten-free, sorghum protein makes it suitable for individuals with diabetes and celiac disease (Prasad & Staggenborg, 2009) [12] . There are many biotic and abiotic factors that affect the quality and quantity of sorghum. Among them, insect pests are major factors that cause appreciable damage that ultimately effects on yield of sorghum. Nearly 150 insect species have been reported as pests on sorghum (Sharma, 1985) [16] . In India, there was a 35 per cent crop loss due to insect pests estimated at $580 million in sorghum and millet fields (Okosun et al., 2021) [11] . The fall armyworm, originally native to the Americas, is a highly polyphagous pest that causes substantial economic losses to crops such as maize, sorghum, beans and cotton (Abrahams et al., 2017) [1] . During migration, adult moths are capable of travelling more than 500 km before oviposition. Initially, outbreaks of this pest caused considerable panic among farmers; however, over time awareness has increased and farmers have learned to identify the pest and adopt appropriate management practices based on guidance and advisories from reliable sources. Fall armyworm is native to tropical and subtropical regions of the Americas (Goergen et al., 2016) [8] . The occurrence and prevalence of this invasive pest were noticed for the first time by Sharanabasappa et al. (2018) [14] from Karnataka, India, which is the first reported infestation in Asia, in July 2018 and subsequently, it has spread into ten other states of our country till the middle of March 2019. Bandi et al. (2023) [4] reported that 16 per cent of yield losses in sorghum due to fall armyworm. For any pest the biology is important step for identifying the various stages and its behaviour for successful IPM. Hence, the present study focused on the biology of FAW on sorghum under laboratory conditions.

Materials and Methods

The investigation on the biology of fall armyworm was carried out during kharif season of 2025 at the P. G. Research Laboratory, Department of Entomology, N. M. College of Agriculture, Navsari Agricultural University, Navsari. The larvae of fall armyworm were collected from a pesticide-free field and brought to the laboratory for rearing under room temperature conditions. The larvae were reared individually in Petri dish and fed on tender sorghum leaves. Fresh sorghum leaves were supplied daily to larvae in each Petri dish until they reached the pupal stage. A biology study was carried out on second generation population. Emerged male and female adults were allowed to mate in glass jar (21 × 15 cm) properly lined with paper and placed in adult rearing cage. The adults were fed with honey solution (5%) soaked on cotton pads in small plastic caps placed inside the jar and replaced daily. Observations were recorded daily 8:00 to 9:00 AM to check the egg laying on leaves and muslin cloth. The incubation period was calculated based on the date of egg laying to the date of hatching. The hatching percentage was determined based on the number of eggs hatched out of the total number of eggs observed. The duration of each larval instar was determined by recording the time interval between successive moults. The number of instars, duration of each instar and total larval period were recorded by observing larvae. Pre-pupal and pupal period were recorded by observing the duration from cessation of feeding to pupation and from pupation to adult emergence, respectively. Adult longevity, fecundity, oviposition period and sex ratio were also recorded under laboratory conditions. Morphometrical measurements of egg, larva, pupa and adults were recorded using suitable microscopic observations and measuring scales. The observations were statistically analysed.

Results and Discussion Incubation period and hatchability

The biological observations of S. frugiperda under laboratory conditions are presented in Table 1. The incubation period varied from 2 to 4 days with an average of 2.38 ± 0.60 days. The present findings are in more or less similar with Kranthi et al. (2021) [10] who observed 2.47 ± 0.05 days of incubation period reared on sorghum. Chaudhary et al. (2023) [5] noted 3.06 ± 0.45 days of incubation period on maize. The hatchability varied from 73.86 to 89.13 per cent with an average 78.96 ± 4.52 per cent. The present findings are in more or less similar with Chaudhary et al. (2023) [5] who observed that S. frugiperda egg hatchability was 73.23 ± 6.72 per cent in maize. Siddhapara et al. (2021) [17] reported that S. frugiperda egg hatchability ranging from 72.00 to 95.54 per cent with a mean of 83.34 ± 9.87 per cent in maize. However, higher hatchability was reported by Gamil (2020) [7] and Vinay et al. (2022) [19] who recorded values up to 97.33 per cent and 96.26 per cent, respectively on maize.

Larval and pupal period

The larva of S. frugiperda passed through six distinct larval instars. The duration of first to sixth larval instar ranged from 1 to 3, 2 to 3, 1 to 3, 2 to 4, 2 to 4 and 3 to 5 days, respectively. The average duration of first to sixth instar larvae were 1.98 ± 0.51, 2.18 ± 0.39, 2.34 ± 0.69, 2.28 ± 0.54, 2.74 ± 0.78 and 3.70 ± 0.61 days, respectively. The total larval period varied from 12 to 19 days with an average of 15.22 ± 1.43 days under laboratory conditions (Table 4.1). The present investigations are supported by Sharanabasappa et al. (2018) [14] who reported that a total larval period on maize ranging from 14 to 19 days with a mean of 15.90 ± 1.45 days. Kalyan et al. (2020) noted that total larval period completed in a mean duration of 16.97 days in maize. Devi et al. (2024) [6] stated that a mean total larval duration of 17.65 ± 1.59 days in maize. The variation in the findings may be due to different environmental conditions and the host used for the study. The duration of pre-pupal stage ranged from 1 to 2 days with a mean of 1.32 ± 0.47 days (Table 4.1). The present findings are in partial agreement with Chaudhary et al. (2023) [5] who recorded that the duration of the pre-pupal stage ranged from 1 to 2 days with an average of 1.57±0.50 days in maize. The pupal period of S. frugiperda ranged from 8 to 10 days with a mean duration of 8.92 ± 0.70 days. The present findings are in partial agreement with Kashyap and Malik (2024) who recorded an average pupal duration of 9.20 ± 1.64 days. Chaudhary et al. (2023) [5] noted pupal period ranged from 8 to 13 days with an average of 9.24 ± 1.60 days. Siddhapara et al. (2021) [17] reported a mean pupal duration of 9.30 ± 0.65 (range of 8-10 days) days

Sex ratio and adult longevity

Out of the total 50 adults emerged, 28 were females and 22 were males, indicating a higher proportion of female adults of S. frugiperda. Hence, the sex ratio (male: female) was calculated as 1:1.27. The present findings are more or less agreed with reports of Ahir et al. (2020) [2] who observed a male-to-female ratio of 1:1.30. Siddhapara et al. (2021) [17] also reported a female-biased sex ratio in S. frugiperda (male: female 1:1.13). Female adults exhibited a longer lifespan compared to males. The longevity of male adults ranged from 7 to 9 days with an average of 8.04 ± 0.84 days, whereas females survived for 8 to 11 days with an average of 10.00 ± 0.82 days (Table 4.1). The studies on adult longevity are more or less in accordance with Siddhapara et al. (2021) [17] who reported that adult longevity of S. frugiperda were 7.90 ± 0.84 days in males and 10.60 ± 1.22 days in females. Similarly, Sharanabasappa et al. (2018) [14] noted that female adults survived longer (mean 10.80 days) than males (mean 8.20 days). Ahir et al. (2020) [2] observed adult longevity in males (mean: 8.63 ± 1.05 days) and females (mean: 10.73 ± 0.72 days).

Pre-oviposition, Oviposition period, Post-oviposition period and fecundity

The pre-oviposition period of S. frugiperda ranged from 3 to 5 days with an average of 3.44 ± 0.65 days under laboratory conditions (Table 4 [17] who reported pre-oviposition periods of 3.47 days and 3.50 ± 0.51 days, respectively. Chaudhary et al. (2023) [5] noted a pre-oviposition period of 3.08 ± 1.24 days. The oviposition period of S. frugiperda ranged from 2 to 4 days with an average of 2.84 ± 0.69 days (Table 4.1). Sharanabasappa et al. (2018) [14] and Kalyan et al. (2020) reported oviposition period recorded as mean of 2.8 days and 2.96 days, respectively. Siddhapara et al. (2021) [17] showed mean oviposition period of 3.20 ± 0.89 days. Thus, the results of the present observation are more or less similar with past findings. The post-oviposition period of S. frugiperda ranged from 3 to 5 days with an average of 3.76 ± 0.78 days (Table 4 [14] reported a postoviposition ranged from 4 to 5 days (mean 4.30 days). Ashok et al. (2020) [3] and Chaudhary et al. (2023) [5] reported comparatively lower mean post-oviposition period of 2.60 ± 0.699 64 days and 1.22 ± 1.52 days, respectively. The variation in post-oviposition period among different studies may be attributed to differences in environmental conditions such as temperature and relative humidity as well as laboratory rearing techniques. Under laboratory conditions, a single female of frugiperda laid an average of 1216.88 ± 152.98 eggs with minimum egg laying of 832 to maximum 1428 eggs (Table 4.1). The fecundity recorded in the present study was accordance with Chaudhary et al. (2023) [5] who revealed that each female of S. frugiperda laid 1272.88 ± 503.15 eggs with a range of 460 to 2424 eggs. Siddhapara et al. (2021) [17] recorded 1145.43 ± 182.15 eggs per female with a range of 766 to 1389 eggs. Devi et al. (2024) [6] observed that an average fecundity of S. frugiperda was 1104.4 ± 93.60 eggs per female.

Total life span

The total life span of male S. frugiperda was an average of 35.08 ± 1.98 days with a range of 30 to 38 days (Table 4 [17] reported total life span of S. frugiperda male was 34.50 ± 1.80 days (range: 32-38 days). Thus, the present findings are closely confirmity with past reports. The total life span of female S. frugiperda was an average of 37.56 ± 2.24 days with a range of 32 to 40 days (Table 4.1). The present findings are in close agreement with Siddhapara et al. (2021) [17] who reported that total female lifespan of S. frugiperda was 37.20 ± 2.52 days (range: 32-40 days). Tiwari and Deole (2021) [18] also recorded total female life cycle with a mean of 36.0 ± 0.75 days (range: 30-45 days).

Morphometrical measurements of egg, larva, pupa and adult

The eggs were dome shaped, dorso-ventrally flattened and pale green in colour initially, which turned golden yellowish and blackish in colour before hatching. The length and breadth of eggs were 0.44 ± 0.04 mm and 0.42 ± 0.04 mm, respectively (Table 2). These measurements are accordance with Siddhapara et al. (2021) [17] who recorded that S. frugiperda egg dimensions were 0.43 ± 0.01 mm in length and 0.42 ± 0.03 mm in width. Similar observations were observed in Prasanna et al. (2018) [13] and Chaudhary et al. (2023) [5] , respectively. The neonate larvae were very small with a comparatively large black head. The larval body was pale green to yellowish in colour, bearing two black setae on each segment. The mature larvae had reddish brown head with cream body bears three white dorsal lines and alight lateral line. An inverted 'Y' shaped epicranial suture on the head and four black spots arranged in exact square on 8 th abdominal segment were the key identification characters of sixth instar larvae. The length of first to sixth instar larvae were 1.09 ± 0.04, 3.72 ± 0.22, 6.58 ± 0.53, 12.84 ± 0.92, 22.55 ± 2.19 and 30.33 ± 2.04 mm, whereas breadth were 0.22 ± 0.03, 0.54 ± 0.07, 0.87 ± 0.08, 1.16 ± 0.15, 2.35 ± 0.38 and 3.14 ± 0.64 mm, respectively. Similar observations were observed in Kalyan et al. (2020). The pre-pupa was pale green to bright brown in colour, darker and wrinkled body. The pupa was reddish brown in colour and with a pair of cremasters at the posterior end. The pupa was obtect type. The distance between genital opening and anal slot was more in female pupa compared to male pupa. The length and breadth of pupa was 17.08 ± 1.28 and 4.87 ± 0.13 mm, respectively. The present investigations are close agreement with Jaba et al. (2020) [9] who recorded an average pupal length and breadth were 16.89 and 4.73 mm, respectively. Sexual dimorphism was clearly observed in adults. In males, the forewings were shaded grey and brown with reniform indistinct spot, faintly outlined in black, with a small Vshaped mark; light brown orbicular spot, somewhat oval and oblique in shape and white patch at the apical margin of the wing. The forewings of females were less distinctly marked, varying from uniform greyish-brown to finely mottled grey and brown patterns and without white patch near the apical margin of the wing as seen in males. In both sex, the hind wings were iridescent silver-white with a narrow dark border. The body length of male and female adults were measured as 17.27 ± 1.46 and 14.23 ± 1.72 mm. These findings are in close agreement with Chaudhary et al. (2023) [5] who noted that the length of male moths ranged from 13 to 21 mm with an average of 16.62 ± 1.66 mm and female moths ranged from 13 to 19 mm with an average of 14.76 ± 1.58 mm. whereas wing expanse were 31.68 ± 0.94 and 32.28 ± 0.27 mm, respectively.

Acknowledgement

Acknowledgements:The authors are grateful to the Department of Entomology, N. M. College of Agriculture, Navsari for their unwavering support.

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