The life history of
H. armigera comprises of four stages,
i.e. egg, larva, pupa and adult. The selected populations exhibited considerable differences in the mean duration of different life stages. The incubation period was recorded to be maximum in the Cooch Behar population (3.72±0.09 days) of the North East Plain Zone which was at par with all the other studied populations except those collected from Hyderabad (3.42±0.07 days), Ayodhya (3.44±0.11 days), Raichur (3.46±0.10 days) and Jabalpur (3.46±0.05 days) (Table 1). Earlier,
Jallow and Zalucki (1998) also observed variations in the oviposition behavior and host plant preference among geographic populations of
H. armigera collected from diverse sources in Australia by offering the same set of host plants. The incubation periods described herein for
H. armigera are also consistent with what has previously been observed by
Deepa and Srivastava (2010);
Chaudhary et al., (2016) and
Chakravarty et al., (2018).
Significant variations were also noticed in the duration of different larval instars and the total larval period ranged from 17.47±0.94 days in the Hyderabad population to 23.37±0.62 days in Cooch Behar population (Table 1). Significantly lower larval developmental periods were also recorded for the other locations of the South Zone and all being at par with the Hyderabad population. These are in accordance with earlier reports of
Sharma et al., (2011); Baikar and Naik (2016) and
Chauhan et al., (2018). The maximum pre-pupal period was recorded from the Samastipur population (2.32±0.11 days) of the North East Plain Zone and it was at par with all the other northern and central zone populations of the country (Table 1). The South Zone populations,
i.e. Hyderabad (1.84±0.04 days), Raichur (1.90±0.07 days), Bagalkot (2.06±0.10 days), Bengaluru (1.92±0.05 days) and Coimbatore (1.86±0.05 days) populations recorded significantly lower pre-pupal period in comparison to populations from other three zones. However, evaluation of this stage has not been considered by many authors, probably due to the difficulty involved in observing this development stage
(Nunes et al., 2017).
The pupal period ranged from 7.96±0.17 days to 13.68±0.24 days with an average of 12.14±0.40 days in males and 10.00±0.33 days in females, which interestingly reveals that in case of
H. armigera, adult female moths appear about two days earlier than males (Table 1). The male pupal period was recorded to be significantly higher in populations from Cooch Behar (13.68±0.24 days), Raipur (13.26±0.43 days) and Pune (13.20±0.75 days). In comparison, it was recorded to be minimum from the Hyderabad (9.56±0.30 days) population closely followed by other South Zone populations, all being at par with each other. The pupal period variation in female populations also exhibited a similar trend and ranged from 7.96±0.17 days in the Hyderabad population to 11.36±0.27 days in the Cooch Behar population.
Deepa and Srivastava (2010);
Sharma et al., (2011) and
Baikar and Naik (2016) found similar results while evaluating the biological characteristics of this pest species on different host plants across India. More recently,
Chakravarty et al., (2018) also conducted biological studies on
H. armigera under laboratory conditions and recorded its average pupal period as 10.24±0.46 days for females and 12.52±0.34 days for males.
Various studies have shown that heliothine females had a greater tendency to emerge out, especially after diapause than males and had shorter pupal periods
(Silva et al., 2017). In the present study also, asynchrony was observed in the emergence of the adults, with the females emerging earlier than males. The female moths are skilled in searching for the best habitat for oviposition, while the males need to locate these females
(Nunes et al., 2017). They explain the importance of the females emerging first as these moths go to find the appropriate host while they mature sexually and prepare to liberate the sexual pheromone to attract the partners, which emerge two days later on average. Similar studies conducted by
Tripathy and Singh (1999) on biological parameters of
H. armigera populations collected from Guntur, Varanasi and Hissar locations of India showed a higher percentage of adult emergence (86.32%) from pupae in Varanasi strain followed by Guntur (84.41%) whereas; only 73.80 per cent emergence was recorded from populations collected from Hissar. The average pupal periods from the three places were 10.53, 12.37 and 39.49 days, respectively.
Further, the studied populations also differed significantly for the longevity of both male and female moths as well as the total life cycle (Table 1). The Cooch Behar population had longest adult longevity (8.82±0.15 days and 11.40±0.21 days, respectively for males and females) and also took maximum duration to complete its entire life cycle (51.82±0.89 days and 52.36±0.84 days, respectively for males and females). The populations from South Zone took a significantly lower duration over others to complete their life cycle. Adults were inactive during most of the day and were active at night (after evening twilight), during which copulation usually starts. Pairs remained in copula for a period of few hours up to one day. The pre-oviposition, oviposition and post-oviposition periods were again recorded to be highest in the Cooch Behar population (2.86±0.04, 6.74±0.25 and 1.78±0.11 days, respectively). The minimum duration for these parameters was recorded from the South Zone populations. The minimum pre-oviposition period (1.90±0.11 days) was found in the Bengaluru population, while Hyderabad and Raichur populations had significantly lower oviposition (4.68±0.30 days) and post-oviposition periods (0.96±0.08 days), respectively (Table 2).
All the studied populations were found to be female-biased with male: female sex ratio varying from 1: 1.10 to 1: 1.46 and the average fecundity of the female moths were recorded to be significantly higher in the Varanasi population (1140.80±161.28 eggs) of the North East Plain Zone (Table 2). The growth and fitness indices of
H. armigera populations also varied significantly (Table 3). The larval, pupal and immature growth indices were found to be maximum (5.12±0.34, 9.87±0.44 and 3.30±0.22, respectively) in Hyderabad population while Varanasi population had significantly higher standardized insect growth index (15.08±0.76) and fitness index (8.71±0.30). The other South Zone populations also exhibited significantly higher growth and fitness. These indexes were recorded to be lowest in Cooch Behar population (3.32±0.16, 6.41±0.21, 2.24±0.08, 11.56±0.35 and 5.84±0.27, respectively). It was also found that the populations having shorter life cycles, in turn, gained adaptive advantages (higher fecundity, better growth and fitness) in relation to other populations. These results are also similar to those described by
Sharma et al., (2011); Amer and El-Sayed (2014) and
Chauhan et al., (2018).
Longevity and reproductive potential of
H. armigera are generally influenced by dietary situations during both the larval and adult stages and poor nutrition experienced by the parents during their development leads to lower fecundity
(Bheemaraya et al., 2019). Several studies have also shown that the diet quality of parents not only affects the production of offspring but can compromise future generations, by reducing immunity and resistance to parasitism
(Sternberg et al., 2015). However, in this study, variations in the biological attributes among the populations were observed, even after being fed with the same artificial diet and keeping all the other variables (temperature, humidity and photoperiod) constant for two consecutive generations. This indicates that such variations are probably influenced by genetic differences among populations, as also hinted by
Chen et al., (2013) and
Chakravarty et al., (2019). However, in contrast with present findings,
Silva et al., (2017) found no significant differences in the adult longevity and fertility parameters of
H. armigera populations collected from different hosts and geographic locations of Brazil. The lack of such differentiation could be due to its recent incursion and expansion in the South American continent.
Variations at the genetic level of this pest species have also been reported by various researchers in India (
Deepa and Srivastava, 2011; Behere
et al., 2013 and
Venkatesan et al., 2016). The pesticide resistance level of the insects may also lead to variations in their life histories (
Gandhi and Patil, 2017). The resistance level of
H. armigera populations used in the present study may also be different because they were collected from different regions of the country having varying pesticide usage patterns and this can also be a probable reason of why they are exhibiting variability in their biological attributes. The cluster analysis of the populations based on the similarity degree of selected biological parameters differentiated them into two major groups/ clusters (Fig 2). Both the clusters showed intermingling of the populations from northern and central pulse growing regions of the country. This supports the presumption that substantial gene flow occurs among populations, probably due to long-range migratory capacity of this species
(Silva et al., 2017). However, the southern zone populations formed a distinct subclade within the second cluster (Y). The topological and temporal barriers must have led to the isolation of these populations from others. However, they also freely interbreed with populations from other parts of India and produce normal fertile offsprings
(Chakravarty et al., 2019).
