Association between temperature and life table development of Fall armyworm Spodoptera frugiperda under control condition


 Fall armyworm Spodoptera frugiperda Smith (Lepidoptera: Noctuidae) (FAW) is one of the devastating polyphagous pests having a wide range of host plants that contribute substantial losses to crop cultivation. For such pests, temperatures exert a significant impact on biology, distribution, and abundance. Hence the present study was executed with the aims of evaluating the effects of different temperatures on the growth and development of fall armyworm. FAW larvae collected from the infested field and were reared in three different temperatures regimes viz. 25±1°C (T25), 30±1°C (T30) and 35 ±1°C (T35) incubator provided with 12:12 photo period. Life table and fecundity tables were calculated using growth parameters. The results indicated that the mean duration of each stage of lifecycle at three different temperatures were significantly different from each other. The accumulated mortality at the adult stage was 95%, 61% and 64% at T35, T30 and T25 respectively. The net reproductive rate (Ro) was 132.17, the average period of generation time (Tc) was less in T30 (32.31) with the daily finite rate of increase in number (λ) was 1.1702 females/female/day and a population doubling time of 3.004 days. Similarly, the intrinsic rate of natural increase in number (rm) was 0.1511 females/female/ day and hypothetical female population in the F2 generation was found to be higher at T30 (17468.20) thus the results indicated that T30 favors the growth and development of FAW compared to the other tested temperatures. Our findings could be useful for the implication in pest management and forecasting.


Introduction
The environmental, economic, and social impacts of invasive alien species are jeopardizing the livelihoods and food security of small-scale farmers in developing countries. (Early et al., 2016;Paini et al., 2016;Pratt et al., 2017). Among them, the Spodoptera frugiperda, also known as the fall armyworm (FAW), is a polyphagous lepidopteran pest native to tropical and subtropical America and mainly associated with maize crops (Cruz et al., 1999;Sarmento et al., 2002). It is a member of the genus Spodoptera, of the Noctuidae family, which is responsible for the world's highest monetary losses to agriculture (Groote et al., 2020) due to the nature of highly mobile insect pests infesting with a wide range of host crops. However, this tropical pest cannot survive in cold temperatures for long periods (Westbrook et al., 2015). Hence high variations in the weather in the Americas allow the FAW, to migrate to other countries, continually which has no diapause (Geogern et al., 2016). In late 2016, the rst case of FAW was reported in West Africa and since spread rapidly across Sub-Saharan Africa, with its presence now con rmed in 44 African countries (Sisay et al., 2019). In 2018 it was reported in Asia damage was primarily in India (Karnataka) (Sharanabasappa et al., 2018). In Sri Lanka, a recent outbreak of FAW caused signi cant yield losses of maize, especially in the Uva, Eastern, and North Central provinces. The damage was rst recorded in October 2018, and the majority of small-scale farming families have been severely (Perera et al., 2019). It was later recorded in the districts of Anuradhapura, Monaragala, Kurunegala, Kandy, Badulla and Rathnapura which belongs to Dry and dry intermediate agro ecological zones in the country (Wijerathna et al., 2020). As such the temperature might have a direct impact on the life cycle of FAW.
Theoretically, the thermal optimum is de ne as the temperature at which a species thrives well in terms of growth, reproduction, and survival (Begon et al., 2006). When the maximum and minimum temperatures are within the species' optimum it promotes, insects' growth and development (Hagstrum & Hagstrum, 1970: Howe, 1967. Further the length and number of each instar that larvae go through before reaching adulthood is in uenced by temperature (Aguilon et al., 2015). Moreover, the relationship between temperature and development rate has a huge impact on pest biology, distribution, and abundance (Tobin et al., 2003). Increased ambient temperature near the thermal optimum of insects causes an increase in metabolic activities (Jaworski et al., 2013).
In ecological perspective, life tables are useful resources for evaluating, predicting and comprehending insect population development rates (Ashok et al., 2020). However, recent publications have focus on revealing the impact of FAW in selected speci c temperatures, though the impact of various temperature has not been revealed. Owing to such limitations, our present study was carried out with the aim of evaluating the life tables of FAW on selected three different temperatures regimes under laboratory conditions.

Insect stock colony
The FAW larvae (F 0 generation) were collected from naturally infected maize elds from the University experimental station at the Agrotech park Malwatte, the South Eastern University of Sri Lanka in Ampara district belongs to the low country dry zone (DL 2b;N 07° 19′ 17.30″ E 81° 43′ 56.19). The collected larvae were reared under constant laboratory conditions (Temperature 26 ± 1 °C, 70% RH and 12L:12D photoperiod) by providing a natural diet (Plessis et al., 2020) and maintained until the pupal stage in the Biosystems Technology laboratory facilities at the Faculty of Technology. The pupae were separated under the optical stereo microscope (LAB 20, Optika, Italy) according to the method previously described by Plessis et al., (2020). Separated pupae were placed in oviposition cages (30 cm H. × 10cm D.) for adult emergence.

Life table study
The uniform three batches of 100 to 150 eggs (F 1 generation) were obtained from the stock (F 0 ) colony and kept in three insect incubators (DI 120 Drag Lab Germany) and were maintained constant different temperature conditions viz. 25 ± 1°C (T25), 30 ± 1°C (T30) and 35 ±1°C (T35) until completion of progeny. The eggs were inspected under the Optical stereo microscope on daily basics to record the egg hatchability and days taken for hatching. As soon as 1 st instar larvae emerged, they were transferred to the individual transparent plastic bottles (10 cm D. × 5 cm H.) and reared on the natural diet according to the Plessis et al., (2020). Then the individual larvae were observed under the optical stereo microscope to determine the duration of 1 st , 2 nd , 3 rd , 4 th , 5 th and 6 th larval instars and were recorded until pupation by observing at their head capsules and exuviae (Plessis et al., 2020). Moreover, the duration of prepupal, pupal and adult stages were recorded.
Determination of oviposition, fecundity and longevity of FAW Ten couples from the emerging population of the above progeny were placed in oviposition cages and maintained in respective three different temperatures to obtain the age-speci c fecundity data. The procedure continued until end of the adult emergence. A moistened muslin cloth was provided as oviposition substrate with 10 % sucrose solution was provided to feed the adults. In each cage, the pre oviposition, oviposition, and post oviposition periods, number of eggs per day were recorded at daily intervals until the death of the female while oviposition substrate was replaced on a daily. Then the number of female birth (mx) was estimated by assuming the 1:1 sex ratio as the number of eggs/day divided by two (Ashok et al., 2020).

Determination of the age-speci c fecundity tables
The age-speci c fecundity tables for different temperatures were prepared using daily observations of con ned FAW couples (Equation 1, 2, 3 and 4) here x = Pivotal age in days; lx = Survival of female at age 'X'; mx = Age schedule for female births at age 'X' and e = 2.71828. The Net productive rate (Ro) in generation as measured in females deliver per generation, Mean duration of generation (Tc) or mean age of the mothers in a cohort at the birth of female offspring, Innate capacity for increase (rm) or at each age interval, the total number of individuals who survived and the mean number of female offspring births were recorded (Atwal & Bains, 1974;Howe, 1953). Subsequently, estimated the precise generation time (T), the nite rate of natural increase (λ) or a number of females per female per day, the population's weekly multiplication and hypothetical F 2 females were calculated.

Cohort life tables
Age-speci c life table or cohort life table for fall armyworm was developed by Kyi et al., (1991) and Krebs (1978a and2001b) Equation 5, 6, 7, 8 and 9 were used. The Krebs formulas were applied to calculate the results, i. e. x = age interval (days) of each stage, egg, 1 st , 2 nd , 3 rd , 4 th , 5 th , 6 th larval instar, pupa and adult, nx = number of survivors at start of the age of interval x, dx = number of dying during the age interval x to x+1, qx is the rate of mortality during the age interval x to x+1, lx is the proportion of organisms surviving to start at age interval x, Lx is the number of individuals alive on the average during the age interval x to x+1, Cumulative number of alive in age interval x to x+1, e x mean expectation of life for organism alive at the start of age x.

Data analysis
The mean duration of each stage of the FAW life cycle in different temperatures was compared using a one-way analysis of variance (ANOVA) followed by Tukey's post hoc test at a 5% signi cant level.

Results And Discussion
Biology of FAW The present results indicated that the mean longevity for each stage of development at three different temperatures was signi cantly (p<0.05, df = 2) vary except for the egg hatching period. Where 1 st to 3 rd instar larvae recorded the longest longevity at T25 and 4 th to 6 th instar larval longevity was longer at T35 (Table 1). However, the total larval longevity was higher at T25 (20.17 days) and T35 (17.89 days) compare to the T30 (14.92 days). Conversely, pupal duration at T35 (5.75 days) was the shortest compared to the other tested temperatures. The longevity of female moths at T35 was signi cantly lower (5.66 days) than T30 and T25 (Table 1). The table indicates the;nx = number of survivors at start of the age of interval x, lx = Survival of female at age x, dx = number of dying during the age interval x to x+1, qx = rate of mortality during the age interval x to x+1, Lx is the number of individuals alive on the average during the age interval x to x+1, Cumulative number of alive in age interval x to x+1, e x = mean expectation of life for organism alive at the start of age x (n = 100).
The cohort life table result of each temperature revealed that the lowest survivorship at T35 resulted from 95% accumulated mortality (dx) from 100 eggs followed by ve adults with a sex ratio (3:1). Besides high mortality was observed in the egg (42) and 1 st instar larval (36) stages with qx values 0.42 and 0.62 respectively ( Table 2). The results further reveal that egg hatchability and survivorship of subsequent larval stage were highly dependent on temperature whereas constant T35 exhibited increase mortality of eggs, early instars and males are more prone to heat stresses and vulnerability. On the other hand, the highest survivorship was observed at T30 (39%) ( Table 3) with less accumulated mortality rate (61%) were resulting in 59 adults from 150 eggs. In addition, egg mortality was less compared to the T35 and the highest dx was in the 1 st instar larval stage (32). Similarly, the qx values are very low in egg (0.08), 5 th (0.01), 6 th (0.03) and pupal (0.06) stages. Though, this temperature range lies between optimum thermal condition recorded the considerably higher mortality percentage due to constant temperature conditions and daily handling the larvae for observation in uencing the natural behavior of the FAW. The table indicates the;nx = number of survivors at start of the age of interval x, lx = Survival of female at age x, dx = number of dying during the age interval x to x+1, qx = rate of mortality during the age interval x to x+1, Lx is the number of individuals alive on the average during the age interval x to x+1, Cumulative number of alive in age interval x to x+1, e x = mean expectation of life for organism alive at the start of age x (n = 150).
Similar to T30, higher survivorship (36%) was observed with less mortality at T25 than T35. It resulted 54 adults with the sex ratio of 2:1. However, as similar to the other two temperature conditions high qx was found in the 1 st instar stage (0.3) meantime the zero qx values were found in the 4 th and 6 th instar larval stages. The present study revealed that, female pre-oviposition period remained at 28 to 30 days and pivotal age was 31 to 33 days at T30. The rst batch of eggs was laid by females after 31 days with mx of 160.27 and lasted until the 38 days (mx = 11), where lx values of 0.28 and 0.03 respectively for 31 and 38 days ( Figure 1). At 32 days of pivotal age, the rst female mortality was recorded on the 4 days after the emergence and the female mortality increased, as evidenced by a steady decrease in the lx values. It had a lx value of 0.24 on the 32 days of pivotal age. The maximum mean progeny production/day (mx = 168.57) was reported on the 32 days.
As same in T25 pre oviposition and oviposition periods took comparatively longer durations while the rst batch of eggs was laid by females on the 34 days. They had mx of 258.86 and lasted until the 42 day (mx = 09), with lx values of 0.17 and 0.03 respectively (  Ashok et al., (2020) by evaluating adult longevity, oviposition period, fecundity and fertility using 30 pairs of FAW at 27 ºC. They reported adult longevity of 12.6 days and 11.1 days for females and males, respectively.
Based on our ndings, the net reproductive rate (Ro) was 480.33 (n = 36) with a generation time (Tc) of 31.24 days. The determined intrinsic rate of natural increase (rm) and the nite rate of increase (λ) was 0.20 and 1.22 females/ female/ day, respectively. The population doubling time was found to be 3.51 days. The FAW life cycle follows complete metamorphosis (holometabolous) consisting of 4 stages viz, egg, larva, pupae and adult stages (Assefa & Ayalew., 2019). The longevity of each stage was varying different day intervals viz, 2 days for egg hatching, larval longevity (12 days), pupal (7 days) and adult longevity (14 days) (Tendeng et al., 2019). However, several authors reported various longevity for different stages of FAW according to the various climatic conditions (Ashok et al., 2020;Plessis et al., 2020). As per the biological parameters of FAW at 25°C Kalyan et al., (2020)  The FAW female moth normally deposits the majority of her eggs during the rst 4 to 5 days of life followed by 3-4 days pre oviposition period, but oviposition can continue up to 3 weeks. Adult life expectancy is estimated to be about 10 days on average, with a range of 7-21 days (Prasanna et al., 2018). According to our study pre-oviposition periods lay between 2.5 to 3 days and total female longevity extends to a maximum of 9 days for both T25 and T30 conditions while 5 days of total life durations are reported at T35.
In higher temperature conditions like T35 results, only 5 adults survive from 100 eggs while the remainder fails to rescue compared to the other two temperature conditions. This may be due to continuous exposure of high temperature to eggs at early development instars tend to increase water losses through the unsclerotized cuticle while on the other hand increasing metabolic rates may lead to rapid depleting the foods reserves. In contrast, other tested two temperatures may lie within the optimum thermal range required for their growth and development. Our observation further con rms that males were more susceptible to rising temperatures than females. Due to the low male to female ratio at T35, the progeny tness became lower and mating was not taken place thereby both males and females were died within 6-7 days without laying eggs. Therefore, it was not possible to construct the life fecundity table, mean length of a generation, innate capacity for an increase in numbers and nite rate of increase in numbers of FAW at T35. Further, our results indicated that the egg hatchability and survivorship of the rst instar larvae were more susceptible to temperature changes. Therefore, the changing temperature can be used in the eld conditions at the stage of eggs and rst instar larvae for the manipulation of FAW populations. Our ndings could be useful implicating in pest forecasting and integrated pest management programs in the eld and control environmental conditions.

Declarations
Author Contribution ADNTK and GKMMKR conceived and designed the study. AMKGM and MBA conducted experimental work, data collected. ADNTK and GKMMKR analyzed the data, and drafted the manuscript. ANM and ADNT supervised the AMKGM and MBA corrected the draft manuscript and improved. SRS supply the planting materials and improved the manuscript. All authors critically revised the draft and approved the nal manuscript for publication.