Influence of Meteorological Factors on Population Dynamics of Fall Armyworm, Spodoptera frugiperda , Lepidoptera: Noctuidae and its Varietal Susceptiblilty to FAW †

: The invasive armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is a key pest of maize that has invaded Pakistan causing severe damage and yield losses. The current investigation was directed to visualize fall armyworm infestation on eight of the different maize varieties. Correlation analysis of FAW infestation with weather factors revealed that temperature (max., mean, and min.) had a weak positive effect on fall armyworm infestation (r = 0.139, r = 0.149 and r = 0.159, respectively) while relative humidity played a weak negative (r = -0.104) role in FAW population build-up. A seasonal mean infestation of the pest found maximum on Malka-16 which was statistically higher than any other variety. The lowest invasion was recorded on the P1543 Hybrid (P < 0.05). Similarly, the infestation was significantly highest in the 1 st half of November (P < 0.05), a decline in pest population build-up was seen in December possibly by cold weather.


Introduction
Fall armyworm (FAW), Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae) is an invasive pest of maize crop.It is native to the Americas and was first reported as an aggressive pest in Africa in the tropical zones of In Pakistan, it has been seen from the recent invasions of fall armyworm that the crops planted by smallholders were more prone to damage.The reason is simple that they cannot afford expensive agricultural inputs.Most of the maize growers in the country are smallholders who always look for low-cost managemental practices to curb pest problems.The use of resistant varieties is an old method and is frequently adopted by smallholders (Abid et al 2019).The studies on the screening of variable varieties having different genomes against exotic pests could be an effective initiative to determine the extent of tolerance found in such varieties (Kasoma et al 2020).
The fluctuations in the population build-ups of insects are directly linked with biotic and abiotic factors that prevail in the micro and macro climates of insect pests (Kamata 2000).Insect populations do not remain the same throughout the year, they get vary with the change in environmental factors like temperature, humidity, or rainfall (Wallner 1987) The poikilothermic nature of the insects makes them highly sensitive to external temperatures.In general perception, temperature is regarded as the most significant environmental factor influencing the development, behaviour, distribution, reproduction, and survival of insects (Briere et al 1999).Temperature-dependent studies of insect pests are always very crucial in understanding the population dynamics of insects and their implementation helps in designing specific pest-control strategies (Ahmad

2016).
Another most important abiotic factor that determines insect survival is the humidity.Variation in humidity levels influences the developmental stages of insects.A high level of relative humidity (RH) could be advantageous for the survival and growth of immature stages of certain insect species (Lu & Wu 2011).A few investigations declare the higher levels of RH as a source of mortality in insects through fungal (Shipp et al 2003) or viral (Fuxa et al 1999) infections.Similarly, low RH may stop embryonic development and loss of softness in the cuticular layer (Guarneri et al 2002).
The present study deals with fall armyworm population density on eight maize varieties grown under agroecological conditions of the Punjab, Pakistan and the relationship of weather conditions e.g., temperature and relative humidity against FAW population build up on eight maize varieties.

Materials and Methods:
Study site description: The study was conducted at Fisheries and Crop Research Station, University of the Punjab (31.498356°N, 74.293647°E).The site was characterized by well-drained clay to loamy soil with a relatively leveled cropping area.In order to study the effects of abiotic factors on FAW infestation upon different varieties of maize, an experiment was conducted following the randomized complete block design (RCBD).Eight varieties including four conventional and four hybrids were cultivated and the sowing was done on the 18 th of September 2019.All of the treatments were repeated thrice and each unit plot size was maintained at (4 x 4.5 m).Seeds were planted using seed driller with 6 inches plant to plant distance while row to row distance was maintained at 1ft.The fertilizers (N, P, K) were applied at the rate of (80, 46, 37) Kg/acre respectively.Irrigation and weed management practices were performed accordingly to the technical recommendations by Agriculture Punjab's Maize Production Technology.

Treatment variables:
Four conventional and four hybrid varieties were selected to screen out against FAW infestation.The seeds were purchased from a registered local seed distributor, Manga Mandi, Lahore.Before cultivation, seeds were treated with Topsin-M (70 WP) which is a fungicide and prevents fungal diseases.The description of varieties is given as:

Data Collection:
From each of the experimental units, the data were collected from 6 different spots in a zig-zag fashion, and from each spot 10 plants were observed.Plants were inspected for the physical presence of larval form in the central whorl of the plant, egg masses, damaging signs including windowing, shot holes, lumps of frass, ragged and torn leaves.Data were taken weekly from 14-Oct-2019 to onward till the crop harvesting.To find out percentage infestation, the following formula was used as described by Mashwani et al (2011).
Percentage Infestation = (No. of Plants Infested / Total No. of Plants Inspected) × 100 The percentage infestation was calculated against each of the 10 plants from the single spot and the average of 6 different spots gave mean infestation present in a single experimental unit.

Meteorological Data:
Data on daily temperature and relative humidity were taken from the observatory officials of the Centres for Integrated Mountain Research (CIMR) department of University of the Punjab, Lahore, Pakistan.

Statistical Analysis:
The infestation data were subjected to ANOVA (Analysis of Variance) by using software Statistics 8.1 while means were compared through Tukey's HSD test at P = 0.05.The percentage of infestation was correlated with meteorological factors i-e., temperature (°C) and relative humidity (R.H.) (%).

Results:
The FAW infestation recorded from different maize varieties were subjected to statistical analyses.The pest infestation remained significantly different on all eight maize varieties as given by the ANOVA (Table 2).Mean comparison test (HSD) explains that the maximum cumulative mean infestation was observed on Malka-16 which was a conventional variety of maize, and statistically, it was different from Gohar-19, Karamat Hybrid, Pearl 2011, CIMMYT Pak, Ghouri Hybrid, 31P41 Hybrid, and P1543 Hybrid.The minimum infestation was recorded on the P1543 Hybrid of maize which showed some sort of resistance against FAW (Table 3).The infestation was monitored for the first time on 14-Oct-2019 and it was the minimum recorded infestation (zero in all treatments) probably because the crop was under five-leaf stage (V5) in all treatments.But as the crop raised its level it put a positive impact on armyworm proliferation on all subsequent treatments.
The mean infestation recorded after the 2 nd week is given in Graph 4.1 and it shows that mean infestation was maximum on the CIMMYT Pak while it was lowest on the 31P41 Hybrid.In the very next week, a decline in the population build-up was seen.But, the higher most infestation of fall armyworm was observed again on Malka-2016.P1543 Hybrid showed 25.55% damage which was the least damage observed in the week.The graphs above showing the periodic variations in the pest infestation from the early days to the maturity of the treatment crops in their respective blocks.They are also showing different susceptibility levels of the different maize varieties.
The Tukey's test on the date wise infestation showed highly significant results (Table 4), it was revealed that infestation was maximum on the 11 th of November 2019 followed by 18 th of November 2019.No statistical difference was found between the invasion recorded on the 17 th   The periodic infestation on different corn varieties viz., Gohar-19, Pearl-11, CIMMYT Pak, Malka-16, Karamat Hybrid, Ghouri Hybrid, 31P41, and P1543 showed a significant and positive correlation with maximum temperature, minimum temperature, and the mean temperature.But the strength of correlation was weak possibly due to cold weather conditions.In contrast to temperature, relative humidity showed a non-significant and negative correlation with pest infestation and its strength was weak also (Table 5).

Discussion:
Spodoptera frugiperda is a polyphagous armyworm affecting various economically important cash crops worldwide (Silva et al 2017).The FAW infestation level in the present study varied significantly on all different maize varieties.Fall armyworm showed maximum infestation on Malka-16 while the attack on P1543 hybrid was limited.In general, conventional maize varieties were susceptible to fall armyworm damage while hybrids showed some sort of tolerance to attack.In a similar study in Eastern Zimbabwe, Baudron et al (2019) visualized different agronomic practices against the invasion of fall armyworm.They inspected a total of 791 maize fields of small landowners and interviewed the farming communities.It was concluded at the end: the cropping areas that strictly followed weeding activity; less likely attacked by fall armyworm, pumpkin intercropping increased the risk of damage, even certain high-yielding maize varieties were found susceptible to the pest.
Planting a diverse range of crops on-farm support the proliferation of natural enemies and reduces the incidence of FAW.The females of S. frugiperda prefer to lay their eggs on maize plants, the maize monoculture provides a sea of maize plants with no diversity, hence it increases the incidence of FAW attack.When maize is intercropped with a diverse range of crops, less feeding is observed with fewer eggs laying on the maize crop (FAO 2018).
Similarly, significant variation in FAW infestation was seen during different weeks ranging from 14-Oct-2019 to 17-dec-2019 possibly due to different growth stages of crops and weather factors i.e., temperature and humidity.Considerably higher populations of the pest were observed on different dates during the 1 st half of November 2019.Perhaps the cropping stage favoured its infestation because FAW develops more rapidly on the growth stages of maize crop rather than reproductive stages (Barfield & Ashley 1987).It could be due to favourable temperature and humidity.From December to onward infestation showed a decline possibly due to cold weather conditions.
The analysis of correlation with weather factors and the pest infestation showed a very weak strength of correlation probably due to a drop in temperature abruptly.The population correlation with maximum, mean and minimum temperatures were weak but positive (r = 0.139, r = 0.149 and r = 0.159, respectively) whereas relative humidity showed a negative but weak relation (r = -0.104)with infestation.Nagoshi & Meagher (2004) studied seasonal fluctuations in populations of fall armyworm and they detected the peak populations of S. frugiperda twice a year; during spring (March-May) and fall (October-December).Early planting and cold weather conditions could be the possible reasons for the pest decline in December 2019 in the current research work.
The results in the present investigation enlightened that in the first five weeks from 14-Oct to 11 Nov 2019 infestation on different maize varieties attained maximum values: the temperature fluctuated between 25-30 ºC while relative humidity remained inside the range of 45-55% during the phase of population build-up.But later on, an abrupt fall in temperature caused the population build-up to decline.In the studies of Valdez-Torres et al (2012) and Simmons (1993), it was revealed, FAW showed maximum fecundity, reproduction, growth and development in the temperature range of 22-32 ºC.In our study, the response of fall armyworm remained parallel to Proceedings 2021, https://doi.org/10.3390/IECE-10609 Nigeria, in 2016 (Akutse et al 2019).Subsequently, it was spread to different parts of the African continent.It was reported that fall armyworm attacks about 350 host plant species belong to 76 plant families (Montezano et al Proceedings 2021, https://doi.org/10.3390/IECE-10609 The seasonal studies about population dynamics, depending on biotic or abiotic factors, provide necessary clues for the management of important crop pests (Aasman 2001).Population ecologists classify the factors, that determine the size of the population, into two major categories; density-independent factors (weather and climate) and density-dependent factors (diseases, competitions and natural enemies).Density independent factors are sometimes regarded as limiting factors while density-dependent factors are known as regulating factors since they keep the population in a narrow range and do not let the population expand freely (Britannica 2017).

Graph 4 . 1 :Graph 4 . 2 :
Infestation after 2 nd week on different maize varieties.Graph 4.2 is showing infestation recorded after the 3 rd week.Statistically, the fall armyworm population was highest on Karamat Hybrid while 31P41 Hybrid showed some sort of tolerance once again.The rise and fall in a population could be determined by variable factors i.e., the infestation rate of FAW is not always the same on different stages of maize crop.It gets varies with the variation in each crop stage.Other than crop development, the abiotic factors like temperature and relative humidity, also determine pest infestation, abundance, and dispersal of insect pests.Proceedings 2021, https://doi.org/10.3390/IECE-10609Infestation after 3 rd week on different maize varieties.A rise in the mean infestation of the pest in individual experimental units was seen from 28-Oct-2019 to 04-Oct-2019.Malka-2016 showed statistically the highest population of fall armyworm while the least infestation was recorded on 31P41 Hybrid once again.

Graph 4 . 3 :
Infestation after 4 th week on different maize varietiesDuring the course of studies, the highest population build-up was seen on the 11 th of November 2019.All the treatment crops observed peak pest were in favour of FAW.The highest damage of 53.88% was recorded on Malka-2016.Statistically, no difference was found among the infestation levels recorded on Ghouri Hybrid, 31P41 Hybrid and P1543 Hybrid.

Graph 4 . 4 :Graph 4 . 5 :
Infestation after 5 th Week on different Maize VarietiesInter-varietal infestation after the 6 th week shows that Malka-2016 showed significantly highest fall armyworm damage.The levels of invasion recorded on 31P41 Hybrid and P1543 were statistically lowermost than the rest of the treatments.Infestation after 6 th week on different maize varieties.

Graph 4 . 6 :
Infestation after 7 th week on different maize varieties.The subsequent reduction in the pest occurrence was seen once again after the 8 th week of investigation.Graph 4.7 shows the mean infestation recorded on 03-Dec-19.Malka-2016 possessed the highest FAW infestation, 31P41 Hybrid showed some sort of tolerance against the pest and unveiled the lowest invasion.

Graph 4 . 7 :Graph 4 . 8 :Graph 4 . 9 :
Infestation after 8 th week on different maize varieties.Proceedings 2021, https://doi.org/10.3390/IECE-10609Temperature drop towards in the middle of December negatively influenced the population build-up of fall armyworm.Additionally, crop maturity was another factor that overlapped with temperature to reduce FAW infestation.CIMMYT Pak significantly showed lowermost invasion of the pest on 10-Dec-2019.On the other hand, Malka-2016, Karamat Hybrid and Ghouri Hybrid possessed the same levels of infestation which were highestmost.Infestation after 9 th week on different maize varieties.Similar to the previous week, invasion recorded on 17-Dec-19 shows a subsequent drop in the pest population.Statistically, Malka-2016, Karamat Hybrid and Ghouri Hybrid showed maximum mean values of pest infestation while CIMMYT Pak showed the lowest infestation in that particular week.Infestation after 10 th week on different maize varieties. https://doi.org/10.3390/IECE-10609 and 21 st of December 2019.The lowest population of FAW recorded on the 14 th of November than 4-Nov-19, 27-Nov-19, 28-Oct-19, 03-Dec-19, and 10-Dec-19.

Table 1 .
Treatment variables and description.

Table 2 .
Analysis of variance (ANOVA) for mean infestation.

Table 3 .
Seasonal FAW infestation on different maize varieties.

Table 5 .
Effect of meteorological factors (Pearson Correlation= r) on population dynamics of FAW (Noctuidae: Lepidoptera) along with different maize varieties and different times.