Prediction of the Annual Generations of The Fall Armyworm, Spodoptera frugiperda

---------------------


INTRODUCTION
The fall armyworm (FAW) Spodoptera frugiperda (Lepidoptera: Noctuidae), has been seen to attack 186 plant species from 42 families, is polyphagous, and is a significant pest of cereal crops and pasture grasses (Casmuz Augusto et al., 2010).It affects a number of nations, including Brazil, Argentina, and the USA (Prowell et al., 2004 andClark et al., 2007), resulting in financial losses in a variety of crops, including maize (Pogue, 2002;Nagoshi, 2007and Bueno et al., 2010and Nabity et al., 2011).Its larvae consume maize at various stages of development, including leaves, stalks, cobs, and tassels, and result in a decrease in the resulting yield (Bakry and Abdel-Baky, 2023).The first appearance of S. frugiperda in Egypt was 2019 on corn crop (Dahi et al., 2020 andHend et al., 2022).
The link between temperature and development rate has a significant influence on biology, spread, and pest abundance (Tobin et al., 2003).Because insects develop within a narrow temperature range, a change in temperature will affect how quickly they develop, how long their life cycles last, and whether they survive (Howe, 1967).As a result, Porter (1991) found that climatic and weather variations had an impact on the status of pest species.Since this will help with risk analyses, forecasting, and management strategies to reduce pest infestation rates, it is crucial to understand how temperature affects the development of target insect species under the present changing environmental circumstances (Calvo and Molina, 2005).In contrast to conditions when insects are exclusively exposed to steady temperatures, oscillations in temperature in natural habitats have an impact on insect population dynamics.When the maximum and minimum temperatures are within their ideal range of growth, insects grow more quickly in environments with temperature fluctuations (Hagstrum and Hagstrum, 1970).However, research investigating disturbance insect species under constant temperatures can be utilized to foretell the phenological and seasonal changes that will occur when temperatures change.The pest reproduction dynamics and timing of management measures were studied by Shanower et al. (1993) and Mironidis (2014).
In order to control pest populations, integrated pest management programs (IPM) employ a comprehensive approach that relies on forecasting the seasonal population cycles of insects.As a result, numerous mathematical techniques have been developed (Clement et al., 1979 andRichmond et al., 1983), some of which explained developmental levels as an indicator of temperature (Wagner et al., 1984).
Chemical pesticides frequently have trouble controlling the S. frugiperda.In order to create a non-chemical technique for its control (Anita et al., 1984 andValand andPatel 1993).Although the most effective way to affect all lepidopterous pests remains the use of insecticides, the creation and development of an alternative program to protect people and/or the environment has become more important.
The current work is aimed at estimating S. frugiperda annual generation peaks on maize plants under field conditions by using the relationship between the seasonal incidence of S. frugiperda and accumulated heat units at Esna district, Luxor region.Many authors have reported predicting the annual generation by using the heat requirements for different insect pests (Emara et. al., 1999;Ismail et al., 2005 andBakry andDahi 2020).

1-Population Estimates of S. frugiperda:
The population fluctuation of S. frugiperda exhibited on maize plants in the field over two successive seasons (2021 and 2022) at Esna district, Luxor region, South Egypt (25º19'31" N, 32º32'08" E).One feddan (4200 m2) was planted with a Single-Hybrid 168 Yellow Corn cultivar of maize plants on the optimum planting date (June, 1 st of every season).Usually, regular conventional farming procedures were used, except for pest control.When the age of maize corn plants reached 15 days, the infestation by pests appeared; random samples of forty maize plants (ten plants from each replicate) were inspected weekly and continued until crop harvesting.The samples represented different strata of the field and were randomly picked in a "W" method in the morning at 7 a.m. to estimate the population size of larvae of S. frugiperda.The counts of larvae per 10 plants ± standard error was calculated and worked out on every investigation date, to exhibit the occurrence of pests on maize plants (Vinay et al., 2022).The weekly mean numbers of larvae per 10 plants were graphically illustrated.

2-The Estimated Number of Annual Generations of S. frugiperda Under the Field Condition:
The annual S. frugiperda larvae population data per 10 plants were represented graphically in figures.The number and interval of annual generations below field circumstances were registered by using the natural curve method.Which is based on the relationship between the numbers of larvae on 10 plants with time (dates of examination).We have a curve for the number of larvae (beginning of the appearance of larvae population per 10 plants and its end) estimated by integration of the population densities curves, and each peak of the curve reflects the activity and strength of the generation.

3-Forecasting of Peaks of S. frugiperda Generations Using Heat Accumulations:
As regards, the prediction of peaks of S. frugiperda generations was done by estimating the connection between the seasonal abundance of S. frugiperda larvae and accumulated heat units expounded (as degree-days) under field status throughout the two seasons of (2021 and 2022).Weekly mean numbers of the larvae population densities of S. frugiperda were graphically demonstrated to determine the counts' peaks (factual observed peaks).Additionally, actual peaks were compared to the prospected peaks as an instrument to assess accumulated heat units for predicting the S. frugiperda generations over each season of maize planting.The observed peaks of S. frugiperda generations that were recorded in the field were expounded, based on the population densities of the larvae during the research period.The time when average population densities of maximum larvae per 10 plants are reached can represent a peak for one generation.The sine-Wave Model was used to calculate heat units for S. frugiperda with horizontal cut-off technique (Allen, 1976) at 30ºC and a lower threshold of 10.39ºC with a mean (364.7 DD's) for generation evolution for coinciding to Dahi et al. (2020).This method was used by applying a Microsoft Excel program evolved at Plant Protection Research Institute, Agricultural Research Center, Giza, Egypt.
The methodology was utilizing the daily mean of maximum and minimum temperatures to compute degree-days and accumulated heat units across an interval of time by applying the above-aforementioned technique.The heat units were computed from 14 th June to 8 th September per season.The daily means of maximum and minimum temperatures, under conditions of Luxor region, were procured by www.wunderground.com .Counting on the averages of heat units needed for completion of the generation (364.7 DD's), evaluated by Dahi et al. (2020), and by comparison between actual peaks (exhibited on the field) and expected peaks (estimated by the technique of Allen, 1976).

4-Association between the Cumulative Larvae Counts of S. frugiperda Per 10 Plants and the Accumulated Heat Units:
The current work objective is to estimate the relationship between the dependent variable represented as (cumulative larvae counts of S. frugiperda per 10 plants and the independent variable (as the accumulated heat units) on maize plants over the two seasons (2021 and 2022).
The data were statistically evaluated using models of simple correlation, regression values, and the explained variance percentage when the counts of mean daily degree days were plotted versus the S. frugiperda larvae counts per 10 plants, and the accumulated degree days, were charted versus the cumulative larvae during the two seasons (2021 and 2022) was corresponding by Fisher method (1950) and Hosny et al. (1972): ŷ= a + bx Where: The all-statistical studies of the data were executed by SPSS (1999).

RESULTS AND DISCUSSION
This study is considered the first study in Egypt to predict the peaks of the expected S. frugiperda generations using accumulated thermal units.

1-Population Fluctuation of S. frugiperda:
The results offered in Figure ( 1) revealed that the seasonal activity of S. frugiperda larvae had three peaks of abundance per season was exhibited in the first week of July, the first week of August, and the first week of September throughout the two seasons (2021 and 2022).Moreover, the larvae of S. frugiperda appeared on maize plants from the third week of June and continued until the maize harvest time each season.These findings are in harmony with those acquired by Kumar et al. (2020) stated the occurrence of S. frugiperda was higher in the second week of July month.Reddy et al. (2020) recorded that the heaviest infestation appeared at the plant age (45 days) of maize cultivation.Supartha et al. (2021) mentioned that FAW counts were detected to be vigorous after 15 days of cultivation of maize.

-Generations Estimation of S. frugiperda:
Data presented in Table (1) showed the approximated number, duration and size of S. frugiperda larvae generations monitored on maize plants under field circumstances in Esna district, Luxor region during the two seasons (2021 and 2022).The data indicated that there are three generations over each season.First generation: The first generation was observed between the period from the June 23 rd and continued until July 14 th in the two seasons (2021 and 2022) and covered a period of 4 weeks below field conditions at (39.86°C, 28.73°C and 22.82%) in 2021 and (40.67°C, 26.52°C and 27.70%) in 2022 for a daily mean of max.temp., min.temp., and relative humidity, respectively.The generation peaked on July 7 th per season.The generation density was 45.00 and 43.88 larvae per 10 plants throughout the two seasons, respectively, Table (1).Second Generation: The second generation was found between the interval from July 14 th and continued until August 18 th and elapsed approximately 6 weeks over each season below field conditions at (40.02°C, 29.95°C and 24.37%) in 2021 and (41.34°C, 27.50°C and 29.58%) in 2022 for a daily mean of max.temp., min.temp., and relative humidity, respectively.The generation peaked on August 4 th across each season.The generation density was 82.88 and 77.99 larvae per 10 plants over the two seasons, respectively, Table (1).Third Generation: The third generation appeared in the interval from August 18 th and extended till Sept., 8 th with a duration of 4 weeks through every season under field circumstances at (40.86°C, 29.77°C and 25.90%) in 2021 and (41.48°C, 27.54°C and 31.43%) in 2022 for daily mean of max.temp., min.temp., and relative humidity, respectively.The generation peaked in September, 1 st per each season.The generation density was 55.88 and 58.50 larvae per 10 plants over the two seasons, respectively, Table (1).
The results mentioned that the population densities of larvae differed from one generation to another.The second generation per season, which started in both of them on July 14 th and continued to August 18 th , was the longest one and biggest in size compared to the other generations over the two seasons.This may be due to the various oscillations of climatic variables.
The conclusions are in coincide with those obtained by Dent (1991) elucidated that the seasonal activity of pests in any region is defined by the climatic variables at that place.Murúa et al. (2009) reported that the counts of S. frugiperda larvae be influenced by the plant's age and its growth.Valdez-Torres et al. (2012) mentioned that through maize planting, S. frugiperda had two field generations per season.Sisay et al. (2019) concluded that the generation interval of S. frugiperda was little, around 20 to 30 days.

3-Heat Units and Seasonal Abundance of S. frugiperda larvae related:
The acquired results in Figure ( 2), exhibit the accumulated heat units and the weekly numbers of the cumulative larvae of S. frugiperda per 10 plants over the two seasons.It was noticed that both of them started to increment gradually to the finish of every season, (Fig. 2).
By calculation of the cumulative larvae of S. frugiperda (as dependent variable) versus the accumulated heat units (as independent variable), the simple regression method pointed out that the numbers of cumulative larvae per 10 plants were more related to the accumulated heat units for the two seasons, demonstrated in Figure (3).
The regression method mentioned that the association offered a logical fit and the coefficient of determination (R 2 ) appeared that the increment in the cumulative larvae numbers happened due to the rise in the accumulated heat units.The associations between them could be defined by the succeeding equations acquired in Figure (3): Y = -10.58+ 010 x R 2 = 99.81% for the first season Y = -10.35+ 0.10 x R 2 = 99.82% for the second season 4-Predicting of S. frugiperda Generation Peaks Using Accumulated Heat Units: By using the lower threshold (10.39ºC) and the degree-days average needed to complete the generation of S. frugiperda larvae (364.7 DD's), that evaluated (Dahi et al., 2020), and by differentiation between actual peaks (that passed in the field) and prospective peaks (which estimated by applying the Sine-Wave method (Allen, 1976) by horizontal cut off technique at 30ºC and lower threshold of 10.39ºC over the two seasons of (2021 and 2022) are depicted in Table (2).
The succeeding results could be discovered; the first generation happened between the interval from June 23 rd and continued until July 14 th and the generation peaked (actual field) on July 7 th per each season.But the expected peak was discovered earlier on July 4 th and July 5 th with 362.70 and 362.70 DD's, over the two seasons, respectively, Table (

2).
The second generation elapsed between the period from July 14 th and continued until August 18 th , the field peak appeared latterly on August 4 th as compared to the awaited peak.The probable peak was attained on July 23 rd and 24 th with 370.87 and 366.06 DD's, for the two seasons, respectively, Table (2).
The third generation per season was observed in the interval from August 18 th and extended to Sept., 8 th , the field peak exhibited on September, 1 st , was dotted delayed as compared to the prospective peak recorded on August 10 th and August 11 th when the Accumulated heat units were 352.92-and 347.66-degree days, during the two seasons, respectively, Table (2).
Moreover, there was a potential predicted fourth peak was achieved on August 27 th and August 31 st , when the accumulated heat units needed 367.74 and 377.33 DD's through the two seasons, respectively.Contrarily, no actual peak was observed in the maize field in this period, Table (2).
Data exposed that the prospected peaks of generations could be discovered when the accumulated thermal units reached (364.7 DD's), which agrees with (Dahi et al., 2020).
Applying obtainable meteorological data handed for the Luxor region, the mean ± STD accumulated heat units per generation for S. frugiperda larvae over the two seasons were estimated to be 364.83± 9.36 DD's.
Eventually, it could be mentioned that the accumulated heat units needed under the climatic circumstances in the Luxor region are important for forecasting the appearance of S. frugiperda larvae, and can assist determine the suitable procedures to control S. frugiperda larvae.The mean ± standard deviation of accumulated heat units per generation for S. frugiperda larvae over the two seasons was estimated to be 364.83± 9.36 DD's.

Fig. 1 :
Fig. 1: Means of weekly counts of daily degree days, and the seasonal incidence of S. frugiperda larvae per 10 maize plants, during the two seasons (2021 and 2022).

Table 1 :
An estimate of the number, length, and size of S. frugiperda larvae generations that occurred on maize plants in the field throughout the two seasons (2021 and 2022).

Fig. 2 :
Fig. 2: Means of weekly counts of accumulated heat units and the cumulative larvae of S. frugiperda per 10 maize plants, during the two seasons (2021 and 2022).

Fig. 3 :
Fig. 3: Relationship between the accumulated heat units (AcHu) and the cumulative S. frugiperda larvae of 10 maize plants, during the two seasons (2021 and 2022).

Table 2 :
Comparison between actual and predicted peaks of S. frugiperda larvae generations on maize plants and accumulated thermal units under field conditions at Esna district, Luxor region over the two seasons (2021 and 2022).