Although management methods for the fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera: Noctuidae), have been explored, monitoring and surveillance in newly invaded regions is problematic. This study investigates the population dynamics and infestation levels of the fall armyworm (FAW), Spodoptera frugiperda, on maize farms in Togo's Guinean and Sudanese climatic zones to improve pest management strategies. Male FAW moths were monitored using pheromone traps over 15 months, and larval densities and plant damage were assessed on 100 maize plants per farm. Results revealed higher moth captures in the Sudanese than Guinean climate zones (2.77 ± 0.16 vs. 2.05 ± 0.13). During the major rainy season (June to August), captures peaked in the Sudanese zone (5.28 ± 0.52) compared to the Guinean zone (3.77 ± 0.55). No significant differences were observed during the minor rainy season (September to November), while dry season (December to May) captures were higher in the Guinean zone (0.54 ± 0.07) than the Sudanese zone (0.24 ± 0.05).  Despite similar larval densities, plant damage was greater in the Guinean zone during the major rainy season, likely due to positive correlations between adult captures, larval counts, and plant damage in this region. Conversely, no such correlations were observed in the Sudanese zone. Additionally, maize stage-specific analyses in Lomé and Toaga found no correlations between FAW captures, larval densities, or damage. These findings highlight the need for climate-specific pest management strategies rather than uniform national approaches, supported by linear regression models for each region.

Distribution of sites and characteristics of study zones

Over 15 months, adult FAW were trapped at 15 maize sites distributed across the Guinean and Sudanese climatic regions of Togo (Fig. 1). During the maize seasons, the sites were expanded to three farms per site, including an on-station experiment within each climatic region, or 45 farms across the country for larval severity data collection.

The Guinean climate covers the three agro-ecological zones (AEZs) of southern Togo and is characterized by bimodal rainy seasons. The first rainy season marks the main maize season from April to July and the second covers the minor maize season from September to November. The two rainy seasons are separated by a short dry season of one month in August, while the main dry season lasts from December to March. The three AEZs of Togo's Guinean climate region include AEZ5 of the southern mosaic coastal savannah mixed with a relic forest and fallow land, AEZ4 of the western mountainous dense and semi-deciduous forest, and AEZ3 of the eastern Guinean woodland savannah. The Sudanese region includes the two AEZs of northern Togo, which has a wet maize growing season from June to September and a dry season from October to May. The two AEZs of this zone include AEZ2 of the lower north, characterized by a mountainous mix of dry forest to savannah, and AEZ1 of the upper north, characterized by a typical Sudanese savannah (Koffi et al. 2020a). The country is dominated by the Harmattan and Mousson winds. The Harmattan is a hot day and cold night wind that blows from the northeast to the southwest from November to April. The Mousson is a mild wind that blows in the opposite direction to the Harmattan from May to October (Amey et al. 2014).

Trapping of male moths

Pheromone-baited traps were used to catch male FAW moths. Standard bucket traps (green top, yellow funnel, white bucket; Unitraps, manufactured by International Pheromone Systems, Neston, UK, distributed by Great Lakes IPM, Vestaburg, MI, USA) were used for 15 months. Rubber septa lures composed of three S. frugiperda pheromone components of (Z)-9-tetradecenyl acetate (Z9-14:Ac), (Z)-11-hexadecenyl acetate (Z11-16:Ac) and (Z)-7-dodecenyl acetate (Z7-12:Ac) (Trécé®, Adair, OK, USA) were placed in green baskets at the top of the bucket traps. The trap design and lure blend used was the most efficient for FAW trapping in Togo and Ghana (Meagher et al. 2019, Koffi et al. 2021). In each bucket, 10 g of cotton soaked in a diluted killing solution of an abamectin containing emamectin benzoate (019 g/l EC) (water/emamectin benzoate 998:2ML mixed solution), marketed as EMACOT 019ECTM (ANTEOR Sarl, Lomé, Togo), was suspended in the trap. The entire system was suspended from wooden poles obtained locally. 

The selection of maize sites was based on the AEZ, with three sites within each AEZ or nine sites in the Guinean climate area and six in the Sudanese climate area. All sites contained three bucket traps attached to wooden poles initially spaced 40 meters apart and three meters from the main maize plots. The traps were initially placed 0.50 m above the ground and adjusted weekly to 0.30 m in relation to the height of the maize plants. The killing cotton strips were replaced monthly, as were the pheromone lures. Caught adults were collected weekly and data were categorized for each month (Koffi et al. 2021). In the on-station trials, maize was grown in four plots of 400 m² each, separated by three meters. Traps were set three weeks before the maize plants germinated.

Population densities and infestations across climatic regions

During three consecutive maize production seasons within the 15 months of adult trapping, data on larval density on 100 plants and infestation estimated from damaged plants within 100 plants were collected on the 45 maize farms in the two climatic regions. The selected farms included those where adult traps were set and two neighboring farms. Farm inspections were carried out monthly during June, July and August. The maize season covers June to August in the Guinean zone, while the maize season starts in the Sudanese zone from July. Inspections of each farm were conducted within the four quadrants of the farm corners and one in the center of the farm. Within each quadrant, 20 maize plants were randomly selected for counting the number of egg masses, larvae and damaged plants. The five quadrants and the selected maize plants were randomly selected during each monthly inspection. The data collected were recorded per farm and tabulated within groups of climatic regions. Comparisons were made between regions and correlations between parameters were assessed within each region.

Population densities and infestations in on-station experiments

Two on-station experiments were conducted in Lomé, in the Guinean region, and Toaga, in the Sudanese region. At each on-station site, the experimental plot was divided into four units of 10 m * 10 m (100 m²), 2 m apart. The two experiments were subjected to the same treatments of weeding and fertilizing without any FAW control. Pheromone trapping was from the first day of maize germination until the day of harvest, and the killing cotton strips were replaced weekly. Pheromone lures were replaced monthly, and captured moths were collected weekly. The numbers of larvae, egg masses, and damaged plants or ears were recorded weekly on 10 randomly selected plants per experimental unit. All data collections took place on the same day to avoid bias. The growth of maize plants was divided into four phenological stages, including the emergence stage, which covered the first week after germination, the vegetative stage, which covered the second week after germination until the week of emergence of male flowers, the tasseling stage, which covered two weeks after emergence of male flowers, and the maturation stage, which lasted until harvest. The data collected were collated into experimental units, grouped into maize growth stages and then subjected to correlations between parameters. The maize yields were determined by the weighing of the total grains per plot, which were then extrapolated into hectares. The direct yield losses per plot were determined as the difference in grain weights between 10 cobs that were found to be healthy and 10 cobs that had been marked during the grain-forming stage and had been fed on by FAW larvae.

Data Analysis

The data collected were arranged by treatment and tabulated in Microsoft Excel version 2013. The raw data were subjected to the Shapiro-Wilk (1965) normality test as applied by Royston (1995). Normal data were compared with ANOVA and means separated using a Tukey test. Multi-variable data that did not pass the normality tests were then tested by the nonparametric Kruskal-Wallis one-way analysis of variance. The two variable data that passed the normality tests were additionally subjected to the Brown-Forsythe test of equal variances, then to the test of assumed equal variances (Student's t-test) or not assumed equal variances (Welch's t-test). The results of the Welch's test not assuming equal variances were used only when the equality of the population variances of the two groups was in doubt. The two variable data that failed normality tests were tested using a Mann-Whitney rank sum test with the U statistic.

Pearson Product Moment Correlation was used to assess the relationship between different parameters. Linear regression models were used to establish the equations linking the independent variable captured adult male moths, and the dependent variables larval density and infestation level. The predictive ability was significant with a probability of P < 0.05. All data subjected to regression were tested for normality (Shapiro-Wilk) and constant variance (Spearman Rank Correlation).

Monthly and spatial plots were generated using simple line and scatter error bars under column means with standard errors. Box plots were obtained with medians and percentiles. The bar graphs comparing catches, larval densities, and infestations between the two regions or yields and yield losses between on-station sites were constructed using the vertical bar graph with error bars from the data means. All data analyses and graphs were performed using SigmaPlot 14.0.

The data collected were arranged by treatment and tabulated in Microsoft Excel version 2013. The raw data were subjected to the Shapiro-Wilk (1965) normality test as applied by Royston (1995). Normal data were compared with ANOVA and means separated using a Tukey test. Multi-variable data that did not pass the normality tests were then tested by the nonparametric Kruskal-Wallis one-way analysis of variance. The two variable data that passed the normality tests were additionally subjected to the Brown-Forsythe test of equal variances, then to the test of assumed equal variances (Student's t-test) or not assumed equal variances (Welch's t-test). The results of the Welch's test not assuming equal variances were used only when the equality of the population variances of the two groups was in doubt. The two variable data that failed normality tests were tested using a Mann-Whitney rank sum test with the U statistic. Pearson Product Moment Correlation was used to assess the relationship between different parameters. Linear regression models were used to establish the equations linking the independent variable captured adult male moths, and the dependent variables larval density and infestation level. The predictive ability was significant with a probability of P < 0.05. All data subjected to regression were tested for normality (Shapiro-Wilk) and constant variance (Spearman Rank Correlation). Monthly and spatial plots were generated using simple line and scatter error bars under column means with standard errors. Box plots were obtained with medians and percentiles. The bar graphs comparing catches, larval densities and infestations between the two regions or yields and yield losses between on-station sites were constructed using the vertical bar graph with error bars from the data means. All data analyses and graphs were performed using SigmaPlot 14.0. Figure 1. Geographical distribution of the trap setting localities with latitude and longitude coordinates within the Guinean and Sudanese climatic regions of Togo. The white dots mark the sites of Guinean zone and the black dots the sites of Sudanese zone. Numbers 1 and 15 show the on-station experiment sites of Lomé (1) and Toaga (15) (d-maps.com). Figure 2: Population dynamics of males of S. frugiperda captured with Universal moth traps baited with Trécé® lures for 15 months across Togo (A), and within the Sudanese (B) and Guinean (C) climatic zones. Kruskal-Wallis One Way Analysis of Variance (P < 0.05). Figure 3: Mean numbers of pooled adults caught per trap per month in the Sudanese (A) and Guinea (B) climatic zones. Kruskal-Wallis One Way Analysis of Variance (P < 0.05). Figure 4: Comparisons between the Guinean and Sudanese climatic regions of Togo: numbers of caught males per month per trap (A), collected larvae (B) and infestation levels (C) in 100 plants per farm during the maize seasons in the. Mann-Whitney rank sum test with the Yates (P < 0.05). Figure 5: Correlations within climatic zones between the trapped adults per trap per month, larval densities on 100 maize plants, and infestation levels of maize in the Guinean (A) and Sudanese climates (B). Figure 6: Correlations between the trapped adults per trap per week, egg masses, larval densities on 10 maize plants, and infestation levels during the emergence (A), vegetation (B), tasseling (C), and mature (D) stages of maize plants in the on-station of Lomé located in the Guinean climatic zone. Figure 7: Correlations between the trapped adults per trap per week, egg masses, larval densities on 10 maize plants, and infestation levels during the tasseling stage of maize plants in the Toaga on-station trial located in the Sudanese climatic zone. Figure 8: Comparisons between the on-stations of Lomé in the Guinean zone and Toaga in the Sudanese zone: maize grain yields (A), and direct yield losses from FAW larval feeding on maize ears during the maturation (B). Table 1. Variation (mean ± SE) of male adults of FAW trapped per month per trap during the major rainy, minor rainy and dry season in the five AEZs of Togo. Table 2. Pearson correlations between fall armyworm variables sampled at the Lomé Experiment Station in the Guinean climatic zone in Togo. Table 3. Pearson correlations between fall armyworm variables sampled at the Toaga Experiment Station in the Sudanese climatic zone in Togo. Table 4: Linear regression of the independent variables larval density (larv) and plant infestation (inf) to predicting number of adult male fall armyworm captured in pheromone-baited traps during maize season in the field (all stages) and plant phenological stages at on-stations in Guinean zone (A) and Sudanese zone (B) in Togo