The Senegalese grasshopper Oedaleus senegalensis (Krauss, 1877): Dynamics and socio-economic impact in Senegal

 Corresponding author: Mamour Toure Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution License 4.0. The Senegalese grasshopper Oedaleus senegalensis (Krauss, 1877): Dynamics and socio-economic impact in Senegal Mamour Toure 1, *, Amadou Fall 2, Amsata Diop 1, Esther Diouf 3, Amadou Bocar Bal 1 and Mady Ndiaye 2 1 Laboratory of Biological, Agronomic, Food Sciences and Complex Systems Modeling, Gaston Berger University of Saint Louis, Senegal. 2 Reproductive Biology Laboratory, Department of Animal Biology, Faculty of Science and Technology, Cheikh Anta Diop University of Dakar, Senegal. 3 French Agricultural Research Centre for International Development (CIRAD), Joint Research Unit, Biology Centre for Population Management, Montpellier, France. World Journal of Advanced Research and Reviews, 2025, 27(01), 415-428 Publication history: Received on 25 May 2025; revised on 30 June 2025; accepted on 03 July 2025 Article DOI: https://doi.org/10.30574/wjarr.2025.27.1.2537 Abstract This study, conducted in Senegal between July and October 2021, investigated the population dynamics and economic impact of the Senegalese grasshopper Oedaleus senegalensis, as well as the effectiveness of a community-based pest control strategy. A total of 250 farmers across four regions (Fatick, Kaffrine, Thies, and Saint-Louis) each managed one fertilized and one non-fertilized (control) millet field of one hectare, using the Souna III variety. Fertilized fields received 150 kg each of Nitrogen-Phosphorus-Potassium (NPK) and urea. Grasshopper densities and development stages were monitored through extensive field sampling and transect observations. Damage was evaluated by estimating leaf and ear attacks, and yield differences between field types were analyzed. In total, 500 hectares were surveyed 1,500 times. Three grasshopper generations were found in most regions, except in Saint-Louis, where only two were recorded. Densities and movement patterns followed the rainfall gradient, decreasing from Kaffrine to Saint-Louis. Control fields showed higher grasshopper densities (20.25 individuals/are) and attack rates (15%) than fertilized fields (8.5 individuals/are; 2% attack). Correspondingly, fertilized fields yielded 813.95 kg/ha, nearly double the 435.30 kg/ha from control fields. The results suggest that fertilization, possibly by enhancing millet’s protein content, helps reduce grasshopper infestation and damage. Keywords: Oedaleus senegalensis; Density; Damage; Millet; Control 1. Introduction The agricultural sector is one of the pillars of Senegal’s economy, contributing an estimated 15% to the gross domestic product (GDP) in 2020. A significant portion of the population continues to rely on it for their livelihood 1. Most of Senegal's agricultural production comes from rainfed crops, totaling 8,771,559 tonnes, including 3,640,545 tonnes of cereals. These cereals, mainly rice, maize, fonio, and millet, account for 31.4%, or 1,144,855 tonnes 2. Millet cultivation is particularly important for food security and nutrition, and its by-products serve as a source of income for farmers, World Journal of Advanced Research and Reviews, 2025, 27(01), 415-428 416 especially in the Groundnut Basin (Fatick, Kaffrine, etc.). Millet is the second most cultivated cereal, with an average production of 1,529 kg per rural household 3. One of the main constraints to optimal food production is losses caused by various crop pests, particularly locusts. Grasshoppers and locusts (Orthoptera: Acrididae) are among the most destructive agricultural pests. They can cause severe damage to food crops, disrupting farming systems and local economies. For millennia, people have faced devastating locust and grasshopper outbreaks. Under favorable climatic conditions, Acridid populations can increase rapidly, resulting in local outbreaks or even regional and continental swarms (4 ; 5 ; 6). In many tropical regions, only a small number of the approximately 6,700 valid Acrididae species pose a serious threat to food security, especially in Africa (7 ; 8 ; 9; 10 ; 11). These insects exhibit morphological plasticity depending on whether they are in solitary or gregarious phases (12 ; 13). Oedaleus senegalensis (OSE) (Krauss, 1877), commonly known as the Senegalese grasshopper, is one of these pests. It is widely distributed across the Sahel, from the southern Sudano-Sahelian zones to the northern SaheloSahelian zones 14. Seasonal movements occur along a north–south gradient, ranging between the 1000 mm isohyet (latitude 11–12°N) and the 150–200 mm isohyet (latitude 17–18°N), following the Intertropical Convergence Zone (ITCZ). This species poses a significant threat to food security, particularly in fragile regions like the Sahel. Damage caused by O. senegalensis is substantial and has led to the loss of thousands of tonnes of millet in Senegal. Launois and Launois-Luong 15 and Krall 16 reported yield losses of up to 40% in the Sahel in 1980 due to this species. In Niger, losses of 20– 40% of millet, sorghum, and rice have been reported 17, while in Mali, crop losses ranged from 70–90% over five years 18. In India, damage was estimated at 30% of crops 19. Cissé 20 also reported damage in Senegal and Mali. O. senegalensis is considered the most economically important grasshopper species in West Africa (21 ; 22). In 2003, O. senegalensis infestations severely impacted crop prospects in sorghum, maize, and millet fields in Senegal 23. Every year, this species swarms in the Groundnut Basin, an area traditionally known for producing food crops and groundnuts. For instance, in October 2005, locust outbreaks caused a 25% loss in the Mbar rural community (Gossas Department, Fatick Region) 24. Later, in 2006, infestations were recorded in the Matam region, followed by outbreaks in 2007 in Ndiassane (Thies Region) and in 2008 in the Noto rural community 25. Various methods are employed to control crop locust pests: mechanical, thermal, chemical, biological, ecological, and integrated approaches (26 ; 27). Among these, chemical control has traditionally been the most widely used and effective (28 ; 29). However, it carries serious environmental and health risks (30 ; 31). In the United States, for example, the widespread use of insecticides has led to the destruction of pollinator populations, resulting in annual yield losses of approximately $1.5 billion due to poor pollination 32. Some insecticides, especially those in the chlorinated cyclopentadiene group, are suspected of having carcinogenic effects (33 ; 34). Sustainable agriculture that protects the environment, human health, and food safety must include preventive strategies for grasshopper and locust control (8 ; 35 ; 36). Many countries affected by locust swarms have developed monitoring and early-warning systems, supported by regional and international institutions 37. Preventive control relies on surveillance 28 and requires in-depth knowledge of the insects' biology and ecology (38 ; 39) to detect phase transitions (40 ; 37). Monitoring and early warning enable timely interventions that are more effective and less costly 41. Mapping dynamics using satellite imagery is also essential in the development of control strategies (42 ; 43). Biopesticides have emerged as the most promising sustainable alternative. The use of fungal conidial formulations has shown great success worldwide in managing agricultural pests (44 ; 45 ; 46). Effective management during both invasion and remission periods is key to controlling grasshopper and locust populations 47. These insects are best identified by their density-dependent phase polymorphism, which can lead to spectacular migratory swarms 48. Over the past 30 years, significant progress has been made in understanding the ecology of the Senegalese grasshopper, monitoring its population, and predicting the risk of invasion. Control strategies are now shifting toward more realistic and sustainable alternatives to chemical pesticides. Efforts must be strengthened by improving monitoring, increasing the number and quality of surveys, and identifying outbreak hotspots and diapause egg fields during the dry season 49. World Journal of Advanced Research and Reviews, 2025, 27(01), 415-428 417 Locust feeding preferences based on plant nutrient content (e.g., nitrogen, carbon, phosphorus) represent a promising area for control 50. Human modifications to ecosystems, whether biological or chemical, inevitably influence locust behavior 51. The main objective of this study is to investigate the population dynamics of the Senegalese grasshopper, assess its economic impact, and evaluate the implementation of a community-based control strategy in Senegal. 2. Study area 2.1. Groundnut basin The research was conducted in the groundnut basin, specifically in the administrative regions of Fatick and Kaffrine (Fig. 1). 2.1.1. Fatick Region In this region, fieldwork was carried out in the rural communes of Mbar and Gossas. Mbar (latitude 14°31′ N, longitude 15°45′ W) is located in the Fatick region, within the Gossas department, Colobane arrondissement. The area has a tropical sub-Saharan climate with irregular rainfall ranging from 300 to 800 mm annually. Monthly temperatures vary significantly, from around 24°C in January to 39°C in May. The soils are tropical ferruginous, and the vegetation is mainly shrub savannah. These soils are well-suited to agriculture, particularly the cultivation of millet, cowpeas, and, to a lesser extent, groundnuts. The presence of fallow land (used as pasture) reflects the area's traditional farming practices. Gossas (latitude 14°30′ N, longitude 16°04′ W) has a tropical-Sudanian climate, with annual rainfall between 300 and 900 mm. The vegetation is diverse, and the soils are classified as dior or tropical ferruginous. Agriculture is the dominant activity, focused mainly on millet, groundnuts, and cowpeas. 2.1.2. Kaffrine Region In Kaffrine, research was conducted in the communes of Gniby, Boulel, and Nganda: Gniby (latitude 14°25′ N, longitude 15°39′ W), Boulel (latitude 14°17′ N, longitude 15°32′ W), and Nganda (latitude 13°83′ N, longitude 15°42′ W) These communes lie within the Kaffrine department, at distances of approximately 43 km, 22 km, and 35 km, respectively, from Kaffrine, the regional capital 59 (DRDR Kaffrine, 2012 Annual Report). The region experiences a Sudano-Sahelian climate, with high temperatures from April to July (ranging from 15–18°C minimum to 35–40°C maximum). The year is divided into two main seasons: a dry season (November to May) and a rainy season (June to October). The area, located between the 800 and 900 mm isohyets, receives relatively favorable rainfall. The soils are mainly of the dior type. Vegetation includes shrub and tree savannahs, pseudo-shrub steppes, and patches of open forest. Agriculture is the primary livelihood for approximately 75% of the regional population. The main crops grown include millet (souna), sorghum, and maize, with groundnuts and market gardening also widely practiced. 2.2. Central Senegal For the central zone, the study was conducted in the rural commune of Touba Toul (latitude 14°49′ North, longitude 16°40′ West), located in the Thieneba arrondissement of the Thiès department and region (Fig. 1). This rural area, like the urban zones, has a Sahelian-type climate (dry and hot) shaped by maritime trade winds from the north and northeast, as well as continental harmattan winds that bring dry, hot easterly air during the dry season. In contrast, humid westerly winds are associated with the monsoon, marking the rainy season. The dry season lasts from October to June, while the rainy season extends from July to October, characterized by the presence of monsoon rains. In the Touba Toul area, the soils are predominantly of the dior type, with low clay content, making them well-suited for crops such as groundnuts, millet, and cowpeas. There are also deck soils, known for being moist, rich, and highly fertile. World Journal of Advanced Research and Reviews, 2025, 27(01), 415-428 418 Vegetation is diverse, comprising a variety of species arranged in three strata: trees, shrubs, and herbaceous plants. Agriculture plays a central role in the local economy, employing around 80% of the working population. 2.3. Northern zone Saint-Louis lies within the Sahelian zone, between 16°02′ North and 16°30′ West (Fig. 1). The region has a Saheliantype climate, characterized by hot, dry continental trade winds (Harmattan) and maritime trade winds from the west. Average annual temperatures are relatively high, with the continental interior experiencing extreme heat year-round, occasionally exceeding 40°C in the Podor department. Rainfall has been low in recent years, though it can reach up to 346 mm annually. Nevertheless, the moderating influence of the nearby ocean to the west is favorable for crop cultivation. The sandy soil is often subject to wind erosion, leading to sandstorms that can last for several days, particularly during the Harmattan period (December to May), which originates in the Sahara Desert. During the rainy season, a thin layer of grass emerges, and bushes begin to regain their foliage. Figure 1 Regions of the study zone in Senegal 3. Material and methods 3.1. Material Two hundred and fifty (250) farmers, each with a one-hectare fertilized field and a one-hectare control field, were selected in the regions of Fatick, Kaffrine, Thies, and Saint Louis. The Souna III millet variety was sown in both types of fields. The fertilized fields received 150 kg of NPK fertilizer applied once, followed by 150 kg of urea applied in two doses spaced 15 days apart. The farmers sowed millet in June, well before the rains, to strengthen the crop against pest attacks and to hasten emergence. The target pests were locusts, primarily the Senegalese grasshopper, Oedaleus senegalensis, which can be either brown or green in color in both the larval and adult stages. World Journal of Advanced Research and Reviews, 2025, 27(01), 415-428 419 3.2. Methods 3.2.1. Study of population dynamics For this study, we conducted 1,500 surveys over 500 hectares, divided into three missions during the 2021 wintering period from July to September. For each mission, we recorded locust density in the fields, locust damage, date, time, temperature, humidity, and vegetation cover. 3.2.2. Densities To assess the density of Oedaleus senegalensis (OSE), we used both the quadrat method and the pedestrian transect method. 3.2.3. Quadrat method This method, used during the first mission, involves randomly placing a 1 m² quadrat in the field and counting the number of locusts found inside, carefully inspecting all the vegetation within the square. Approximately fifty repetitions are performed, and the average count represents the density for the given field. We calculated the locust density for each field using the following formula: 𝑑𝑙𝑜𝑐𝑢𝑠𝑡𝑠 = 2𝑁𝑙𝑜𝑐𝑢𝑠𝑡𝑠 • dlocusts: locust density (number of locusts/100m2) • Nlocusts: total number of locusts in fifty replicates Senegalese grasshopper density is obtained after correcting locust density for net sampling. 3.2.4. Pedestrian transect method This method involves walking through the field along a 100 m long and 1 m wide path, counting any grasshoppers that take flight. The surveyor performs about 50 repetitions and calculates the average, which corresponds to the density of the surveyed field. The actual density of Senegalese grasshoppers is obtained by adjusting this value using the percentage derived from net sampling. 3.2.5. Net sampling The purpose of net sampling is to determine the instantaneous population structure of the locusts. Repeated sampling over time and space allows us to study the population dynamics. We thoroughly search the surveyed fields and capture grasshoppers spotted jumping or resting on the ground using a sweep net. After identifying individuals of Oedaleus senegalensis (Krauss, 1877), we calculate the percentage of Senegalese grasshoppers, which is then used to correct the densities obtained during the surveys. The following formula is applied: 𝑑𝑂𝑆𝐸 = 𝑥𝑑𝑙𝑜𝑐𝑢𝑠𝑡𝑠 • dOSE: Senegalese grasshopper density, • x: percentage of OSE density at sampling, • dlocusts: density of all locusts at survey. 3.2.6. Developmental stages of generations For each survey mission, we identified the generations of O. senegalensis populations present in the study areas. During the week, three sampling sessions were conducted by six surveyors between 7:00 and 8:30 a.m. The samples were collected and their composition analyzed. The captured Senegalese grasshoppers were examined, and the different developmental stages were characterized using an identification key. From July to September 2021, we established the generations and larval stages present in the four surveyed regions of Senegal. World Journal of Advanced Research and Reviews, 2025, 27(01), 415-428 420 3.3. Socio-economic impact 3.3.1. Assessment of attack rate Over a ten-meter length of millet field, which normally contains about 10 tiller clumps, we first counted the number of tillers, then the total number of leaves, and finally the number of leaves damaged by locusts. For the last mission, when the millet was at a very advanced heading stage, damage was assessed on the ears. Fifteen repetitions were conducted for each field. 3.3.2. Recording production After harvesting, farmers threshed the millet in front of our supervisory team, who recorded the yields from both fertilized and control fields. The harvest data allowed us to assess the socio-economic impact of grasshopper attacks. All results were processed using R software (version 4.2.3, dated March 15, 2023). We examined whether there were any effects of region and field fertilization on the observed densities. An ANOVA was conducted for each mission, followed by a comparison of group means using the Tukey Honest Significant Difference (TukeyHSD) test. The ANOVA test was used to analyze differences between regions and field types. The TukeyHSD test assessed the significance of mean differences in OSE densities based on field type and region. 4. Results 4.1. Population dynamics 4.1.1. OSE density Monitoring of fertilized and control fields throughout the rainy season showed that Senegalese grasshopper densities were similar during the first mission. Only in Kaffrine did the fertilized fields have fewer locusts than the unfertilized ones during this mission. Significant differences were observed between regions, mainly driven by Kaffrine (Fig. 2). Figure 2 Senegalese grasshopper density in fertilized (green)and control (red) Fields during mission 1 Subsequent missions revealed a clear difference between the two types of fields. Fertilized fields had fewer Oedaleus senegalensis (OSE), with densities almost half those of unfertilized fields across all regions. However, during the second mission, the Kaffrine region was the only one showing no significant difference between fertilized and unfertilized fields (Figs. 3 and 4). The highest density was recorded in unfertilized fields in Kaffrine at 40.35 OSE/are, while the lowest was in fertilized fields in Saint Louis at 7.57 OSE/are. This indicates that Senegalese grasshoppers tend to prefer fields not enriched with nitrogen. World Journal of Advanced Research and Reviews, 2025, 27(01), 415-428 421 The results also showed that Oedaleus senegalensis movements and densities decreased along the rainfall gradient from Kaffrine, Fatick, Thies to Saint Louis (Table 1). The species was more abundant in unfertilized fields, with an average density of 20.25 insects per year, compared to 8.5 insects per year in fertilized fields. Figure 3 Senegalese grasshopper density in fertilized (green) and control (red) fields during mission 2 Figure 4 Senegalese grasshopper density in fertilized (green) and controls (red) fields during mission 3 Table 1 Annual average density of Senegalese grasshoppers by region and filed type Regions Saint Louis Thies Fatick Kaffrine Fertilized fields 7.57 9.97 12.28 26.81 Control fields 13.34 18.74 22.68 40.35 The presence of vegetation plays a crucial role in grasshopper distribution. For the same percentage of plant cover, Senegalese grasshoppers are more abundant in control fields than in fertilized fields. However, the highest number of insects is observed when vegetation cover ranges between 50% and 60% across all the regions studied (Fig. 5). World Journal of Advanced Research and Reviews, 2025, 27(01), 415-428 422 Figure 5 Senegalese grasshopper density as a function of plant cover 4.1.2. generations and development stages Locust captures and identifications during the surveys enabled us to determine the developmental stages and generations of Oedaleus senegalensis present during each mission. We identified three generations of OSE (G1, originating from the eggs of the previous season’s last generation, and G2 and G3 indigenous) in the Thies, Fatick, and Kaffrine regions. In Saint Louis, only the G1 and G2 generations were observed. All developmental stages were identified, from eggs (hypogeal stage), through larvae (epigeal stage), to adults (imago) (Table 2). Table 2 Development stages and generation during mission (A. adult, G generation, L larva, O egg, Example L1G1, stage 1 larva of first generation) Regions Mission 1 (in july) Mission 2 (in august) Mission 3 (in september) Saint Louis L1G1 L2G1 AG1 OG1 L1G2 AG2 OG2 Thies AG1 OG1 L1G2 AG1 OG1 L1G2 AG2 OG2 L1G3 L2G3 Fatick L4G1 L5G1 AG1 OG1 AG1 OG1 L1G2 AG2 OG2 L1G3 L2G3 Kaffrine L4G1 L5G1 AG1 OG1 AG1 L1G2 L2G2 AG2 L4G2 L5G2 L1G3 World Journal of Advanced Research and Reviews, 2025, 27(01), 415-428 423 4.2. Socio-economic impact 4.2.1. Damage to millet Attacks were observed at all stages of millet development, but those on young seedlings and ears caused much more severe damage (Figs. 6 and 7). Damage on young seedlings was primarily caused by the G1 generation, while the G3 generation was responsible for damage to the ears. During the first mission, the difference in attack levels between fertilized and unfertilized fields was not statistically significant (p = 0.0683). However, subsequent missions showed a highly significant difference in millet damage between the two field types (p = 0.0367). Damage rates recorded were 24.85% in unfertilized fields compared to 5% in fertilized fields. More attacks were observed during the first and last missions than during the second. Figure 6 OSE damage on millet leaves Figure 7 OSE damage on millet ears 4.2.2. Millet yields Yields varied significantly according to field type. The yields recorded in fertilized fields were 138,734 Kg compared to 55,808 Kg for the unfertilized fields, i.e. 2.5 times more. Average yields were 813.95 kg/ha in fertilized fields compared to 435.30 kg/ha in control fields. Standard deviations for fertilized and control fields are 394.50 and 275.39 respectively. Depending on the region, we observed variations in harvests. Fertilized fields produced 61%, 63%, and 72% more than control fields in Fatick, Kaffrine, and Thies, respectively. Comparison of production by region, we have seen that for fertilized fields, the Fatick region showed the highest average production (856.86), followed by Thies (803.08), and Kaffrine (781.89). For non-fertilized fields, Fatick again recorded the highest average production (537.23), followed by Kaffrine (449.58), and finally Thies (319.10). Harvesting did not take place in Saint Louis because the millet failed to mature due to an early cessation of rainfall. 5. Discussion Unfertilized fields are more vulnerable to locust attacks, despite some of these fields having low locust populations, particularly in July. After the first rains, millet germination coincides with the massive hatching of Senegalese grasshopper eggs, which had been in embryonic diapause during the dry season. Thus, rainfall triggers the resumption of Oedaleus senegalensis development. The water factor is a complex element that acts directly on the species and indirectly on its environment. The Senegalese grasshopper is an acridid species found in semi-arid zones, with a distribution range located in areas receiving between 250 mm of annual rainfall in the north and 1000 mm in the south. The environment is optimal for the species’ survival when monthly rainfall ranges between 25 and 50 mm. Between 50 and 100 mm, conditions remain compatible with the insect’s normal development. Below 50 mm or above 100 mm, the environment becomes respectively too dry or too wet for the species [15].