In Niger, agriculture employs 80% of the working population, for which it is the main activity and source of income to meet food and economic needs. Agriculture contributes more than 40% to the Gross Domestic Product (GDP) [1] . Among the cultivated plants, cereals (millet, maize, sorghum, rice) not only occupy an important place in national agricultural production but also constitute the food base of the population and are therefore the most consumed by the population [2] . Millet (Pennisetum glaucum (Leek) R. Br.), is the most widely grown cereal in Niger, with a planted area of more than 65% and; it constitutes 75% of the country’s total cereal production [3] [4] . Millet is produced in all regions of the country, with an estimated average annual production of 2,146,706 tons [5] [6] . As for maize (Zea mays L.), it is ranked as the fourth largest cereal grown in Niger with an average annual production at the national level of 5681 tonnes after millet, sorghum and rice [5] . Despite their importance, both in terms of food and economics, millet and maize crops continue to face many biotic constraints, including insect pests. In 2016, a new species called Fall armyworm, Spodoptera frugiperda, appeared in Sub-Saharan Africa [7] , a brand-new pest. This pest colonized almost all the countries of the African continent only two (2) years after its appearance [8] . Fall armyworm (CLA) is a voracious and polyphagous insect that forms large populations at a very high rate of spread [9] [10] . This species is considered an extremely dangerous pest; one of the most destructive on the American continent [10] . Severe infestations, especially early ones, lead to significant reductions in yield, the consequences of which are disastrous on the economic, social, and ecological levels [11] . In Niger, this pest has been present since 2016 and is currently found in all regions of the country [12] . Given its rapid spread and the number of crops on which Fall armyworm is present, it is important to monitor the population dynamics of this pest through monitoring tools and larval detection in the field. These tools usually come in the form of the various traps such as simple glue traps, light traps, chromatic traps, and pheromone traps. The latter has particular characteristics of specificity to insects [13] . This sex pheromone trap is a better way to monitor Fall armyworm population dynamics [14] - [16] . The objective of this study is to assess the incidence and severity of damage caused by the Fall armyworm, Spodoptera frugiperda (J. E. Smith) on maize and millet crops in the Maradi region. More specifically, it is a question of monitoring the population dynamics of Fall armyworm in the fields.

This study was conducted in Niger in the Maradi region. The Maradi region is located at 13˚27' and 15˚26' north latitude and 6˚16' and 8˚36' east longitude; it is located in the southern strip of Niger with an area of 41,796 km² ( Figure 1 ), bordered by the region of Tahoua to the west, Zinder to the east, Agadez to the north and the Federal Republic of Nigeria to the south (PDR, 2015).

The results of this study show that the Fall armyworm, S. frugiperda, is present in the Maradi region where all the fields of sites concerned by the study were attacked. This study is one of the first of its kind to address the extent of S. frugiperda damage to maize and millet. The infestation rate of 71.7% recorded as the highest in the present study appears low in view of the literature in many of the studies. This rate is lower than those reported by [18] in a similar study in East African countries, 90.83% in Kenya and 96.5% in Tanzania. [19] noted a 100% infestation rate in Maputo province in Mozambique. This low infestation rate could be explained by the misuse of chemical pesticides on the main pest. These pesticide applications significantly affect S. frugiperda populations at production sites. Other factors such as temperature, relative humidity and photoperiod may also explain the low infestation in the Maradi region. [20] pointed out that longevity and fecundity in Fall armyworm are highest between 21˚C and 25˚C, while in the Maradi region, at least an average temperature of 35˚C is recorded. This rate is comparatively higher than 37.31% obtained by [21] in the rural commune of Djirataoua in a similar study in the Maradi region and 42.81% S. frugiperda infestation rate obtained on maize by [22] in the same commune. However, an increase of about 35% was observed in the same region. It also proves a gradual adaptation in the region since its introduction in 2016 in the country, despite the use of synthetic pesticides. However, the millet treatments of INRAN and Djirataoua recorded an average rate respectively. These low S. frugiperda infestations on millet could be explained by cultural practice, and natural enemies, especially predators present in the environment. The most common predators of millet belong to the Formicidae, Forficulidae and Vespidae families. Species of the family Vespidae and Formicidae are formidable predators of S. frugiperda larvae, while insects of the family Forficulidae are known as egg predators according to [11] [23] .

All fields at the sites were attacked by armyworm but the rate is not as high compared to other countries on the continent, as it was reported in a similar study that about 80% to 100%, 93% to 100% and 100% of maize fields infested with the pest in Ethiopia, Tanzania and Kenya respectively, [18] . The total average of the level of leaf damage of the two (2) sites Djirataoua and INRAN on the Davis and William rating scale shows a higher level of severity of score 2. Despite the evolution of the pest infestation rate in the fields, the low level of foliar damage observed could be due to the misuse of pesticides against the pest, which could then reduce the damage to the leaves by killing the larvae of S. frugiperda. This low degree of attack could be linked to the abundant rainfall in the Maradi region. According to [24] , heavy precipitation causes mortality of S. frugiperda larvae by filling whorled leaves with water, especially young larvae, and can fall from larvae to the ground where possible contact is established with predators or entomopathogenic microorganisms. [25] have developed other parameters that influence the incidence and severity of S. frugiperda, among which the use of chemicals, cultural practices, varieties, are important parameters.

The average number of 32 S. frugiperda adults captured in Djirataoua_Maïs appears to be higher in this study. This number of adults is comparatively higher than that noted by [26] of 6 adult male butterflies of S. frugiperda captured in 2019 in a study in the Centre-West region of Burkina Faso. This number could be explained by the duration of the captures about three (3) months to Djirataoua_Maïs and the favorable conditions for the survival of the armyworm in the irrigated perimeter of Djirataoua. It could also be related to the phenomenal quality and the necessary time of activity of the substance during monitoring. According to [27] , the number of adult S. frugiperda in a given area is influenced by factors such as climate, cultivated hosts and cropping system. [28] pointed out that adult catches are relative to temperature, humidity and wind; and vary from one moment to another or from one site to another. [29] also reported that the number of males captured in traps varies by geographic area.

The average number of S. frugiperda eggs (14 lots) and 20 larvae recorded in Djirataoua_Maïs has the highest number for this survey. These results are similar to that of [30] which obtained in 2019 an average of 174.8 eggs per egg mass and 280.5 larvae in the conditions of temperature of 24˚C ± 1˚C, relative humidity of 50 in 2019 in a similar study in the Democratic Republic of Congo. This low number could be due to the application of pesticides for phytosanitary treatment and the large number of telenomus remus in the area.

This study has shown that the armyworm attacks the main cereals grown in Niger. Since its introduction, this caterpillar has posed a major threat to the country’s food security, as millet, being the most widely grown cereal, is not spared from its attacks, although the level of damage observed is not worrying. Natural infestation and the severity of damage caused by armyworms are higher on maize than on millet. In addition, the sex pheromone traps used to monitor population dynamics captured large numbers of adult male S. frugiperda in the fields at the study sites. FAW poses a threat to cereal production, the observed damage levels are not alarming, possibly due to pesticide use by farmers and natural enemies. This study confirms the presence and attacks of armyworm on millet, the main cereal crop in Niger, already undermined by attacks from the ear leaf miner. This pest pressure on millet could further complicate millet production in Niger. This situation could further complicate also the biological control efforts against millet earworm in Niger.