Biology, Distribution and Management of Fall Armyworm, Spodoptera frugiperda J.E. Smith (Lepidoptera: Noctuidae), on Maize (Zea mays L.) in Ethiopia

Abstract

Fall armyworm (FAW) Spodoptera frugiperda J.E. Smith (Lepidoptera: Noctuidae) is a polyphagous insect pest and has long been known as a serious economic pest in the tropical and subtropical regions of America. However, outside of the native range in North and South America, FAW was recently invaded different African countries and caused significant damage to maize. The invasive FAW first invaded Ethiopia in February 2017 and has since become established in many areas across the country and threatened food security of smallholder maize growing farmers. Maize, Zea mays L. is one of a strategic and staple crop that is prmarlly grown as a source of food in several Sub-Saharan Africa (SSA) countries including Ethiopia. In SSA, maize is essential for economic and food security for over 208 million people. In Ethiopia, maize occupies about 2.1 million hectares of land, of which, smallholder farms accounted for more than 95% of the total area and production. However, maize has a very low average grain yield in SSA due to a variety of abiotic and biotic constraints. Among the biotic constraint the damage caused by the recently introduced FAW is the most significant biotic component. Therefore, the present study aimed at determining the biological characters of FAW, their current distribution, damage level and its associated natural enemies which may provide further insights for future effective integrated management options. Recently, the damage caused by FAW has increased year after year. The current assessment of FAW distribution and infestation in different parts of Ethiopia confirmed that during the main season, the overall mean average infestation of FAW for the 2018/19 and 2019/20 crop seasons was 16.38 and 21.41%, respectively. Moreover, the dry season assessment recorded an average infestation of 42.5%. The results of natural enemy assessment revealed that six different types of indigenous natural enemies namely Entomopathogenic fungi (EPF), Entomopathogenic nematodes (EPN), Ladybird beetle, Earwig, Trichogramma sp., and Cotesia sp. that cause natural mortality on FAW were recorded and identified. The identified EPF and EPN were further screened against FAW to develop a biocontrol program against FAW. Among the EPN isolates, Steinernema sp. (Aso-Tes-287) and Heterorhabditis sp. (Am-Ger-Tes-74, Am-Adm-Tes-369, Z9) caused larval mortality within ten days under laboratory and lath house pot experiments. Particularly, based on the pot experiment, 74.7 (Aso-Tes-287), and 78.3% (Am-Ger-Tes-74) larval mortality were obtained at 600 IJ/ml. Most of the isolated EPF from different maize growing areas are virulent xxii against the 3rd instar larvae of FAW larvae and caused 50% mortality within 5.6 days. The four most virulent EPF isolates were further tested in a wire-house pot experiment for dose response at four different conidial concentration levels. Accordingly, two of the EPF isolates (ABe28 and SMe162) caused significant larval mortality at the higher conidial concentration (i.e., 1x109 conidia ml–1). Therefore, two EPN (Aso-Tes-287 and Am-Ger-Tes-74) and two EPF (ABe28 and SMe162) isolates, which caused higher mortalities within shorter periods, were promising bio-agents for the management of FAW. Future work would therefore be focused on field validation of these isolates against different stages of FAW in different agro-ecologies. The biology of FAW was studied in the greenhouse (usingr no-choice test) on different crops namely, maize, sorghum, chickpeas, barley, and wheat. Thus, the life cycle of FAW ranged between 31 to 38 days at rearing room temperature of 25.5 to 37.4°C and relative humidity of 35.33 to 45.46%. The highest number of days from egg to adult was recorded on sorghum and maize, while the lowest duration of on chickpea, barley, and wheat. The host preference study was also conducted using both choice and nonchoice tests on the major crops grown in Ethiopia. The host preference study result discovered that maize, sorghum, Swiss chard, teff, elephant grass, and cabbage were the most preferred hosts for egg-laying and larval development.