Temporal Dynamics of Fall Armyworm, Spodoptera Frugiperda (J.e. Smith), Stemborer Pests and Associated Natural Enemies in Maize Fields in Semi-arid Zone, Machakos, Kenya

Abstract

Maize is one of the principal food security crops in sub-Saharan Africa (SSA). In Kenya maize is a staple food crop and source of employment and income for millions of farming communities. Due to poor maize yields, production rarely meets local population demand. The low yield of maize production is associated with many challenges including stemborer species. Key among them include Busseola fusca (Fuller), Sesamia calamistis (Hampson) and Chilo partellus (Swinhoe). In addition, the recent invasion by Fall armyworm (FAW), Spodoptera frugiperda (J.E Smith) in SSA from America poses a serious food security threat in the continent. However, little is known about FAW seasonal dynamics in different Agro-ecological zones and in particular, its interaction with stem borer pests. Thus, this study was initiated to determine the temporal dynamics of FAW and its interaction with stem borer pests and associated natural enemies in the semi-dry region of Machakos County, Kenya. The study was conducted at KALRO Katumani and two other farmers’ fields in Mwania and Mikuyu villages during the short rain growing season (Oct, 2019-Jan, 2020). In a pre-planting survey of FAW in the cultivated and non-cultivated areas, 2 FAW egg batches and 90 larvae were collected in the cultivated maize area in Mwania adjacent to the research farm. Males were monitored using sex pheromone traps deployed across the farms but no catches were made. However, during the study a total of 449 FAW males were caught in six sex pheromone traps in deployed in the three sites. The FAW male catches per trap varied among different maize growth stages. The first males were detected during 2-4 leaf stage in Mikuyu and later during 5-7 leaf stages of maize plant in Mwania and Katumani, and later peaked among tasselling stages (11.5±11.8 and 0.85±0.95) in Mwania and Mikuyu respectively, and later among maturity stage (2.42±2.18) in Katumani. In a random zigzag transect a total of 1593 FAW larvae were collected in all sites. After 2-4 leaf stage, larvae increased steadily and high densities were observed among 8-11 leaf (1.78 ±0.33 and 2.63±0.41) larvae per plant in Mwania and Mikuyu respectively, and later among 12-15 leaf stage (3.13±0.52 in Katumani. Among the natural enemies of FAW identified included Zygobothria ciliata (Diptera: Tachinidae) and two other predators, Myrmicaria opaciventris (Hymenoptera: Formicidae) and Paederus sabaeus (Coleoptera: Staphylinidae). In four plots per farm selected randomly, two stem borers, B. fusca and S. calamistis were recovered in all the farms and were found in association with FAW notably S. calamistis species which were observed in maize cobs of the same plants. The implication of the findings is that FAW larval densities varied greatly among the different maize phenological stages despite recruitment of its local parasitoid species in the studied areas. This suggests that not one single control strategy is viable against fall armyworm management but rather it will form the basis upon which realistic Integrated Pest Management (IPM) package could be explored. More so, similar studies need to be extended to other agroecological zones of Kenya for broader understanding. Lastly, the few recoveries of stemborers (B. fusca and S. calamistis) among other factors suggest that FAW is capable of displacing stemborer pests from maize fields due to its cannibalistic nature hence expansion of its range and this necessitates future studies on its interaction with other key lepidopteran pests in other similar agroecological zones in Kenya.