Impact of altitude variation and grassland composiition on stemborer and fall armyworm population in maize farms and surrounding grasses in western Kenya during 2019-2020 rain season

Abstract

Grassland ecosystems adjacent to maize farms are often habitats for pests that pose significant risks to maize crops. Among these pests, lepidopteran stemborers and fall armyworm are the most detrimental, negatively impacting maize productivity. Although pests are known to thrive in grasslands, the influence of wild grassland composition on pest populations in maize fields and surrounding grasses across varying altitudes remains poorly understood especially in western Kenya. Given that altitude can significantly affect both plant and insect communities, studying this factor is crucial for developing effective pest management strategies. This study investigated the relationship between altitude and pest dynamics in maize farms and adjacent grasslands, focusing on four specific objectives, namely to: (i) assess the seasonal composition, distribution, and characteristics of grasses in grasslands near maize farms across four altitudes (low -1100 metres above sea level (masl); medium-1300 masl; high- 1500 masl; very high- 1700 masl) in western Kenya; (ii) examine the impact of altitude on grass species diversity and stemborer (Busseola fusca and Chilo partellus) and fall armyworm (Spodoptera frugiperda) populations across the four altitudinal zones; (iii) explore the influence of grass composition and characteristics on stemborer and fall armyworm populations in grasslands surrounding maize farms across the four altitudes; and (iv) investigate how surrounding grassland composition and characteristics affect stemborer and fall armyworm populations in maize farms across the four altitudes. The experimental design involved purposive sampling at four distinct sites of varying altitudes Mt. Elgon (very high elevation), Vihiga (high elevation), Homabay (medium elevation), and Lambwe (low elevation). Data were collected during the 2019 and 2020 short and long rain season respectively focusing on eight maize farms per site. Key parameters measured included grass diversity, percentage cover, species richness, and the abundance of stemborers and fall armyworms. Grass species were sampled using quadrats randomly placed in grasslands adjacent to maize farms, while inspecting 10 randomly selected maize plants from each farm assessed pest abundance. Environmental variables, including altitude, temperature, and soil moisture, were measured to contextualize grass and pest dynamics. Data analysis utilized the BiodiversityR package to rank grass species richness, Non-Metric Multidimensional Scaling (NMDS) for visualizing grass distribution, and Principal Component Analysis (PCA) to assess variable relationships. Spearman’s correlation explored the influence of grass characteristics on pest abundance, while Tukey’s HSD, linear regression, and Generalized Linear Models (GLM) evaluated altitude-pest dynamics relationships. Results showed a total of 55 grass species in 2020 versus 32 in 2019 across the four elevations. NMDS showed significant variation in grass composition by elevation (P < 0.005), with elevation affecting species composition more in 2020 (global R² = 0.454). Grass percentage cover was highest in Homabay in 2019, while Mt. Elgon showed increased cover in 2020. Lambwe displayed significantly higher grass fresh weight in both years. Additionally, fall armyworm abundance on maize farms correlated strongly with grass characteristics, especially grass cover, which negatively affected pest populations (Homabay rs = -0.93, P < 0.001). These results underscored the need for pest management strategies that account for interactions between grassland ecology, pest biology, and agricultural landscapes, emphasizing the importance of grass species composition, diversity, and abundance in shaping pest populations.