Occurrence of Soil-dwelling Pests of Cabbage and Onions in Kenya and Management of Onion Fly Using Fortified Black Soldier Fly Frass Fertilizer

Abstract

Soil-dwelling insect pests constitute a significant challenge to vegetable production and food security, with their damage being exacerbated by climate change, land degradation, and poor monitoring due to soil heterogeneity. Chitin and chitosan from sea arthropods, such as crabs, lobsters, shrimp and krill, and mass rearing of the black soldier fly (BSF) have been explored for promoting soil nutrients and crop yield. However, there is limited knowledge about the agronomic and plant health potential of BSF frass fertilizer and chitin-rich pupae exuviae in crop production. Therefore, this study determined the incidence and severity of soil-borne insect pests of onions and cabbage and their management practices in Kenya, the insecticidal potential of fortified BSF frass fertilizer on onion flies and the agronomic performance of onion grown on soils enriched with fortified BSF frass fertilizer. A field survey was conducted to assess the occurrence, incidence and management of soil-borne insect pests of cabbage and onions in Nyandarua, Nakuru, Kiambu, Nyeri, and Kajiado counties targeting 45, 65, 32, 34, and 34 fields, respectively. Crops were randomly selected and assessed for signs and symptoms of infestation, and those affected were carefully uprooted together with the attached pests for morphological and molecular characterization. Chi square, t-test and analysis of variance (ANOVA) were performed to compare variables among counties and AEZs using SPSS. Onion flies which had the highest damage score were reared for three generations and used in contact, residual and ovicidal bioassays using the liquid fertilizer made from BSF frass fertilizer and pupae exuviae. Nevertheless, a field study was also conducted to determine the impact of amending the soil with BSF frass fertilizer and chitin-rich pupae exuviae on onion crop growth and yield. The BSF frass fertilizer was applied at a constant rate of 4.12 t ha-1 based on the N requirement of onion plants which is 120 kg nitrogen (N) ha-1. Black soldier fly pupae exuviae was applied as a percentage of frass fertilizer based on chitin rates. For the laboratory bioassays and the field experiment, the data were subjected to ANOVA followed by mean separation using Fisher’s LSD at α = 0.05 significance level in R statistical software...