The Interactive Effctive of Blackjack extract and biostimulants on fall armyworm damage maize yield, and selected soil chemical properties

Abstract

Fall armyworm, Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae), poses a significant threat to maize production. Smallholder farmers in Africa have relied heavily on synthetic pesticides to control the fall armyworm. However, the overuse of synthetic pesticides affects the economic viability of smallholder farming systems. It also contributes to the emergence of secondary pests, insecticide resistance and environmental pollution. Therefore, sustainable and environmentally safe pest control approaches, such as botanical pesticides and bio-stimulants, are required. This study aimed to evaluate the interactive effect of blackjack extract (BJE), biochar (BC), and arbuscular mycorrhizal fungi (AMF) on fall armyworm damage, maize yield and selected soil chemical properties. A laboratory cage experiment assessed fall armyworm leaf damage, while a complementary greenhouse experiment conducted in spring 2024 and 2025 further evaluated the interactive effects of these treatments on fall armyworm leaf and cob damage, maize yield and selected soil chemical properties. Both experiments were laid out using a completely randomised design (CRD) with eight treatments: control, individual and combined applications of BJE (10% w/v), BC (10t/ha), and AMF (5.3 g), each replicated three times. BC and AMF were applied to the soil before planting, while BJE was sprayed as an aqueous extract. Leaf and cob damages were analysed using ordinal regression in SPSS Statistics. Yield components (shoots and root biomass, root length, grain yield, harvest index) and soil chemical properties (pH, EC, NO3--N, NH4+-N, P, K, OC, CEC, Na, Ca, Mg, S) were analysed using a three-way ANOVA in Minitab. The results of this study showed that, on average, BJE significantly reduced leaf damage by approximately 19% against 1-day-old larvae and approximately 12% against 7-day-old larvae in the laboratory experiments. In the greenhouse experiment, BJE consistently reduced leaf damage by 16.94% in 2024 and 14.63% in 2025, and cob damage by 18.83% in 2024 and 11.54% in 2025. BC and AMF, whether applied individually or together, did not significantly reduce damage. However, when either was combined with BJE, an additive effect was observed, resulting in notable reductions. On the contrary, the BJE treatment did not significantly affect maize yield components except for yield, which increased by 21.81% in 2024. BC showed variable effects, reducing yield and biomass in 2024, but increasing root and shoot biomass by 21.5% and 17.2%, respectively, in 2025. AMF had a significant effect on plant height (13.6%), root length (6.8%), and shoot biomass (14.3%) in 2024, but in 2025, it did not show significant differences in any yield component. In terms of soil chemical properties, BC increased soil pH, K, and Na by up to 3.4%, 102.2% and 30.3%, respectively, while reducing NO3--N by up to 52.1%. Conversely, AMF increased NO3--N (66.6%) in 2024. Combined treatments did not produce synergistic effects; however, additive effects were observed in BJE + BC + AMF and BC + AMF, which significantly increased root biomass, harvest index and enhanced soil K and Mg. These findings indicate that BJE is the primary driver of fall armyworm suppression, while the bio-stimulants (BC + AMF) mainly improved soil fertility and maize growth depending on experimental year. The results provide evidence-based recommendations to optimise sustainable, environmentally friendly pest management strategies for maize production while reducing reliance on synthetic pesticides.