Efficacy of varying concentrations of agroforestry-derived botanical extracts on aphid (Brevicoryne brassicae) populations and yield performance in rape (Brassica napus L.)

Abstract

<title>Abstract</title> Synthetic herbicides are becoming unaffordable for many smallholder farmers in southern Africa. In controlling pesticides, most resource-poor farmers use plant extracts, which are less expensive and environmentally friendly. The objective of this study was to evaluate the use of agroforestry crude plant extracts in the control of aphids, and growth and yield performance of <italic>Brassica napus</italic> Linnaeus. The experiment was performed in a randomized complete block design with seven treatments replicated three times. The results revealed no significant effect (p &gt; 0.05) on aphid number per plant from the use of various concentrations of the botanical extracts and Dimethoate. However, the use of 30 g L^{− 1} <italic>Tephrosia vogelii</italic> or 30 g L^{− 1} <italic>Tagetus minuta</italic> decreased the number of aphids per plant over time. The use of <italic>T. vogelii</italic> or <italic>T. minuta</italic> for 2–8 weeks had a significant effect on <italic>Brassica napus</italic> yield at different concentrations. The highest fresh yield (9.26 ± 0.02 t ha^{− 1}) of rape was obtained from the 30 g L^{− 1} <italic>T.v.</italic> concentration, and the lowest fresh yield (0.80 ± 0.64 t ha^{− 1}) was recorded from the 10 g L^{− 1} <italic>T. vogelii</italic> concentration but was not significantly different from that of dimethoate (8.85 ± 0.12 t ha^{− 1}). The <italic>T. vogelii</italic> and <italic>T. minuta</italic> concentrations reduced the aphid population and improved fresh rape yield. However, farmers are encouraged to use <italic>T. vogelii</italic> and <italic>T. minuta</italic> botanical extracts at 30 g L^{− 1} concentration or higher, as an economically viable aphid management strategy.