Evaluation of Rosemary (Rosmarinus officinalis) as a potential push plant for the management of cabbage aphid (Brevicoryne brassicae), and its impact on natural enemies in smallholder Kale (Brassica oleracea) production systems

Abstract

Kale (Brassica oleracea var acephala (L.) production holds global significance as a result of its exceptional nutritional and economic value. However, the cabbage aphid (Brevicoryne brassicae), poses an important challenge to B. oleracea var. acephala production, with substantial direct and indirect yield losses. Due to its rapid reproduction and resistance development, farmers overly rely on synthetic pesticides to eliminate the pest. Integrated pest management (IPM) strategies, including cropping systems and the use of natural enemies that minimize reliance on synthetic pesticides, are essential for sustainable and environmentally friendly pest control in vegetable production systems. This study hypothesized that R. officinalis plant odour disorients B. brassicae through specific volatile organic compounds (VOCs). It tested this by assessing B. brassicae and its parasitoid (Aphidius colemani) responses to R. officinalis and B. oleracea var. acephala volatiles using a four-arm olfactometer. Gas chromatography-mass spectrometry (GC-MS) identified compounds triggering these behaviors, while gas chromatographyelectroantennographic detection (GC-EAD) pinpointed the responsible VOCs. Finally, the impact of intercropping B. oleracea var. acephala with R. officinalis on the population dynamics of B. brassicae, A. colemani, and spiders was evaluated. The collected data was analyzed using R statistical software version 4.2.2. Brevicoryne brassicae preferred the olfactometer arm with B. oleracea var. acephala plant and its volatiles over those with R.officinalis and its volatiles. Aphidius colemani preferred the arms with B. oleracea var. acephala plant and its volatiles, over the arm containing R. officinalis and its headspace volatiles. Gas Chromatography-Mass Spectrometry analysis revealed that R. officinalis emitted more and diverse compounds such as camphene, α-Phellandrene, δ-2- Carene, (Z)-Sabinene hydrate, linalool, borneol, α-Terpineol, verbenone, citronellol, geraniol, bornyl acetate, β-Caryophyllene, α-humulene, and caryophyllene oxide. GC-EAD analysis indicated that B. brassicae antenna responded to Linalool, α-Terpineol, Verbenone, Geraniol, Camphor, and Borneol from R. officinalis, and Sabinene, γ-Terpinene, and β- Caryophyllene from B. oleracea var. acephala. Intercropping R. officinalis with B. oleracea var. acephala significantly reduced B. brassicae populations, while increasing the numbers of A. colemani, highlighting R. officinalis potential as a push plant in sustainable B. brassicae management systems.