Potential of some botanical extracts, soap solutions and companion plants against cabbage aphid (Brevicoryne Brassicae L.) on rape (Brassica napus L.)

Abstract

Rape (Brassica napus L.) is a popular leafy vegetable grown for home consumption and for sale in Botswana and other African countries. It significantly contributes to the livelihood of both smallholder and commercial farmers. Cabbage aphid (Brevicoryne brassicae I..) is one of the most important sap sucking pests of Brassica crops in Botswana, and if not controlled, can cause yield losses of rape and other brassicas. There is public concern related to pesticides use in crop production and this has led to the rise of consumer interest in organically produced foods. Net-shade and field experiments were conducted at the Botswana University of Agriculture and Natural Resources, Gaborone to test the potential of some botanical extracts, soap solutions and companion plants against cabbage aphid (Brevicoryne Brassicae L.) on rape (Brassica Napus L.) to determine the (i) effect of spraying water-based botanical extracts and soap solutions on mortality of cabbage aphid on rape; (ii) the effect of companion planting rape with garlic (Allium sativum L.), peppermint (Mentha canadensis L.), french marigold (Tagetes patula L.), basil (Ocimum basilicum L.) and imidachloprid application on cabbage aphid populations and yield of rape. In the net-shade study, water­ based botanical extracts of garlic, peppermint, French marigold, and basil at 10% and 20% (fresh w/v) concentrations and 5% and 10% (v/v) liquid soap solutions were sprayed against cabbage aphid on rape plants in insect-proof cages. The results of the net-shade study indicated significantly higher aphid mortality in summer than winter (P=0.000l) and imidacloprid had the highest mortality followed by 10% liquid soap, 20% basil and garlic, marigold and mint In the field experiment, imidacloprid spray significantly controlled the aphid populations (P=0.0001) compared to the companion plating treatments. Among the companion planting treatments, basil had the lowest aphid population followed by marigold, mint, garlic and the untreated control which had the highest population. Therefore, spraying rape with water-based extracts of basil and marigold and liquid soap has lite potential to control cabbage aphids in place of synthetic insecticides. This study, therefore, has shown that botanical can be incorporated as constituents of a pest management strategy. More so, smallholder farm producers who cannot afford expensive and dangerous synthetic insecticides can utilize basil and marigold for pest management, in addition to selling these herbs for profit.