Dual ecosystem services from non-food crop vegetation: benefits for cabbage pest management

Agricultural intensification has enhanced crop productivity and global food security but natural processes that underpin agroecosystem function have been eroded. Destruction of natural habitats has caused biodiversity loss and diminished valuable ecosystem services including natural pest suppression. Injudicious use of synthetic insecticides negatively affects human health, food safety, pest resistance and beneficial fauna. As a response, this thesis assessed the potential of native/naturalized plants (two Australian and six Ghanaian) in supporting the dual ecosystem services of habitat manipulation for conservation biological control and botanical insecticide provision to manage pests of cabbage. Aqueous extracts of two Australian plants, Nicotiana megalosiphon (Solanaceae) and Mentha satureioides (Lamiaceae) at 1, 5 and 10% (w/v) were tested against Plutella xylostella (Lepidoptera: Plutellidae), Brevicoryne brassicae and Myzus persicae (Hemiptera: Aphididae) in Australia. All concentrations of N. megalosiphon controlled P. xylostella better than tau-fluvalinate insecticide (positive control) and M. satureioides whilst both 5% and 10% concentrations of N. megalosiphon controlled both aphid species comparable to tau-fluvalinate. Six Ghanaian plants, Ageratum conyzoides, Tridax procumbens (Asteraceae), Crotalaria juncea (Fabaceae), Cymbopogon citratus (Poaceae), Lantana camara (Verbenaceae) and Talinum triangulare (Talinaceae) were also tested against P. xylostella and B. brassicae in Ghana. The 5% and 10% concentrations of A. conyzoides extract was as effective as emamectin benzoate insecticide (the positive control) against both pests. Extracts of C. juncea, C. citratus, L. camara and T. procumbens at 10% concentrations were active against both pests compared with a water control, while T. triangulare extract was inactive.
In habitat manipulation field experiments over three consecutive growing seasons, the six Ghanaian plants were grown alongside plots of cabbage. Infestations of P. xylostella, B. brassicae and Hellula undalis Fab. (Lepidoptera: Crambidae) were significantly lower in all habitat manipulation treatments, and crop yields and quality were superior, compared with the control treatment with no habitat manipulation plants. Economic analysis revealed higher net incomes in the habitat manipulation treatments than the control, with higher cost: benefit ratios ranging from 1:2.8 for C. citratus to 1:46.6 for A. conyzoides.
In a second, three-season field experiment, habitat manipulation and botanical insecticides were tested simultaneously to manage cabbage pests in a three-way split plot design, such that the crop in each plot was sprayed with either a 10% w/v aqueous extracts from the habitat manipulation plant species, a negative control (water) and a positive control (emamectin benzoate). Pests were suppressed in all habitat manipulation treatments and habitat manipulation plus pesticide treatments compared with the no habitat manipulation plus water control. Natural enemies were depressed in all treatments sprayed with emamectin benzoate showing effect of pesticide toxicity. Economic analysis revealed cost: benefit ratios as high as 1:49 (for T. triangulare border plants) and as low as 1: -2.23 (for T. procumbens border plus emamectin benzoate spraying) showing that combining habitat manipulation and application of synthetic insecticides can be costly.
Overall, this thesis has shown the potential for wild plants to deliver dual ecosystem services to suppress pests, cascading to higher crop yield and profitability.