Abstract
Plants with pesticidal properties have been investigated for decades as alternatives to synthetics, but most progress has been shown in the laboratory. Consequently, research on pesticidal plants is failing to address gaps in our knowledge that constrain their uptake. Some of these gaps are their evaluation of their efficacy under field conditions, their economic viability and impact on beneficial organisms. Extracts made from four abundant weed species found in northern Tanzania, Tithonia diversifolia, Tephrosia vogelii, Vernonia amygdalina and Lippia javanica offered effective control of key pest species on common bean plants (Phaseolus vulgaris) that was comparable to the pyrethroid synthetic, Karate. The plant pesticide treatments had significantly lower effects on natural enemies (lady beetles and spiders). Plant pesticide treatments were more cost effective to use than the synthetic pesticide where the marginal rate of return for the synthetic was no different from the untreated control, around 4USD/ha, compared to a rate of return of around 5.50USD/ha for plant pesticide treatments. Chemical analysis confirmed the presence of known insecticidal compounds in water extracts of T. vogelii (the rotenoid deguelin) and T. diversifolia (the sesquiterpene lactone tagitinin A). Sesquiterpene lactones and the saponin vernonioside C were also identified in organic extracts of V. amygdalina but only the saponin was recorded in water extracts which are similar to those used in the field trial. Pesticidal plants were better able to facilitate ecosystem services whilst effectively managing pests. The labour costs of collecting and processing abundant plants near farm land were less than the cost of purchasing synthetic pesticides.
Highlights
Tanzania is among the top twenty largest producers of common beans, Phaseolus vulgaris L., in the world, and is the second largest producer after Kenya in sub-Saharan Africa.[1]
Due to the severity of different insect pests affecting beans, many African farmers increasingly resort to frequent use of commercial synthetic pesticides.[5]
The synthetic pesticide was superior in reducing abundance of all pest insects in comparison to the plant extracts with
Summary
Tanzania is among the top twenty largest producers of common beans, Phaseolus vulgaris L., in the world, and is the second largest producer after Kenya in sub-Saharan Africa.[1]. Plants with pesticidal properties have been investigated for decades as alternatives to synthetics, but little progress has been made to develop new products.[9,10] research on pesticidal plants is increasing, it is failing to address gaps in our knowledge that constrain their adoption.[11] One of these gaps is their evaluation under realistic field conditions to assess their efficacy as well as whether their use can be economically beneficial to farmers. In comparison to concentrated synthetic products, pesticidal plants should be more environmentally benign due to their short persistence, naturally low concentrations of a more diverse suite of active ingredients and anti-feedant/repellent modes of action. The aims of our study are to: 1) investigate optimal application methods of pesticidal plants, weed species that are widely available and abundant in bean production ecosystems, for insect pest control on common bean; 2) compare the effects of a common synthetic pesticide and pesticidal plants on the level of pest control and their potential effects on predatory insect species; and 3) a cost-benefit analysis on these pest management options
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