Effect of application of biopesticides on pests and yield of snap beans in Rwanda.

摘要

Snap bean (Phaseolus vulgaris) is a potential income earner for subsistence farmers in Rwanda. The crop realises a higher income per unit dry weight than dried beans. Constraints of pests and the problems of pesticide use call for alternatives that are safer for consumers. The objective of the study was to evaluate the effects of neem, tephrosia, tobacco and cow urine on pest infestation and yield of snap bean in Rwanda. The treatments were sprays of leaf extracts of neem, tephrosia, and tobacco, cow urine, and a control (water). The treatments were replicated four times in a randomized complete block design. Biopesticide and water applications and insect counts were carried out every one and a half to two and a half weeks. Insect counts were done on a sample of 10 or 20 plants. All whiteflies observed on whole plants were counted. Aphids were counted on a randomly selected shoot while all thrips found on a random sample of five leaves or flowers were counted. Yield data (fresh weight) per hectare was calculated from the middle rows of each plot, leaving out the border rows. Insect counts were totaled across the season for analysis. Major pest species observed were whitefly (Bemisia tabaci), black bean aphid (Aphis fabae), and thrips. In the first season, the number of whiteflies was not significantly different among treatments. The tobacco treatment resulted in a significantly lower number of aphids than all other treatments. The lowest populations of thrips were observed in plots treated with neem and tephrosia. The cow urine treatment gave the highest total and marketable yield. In the second season, tephrosia treatments significantly reduced populations of thrips. However, total and marketable yield were not significantly different among the treatments. It is concluded that tobacco has potential for control of aphids while neem and tephrosia could be used to control thrips. The insect populations attacking snap bean were probably too low to influence yield. However, differences observed in populations of aphids and thrips may translate into differences in yield in seasons when insect populations are high.