Cultural and botanical methods for the management of thrips on french beans Phaseolus Vulgaris

Abstract

French bean is the most important export vegetable crop cultivated in Kenya. Pests and diseases are the major constrains to its production. The major pests of French beans are bean flies, thrips, and bean aphids. Amongst these pests, thrips are the most notorious and account for 63 – 68% yield loss of fresh marketable pods. Frankliniella occidentalis is the most widespread thrips species which has developed resistance to the commonly used synthetic pesticides. Farmers rely heavily on these synthetic pesticides in order control the thrips and up to sprays of 15 times have been reported per growing season. This act has lead to the contamination of the fresh French bean pods with pesticide residues. The toxicated fresh pods may not be accepted in lucrative markets, more so in Europe. This study therefore, aimed at developing an alternative method of controlling thrips on French beans by use of cultural and botanicals. Laboratory and greenhouse experiments were conducted in two trials to assess the efficacy of botanicals against Frankliniella occidentalis infesting French beans. Field experiments were also conducted in two trials to evaluate the effect of different mulches on infestation and damage of French beans by thrips, and to evaluate the effect of integrating intercropping, mulching and use of botanicals on the infestation and the damage of French beans by thrips. The laboratory and greenhouse experiments were carried out at the International Centre of Insect Physiology and Ecology Nairobi in a complete randomized design. Field experiments were conducted at the Kenya Agricultural and Livestock Research Organization, Thika, Kenya in a randomized complete block design with five replicates for mulching experiments and three replicates for the integrated experiments. The laboratory results showed that L-Cyhalothrin caused the highest mean percentage mortality of the first instar F. occidentalis followed by Pyrethrum. Neem and Garlic caused the lowest mean percentage mortality. The result from greenhouse experiments showed that the lowest mean number of thrips was recorded on the French beans sprayed by L – Cyhalothrin followed by pyrethrum (soil + foliar), neem (soil + foliar), neem (soil) pyrethrum(soil), garlic (soil + foliar), garlic (soil) and control. The lowest damage score on fresh pods was recorded on the leaves treated with Pyrethrum (soil + foliar) and neem (soil + foliar) while the highest damage scores were recorded on the French beans treated with garlic (soil) and the control. The result from the experiments involving mulching showed that the lowest mean number of thrips was recorded on French bean leaves and flowers mulched with transparent plastic sheets followed by those mulched with dry grass, black plastic mulch, tithonia green mulch and the control. The results from integrated field experiments revealed that maize + dry grass + pyrethrum had the lowest mean number of thrips in flowers followed coriander + dry grass + pyrethrum, dry grass + pyrethrum, French bean a lone, coriander, pyrethrum and dry grass. In flowers the lowest mean number of thrips from French bean plant sprayed by LCyhalothrin followed by maize, maize + dry grass + pyrethrum, coriander + dry grass + pyrethrum and dry grass. The most abundance thrips species identified on French beans leaves was Hydatothrips adolfifriderici followed by Megalurothrips sjostedti while Frankliniella schultzei and F. occidentalis were very low in number. On French bean flowers, F. schultzei was the most abundance followed by M. sjostedti while F. occidentalis and H. adolfifriderici were very few. This study showed pyrethrum and neem pesticides can be used as an alternative chemical management for F. occidentalis on French beans in greenhouses. It also revealed that cultural and botanical methods can be integrated to provide an alternative pest management system. The system involves a combination of pyrethrum + maize + dry grass. However, these methods are not as effective and fast acting as synthetic pesticides, but are safer for the environment and consumers.