Evaluation of Various French bean Cultivars for their Susceptibility to Thrips and the Effect of Nitrogen Fertilizer and Natural Enemies on Population Dynamics of Thrips

Abstract

Western flower thrips; Frankliniella occidentalis (Pergande) and Megalurothripssjostedti (Trybom) (Thysanoptera: Thripidae) are among the most important pests ofFrench beans, (Phaseolus vulgaris) in Kenya. Attack by thrips leads to flowermalformation, distortion, and discoloration while in severe infestations, flower budsdo not open and may abort prematurely, hence, the need for urgent and effectivealternative intervention strategies. The study was aimed at evaluating various Frenchbean cultivars for their susceptibility to thrips and the effect of nitrogen fertilizer andnatural enemies on population dynamics of thrips and hence come up withinnovative integrated pest management (IPM) strategies for management of thrips inFrench beans. This was done through a study of changes in the population of flowerthrips based on different French bean cultivars: Paulista, Army Star, Julia, Samanthaand Alexandra, grown under varied top dressing nitrogenous fertilizer regimes. Thiswas done between August 2007 and January 2008 in, JKUAT farm and KARIMwea. The experiment was laid in a Randomized Block Design with two treatmentsand four replications. Ten flowers and leaves were sampled at random from each ofthe experimental plots and taken to the laboratory for counting of thrips and Naturalenemies. There was significant difference in the mean number of both F.occidentalis and M. sjostedti population among the six French bean cultivars.Results from this study demonstrated that some French bean cultivars were moreresistant to flower thrips. Julia had the highest overall mean while Paulista had theleast number of thrips. This suggests that the most susceptible variety among the sixcultivars was Julia and Paulista was the least susceptible both species of thrips viz:F. occidentalis and M. sjostedti. Assessment of Trichome densities per square cm foreach cultivar showed that Paulista had a higher density while Julia had a lowerdensity. This could be a reason for thrips resistance in Paulista. This studyconcluded that resistance to thrips is cultivar dependant in French bean c. Furtherstudies are, however, recommended on assessment of yield effect by quantificationof damage on the pods, since a particular cultivar could have high thrips count butless pod damage, an indication that the cultivar might be thrips tolerant thoughsusceptible. An evaluation of trichome on these cultivars is also recommended toidentify the types and their phytochemical components and their effect on thrips.Orius spp. were the only natural enemies encountered, and had no effect onpopulation of thrips. Increasing the level of nitrogen fertilizer on French beans didnot affect the abundance of thrips in each of the varieties and therefore, the differentFertilizer levels had no effect on the overall number of thrips