Biological Control of Western Flower Thrips, (Frankliniella occidentalis Thysanoptera: Thripidae: Frankliniella) in French Beans Using Plant and Soil Dwelling Mite

Abstract

Western Flower thrips (WFT), Frankliniella occidentalis pergande (Thysanoptera: Thripidae), is one of the most important pests damaging a wide range of economic important crops in protected and open cultures worldwide. Its cryptic life cycle, combined with a very short generation time and the ability to rapidly develop resistance against insecticides, are characteristics that make control of this pest extremely difficult. Moreover, commonly used natural enemies often do not lead to sufficient control levels. However, it has been reported that the life cycle of F. occidentalis includes a soil passage, which is still neglected in biocontrol strategies. Studies were conducted from 2017 to 2018 at Real IPM Company with aim to evaluate the efficacy of Plant Predatory and Soil swelling Mite for control of pupae stage of western flower thrips in French beans (Var. Samantha) crop. A laboratory study to examine the rate of consumption of WFT pupae in potting media, by the soil dwelling mites Hypoaspis sclerotarsa (Costa)(Soil dwelling mite) was designed. Five Hypoaspis sclerotarsa predator densities were evaluated (0, 2, 6, and 8) against five densities of WFT prey (5, 10, 10 15 and 20 pupae) Pupal consumption was assessed at 2 hourly intervals over a six –hour period. The study confirmed that H. sclerotarsa did consume WFT pupae and that the rate of consumption increased with increasing densities of H. sclerotarsa. However, this was not consistent because, as the numbers of WFT pupae increased, so did the ratio of WFT pupae remaining to those consumed, increase. To evaluate the effect of combining plant and soil dwelling mite, three separate greenhouse experiments were conducted: a) Aim to evaluate different densities of the mite Amblyseius Montdorensis (Foliar predator; AM at 0, 5, 10 or 15 per pot); b) To assess different densities of H. sclerotarsa (ground predator; HS at 0. 50, 100 or 150 per pot); c) To assess a combination of the two (0AM, 0HS; 15AM, 50HS; 15AM, 100HS; 15AM, 150HS) on emergence) of WFT from soil; initial start populations of WFT were either small (10) or large (20). A complete randomized design was used and for each experiment, there were three replicates per treatment, repeated twice over time. Single applications of A. Montdorensis, H. sclerotarsa or a combination of both all had an impact on the number of WFT emerging compared with the control. There was a significant effect of A. Montdorensis densities on the number of WFT emerging from the soil (F=0.31, P= 0.420 df =1). There was no significant difference in the population densities of WFT emerging from soil in the control and following release of H. sclerotarsa when initial release densities of WFT at the two initial prey densities of 10 and 20. Combined use of A. Montdorensis and H. sclerotarsa at a density of 150 with 15 A. Montdorensis reduced adult WFT emergence at density of 20 WFT, by 93.35%. To determine appropriate timing and frequency application of H. sclerotarsa, different densities of H. sclerotarsa (0,50,100 and 150, Foliar A. Montdorensis 15), were released in the soil media solely before pre-pupation and at pupation stage of WFT development. There was significant effect in mean number of emerging thrips from the soil when compared to control in all tested densities; of 0 (54.71%), 50(41.38%), 100(24.72%) and 150(5.83%), However H. sclerotarsa proved better when applied as a pre-pupation rather than as a post pupation treatment with emerging thrips number recorded when H. Sclerotarsa was applied at different densities; 0(86.9%), 50(50.83%), 100(29.16%) and 150 (7.75%). This is the first report on the potential of H. sclerotarsa as a biocontrol agent of WFT in Kenya. Over all, the study confirmed that both H. sclerotarsa and plant predatory mite can substantially reduce thrips population and might be important antagonists for F. occidentalis control in protected crops.