Eco-Friendly Management of Thrips Tabaci Lindeman (Thysanoptera: Thripidae) on Onion

Abstract

Onion occupies a prominent place among all the vegetables grown in Pakistan. Despite of high yield potential, its average yield is low in Pakistan due to severe attack of Thrips tabaci. In order to control Thrips tabaci in quick and short time, most farmers apply broad spectrum insecticides which have created health hazards and environmental pollution issues. This situation demands that efforts should be made to develop an alternative technonology for Thrips tabaci, which should has less health hazards and more environmental friendly. In this regards, a study comprised of cultural, biological and selective chemicals against Thrips tabaci management on onion was initiated. The cultural practices i.e. intercropping and plant spacing was evaluated in field plots. For biocontrol, phytoseiid mites Neoseiulus barkeri was evaluated against onion thrips in controlled conditions. While the efficacy of botanicals and synthetic selective insecticides were tested against onion thrips in laboratory and field crop. Four intercrops (chili, tomato, okra and cotton) were used in onion field crop and minimum thrips density (8.21±1.6 in 2011 and 10.72±1.94 in 2012) per plant was recorded in onion plots intercropped with cotton during both years of study. The maximum thrips density (26.19±5.67 in 2011and 33.15±6.73 in 2012) per plant was recorded in control plots where onion was planted as alone. Onion plots intercropped with cotton yielded the highest bulb weight and more yield/plot followed by tomato, chili and okra. In onion, five plant spacings (10, 15, 20, 25 and 30 cm) were used and minimum thrips density (14.85±3.16 in 2011 and 19.55±4.60 in 2012) per plant was recorded in onion plots in which plant to plant spacing was 30 cm followed by 25, 20, 15 and 10 cm during both experimental years. The thrips density was increased with decrease of plant to plant spacing. However, yield per unit area was significantly lower in maximum plant spacing (30 cm) but single bulb weight was higher as compared to lowest plant spacing (10 cm). Predatory efficacy of phytoseiid mite, Neoseiulus barkeri was evaluated by providing different stages of thrips larval instars as prey in combination (choice test) and separately (no choice test) in controlled condition. In choice test, non significant difference in total thrips consumption per day was observed. Whlie in no choice test, a significant difference in total thrips consumption per day was observed in each prey group. Predatory mite fed more number of thrips 1st larval instar as compared to 2nd larval instar and adult in both tests. viii During both observation hours (12-hrs or 24-hrs) a significant difference was also found on preference on immature and adult thrips. However, consumption rate was higher in initials hours of observation (12-hrs) and slow down later on (24-hrs), there may be reason of aggressive predation initially. The effectiveness of selected insecticides (Acephate® , Movento® and Radiant ® ) and botanicals (neem, datura and kortuma (bitter apple) to control Thrips tabaci was evaluated. Bioassay tests were performed using leaf dip and spray methods in the laboratory. During both bioassay methods, Acephate® caused 100% mortality of nymphs within 48-hrs of treatment. However, Movento® caused 100% mortality at 72-hrs of post treatment. The maximum mortality caused by Radiant® after 72-hrs of post treatment was 95.92±2.04%. The mortalities caused by neem, datura and kortuma at different time intervals were also significantly higher than control. Similar response was observed in adult thrips for all insecticides and botanicals Three botanical insecticides (neem, datura and kortuma or bitter apple) and three new chemistry synthetic insecticides (acephate, spirotetramat and spinetoram) against onion thrips was evaluated in experimental field plots. The influence of these botanical and chemical insecticides on natural predators and crop yield was simultaneously investigated. All the botanicals and chemical insecticides caused significant reductions (45-70%) in thrips population. The botanicals gave more than 60% control of thrips while among chemical insecticides, acephate was found to be the most effective followed by spirotetramat and spinetoram and these insecticides gave better control than the botanicals. The adverse effects of the botanicals on predator population were negligible compared to chemical insecticides. Study findings showed that onion intercropped with cotton having proper plant spacing should be practiced for reducing thrips infestation and obtaining suitable bulb size with optimum onion yield. All tested insecticides and botanicals were effective for the control of thrips. Therefore, it is recommended that treatment with botanical pesticides or chemical insecticides should be used to conserve biological control agents and to keep thrips population below economic injury levels until crop maturity.