Studies on the biology, population density, distribution pattern and management of Thrips palmi Karny (Thysanoptera: Thripidae) on watermelon, Citrullus lanatus (Thunberg)

The biology, population density, distribution pattern and management of Thrips palmi Karny were studied on watermelon from 1998 to 2000. The average incubation period was 6 days for all three host plants (watermelon, eggplant and cucumber). Developmental period of larvae and pre-pupae/pupae ranged from 4-5 days on all host plants. Amaranth leaves did not support larval growth and development. Adult thrips lived for 11.11 days on the average when reared on cucumber and 7.57 days on eggplant. Total developmental period (egg-adult) was significantly longer on cucumber (18.90 days) than on eggplant (15.24 days). Thrips reared on watermelon had a total developmental period of 17.36 days. There were more females than males on all the host plants used, the sex ratios (male:female) being 1:3.15 (watermelon), 1:2.24 (cucumber) and 1:1.90 (eggplant). Survival ranged from 72 to 83% for all the three host plants. In field sampling for thrips, the Berlese funnel method had consistently high population estimates throughout the growing season for watermelon. It is the best among the three, having the highest number of population counts per leaf (17.13) compared to bagging (3.03) method. The ocular method (12.62) lies intermediate but it is the fastest and most practical method. Three plant species were found positive for T. palmi in the field, namely, eggplant, okra and amaranth. Contrary to earlier reports, corn and peanut growing near watermelon field were negative for thrips. Thrips populations were high during the vegetative and reproductive stages of watermelon. At seedling stage, thrips were significantly low throughout the cropping season. T. palmi distribution varied according to leaf location and sowing period. In the first sowing, thrips populations were evenly distributed on all the leaves within the plant. The middle leaf harbored significantly high number of thrips. Natural enemies, temperature and thrips density appear to have significant correlation at the early stage of the cropping season (first sowing). Percent relative humidity showed negative correlation during the second sowing period. On the third sowing, the abiotic (temperature, rainfall and relative humidity) and biotic (natural enemies) factors did not show significant correlation with thrips density. Artificially infested watermelon plants had significantly shorter vines than the healthy (uninfested plants). Average leaf sizes of top and middle leaves were similar regardless of the number of thrips found on it. The basal leaf sizes were significantly reduced. Thrips populations on biocontrol plots were significantly lower than on the chemically sprayed and mulched plots. The fruit weight was significantly affected by three management tactics used but not quantity. The highest total yield (kg/ha) was obtained using biocontrol. High additional net benefit was achieved using biocontrol, followed by cultural and chemical