Seasonal incidence of onion thrips on onion during rabi and kharif season in Punjab conditions
The first incidence of thrips was started from 8th standard meteorological week (SMW) in mid- December transplanted onion crop during rabi season (Fig. 1).
Average adult thrips population was ranged from 1.69 to 17.36 per plant whereas average nymph population was ranged from 6.36 to 45.49 per plant. Peak population (17.36 adult thrips per plants) was attained at 13th SMW whereas peak population of nymphs (45.49 nymphs per plant) was attained at 15th SMW. Thrips population gradually increased in month of March and suddenly declined in April month which may be due to leaf hardening and crop attains maturity stage. During kharif season (Fig. 2), the incidence of thrips commenced in the first week of September (3.09 adult thrips/ plant) which gradually increased and reached to peak in the first week of October with 37.79 nymphs and 11.03 adult thrips per plant. Thereafter, immature and adult thrips population declined and completely disappeared in the first week of December. Overall, onion thrips load was higher during 39th (27.33 nymphs/plant), 40th (37.79 nymphs/plant), 41th (34.39 nymphs/ plant) and 42th (28.26 nymphs/plant) SMW. The 48th SMW of November month registered least number of immature (9.09 nymphs/plant) and adult thrips per plant (2.76 adult thrips per plant). During early September and late November, only moderate level of pest incidence was recorded, while maximum immature and adult thrips infestation coincided during September and October months.
Weather factors during rabi and kharif season 2021-22
The temperature showed a sequential increase up to time of harvesting in rabi season (Fig. 3). The range of maximum temperature was 10.8-35.6oC, minimum 3.5-20.0oC and the average 8.6-26.0oC. In addition, total average of temperature was 23.82, 10.75, and 17.26.0oC for maximum, minimum and average temperature, respectively. However, relative humidity was found to be in a decreasing trend up to time of harvesting. It ranged between 27 to 96 per cent RH with a substantial fluctuation. Total average relative humidity was 68.05 per cent RH. Wind velocity was ranged 1.4 to 7.0 km/hr with total average of 3.71 km/hr. Sunshine hours showed gradual increase with time that ranged between 0 to 11.7 hrs with a total average of 6.05 hrs. However, rainfall was in negligible amount during rabi season. The temperature showed a sequential fall to time of harvesting during kharif season (Fig. 4). The range of maximum temperature was (24.4 to 35.6oC), minimum (7.6 to 27.2oC) and the average (16.9 to 31.4oC). In addition, total average of temperature was 30.5, 19.6, and 25.07oC for maximum, minimum and average temperature, respectively. However, average relative humidity was found to be almost stable throughout the crop season. Relative humidity ranged between 51 to 81 per cent RH with a substantial fluctuation. Total average relative humidity was 67.35 per cent RH. Wind velocity was ranged 1.4 to 7.3 km/hr with total average of 2.78 km/hr. Sunshine hours showed stability throughout crop season that ranged between 1.9 to 10.8 hrs with a total average of 7.05 hrs. However, rainfall was in negligible amount during kharif season.
Correlation And Regression Analysis
Population dynamics of T. tabaci on onion in relation to weather factor was clearly shown that immature and adult T. tabaci incidence with temperature, wind velocity, sunshine hours was positively correlated in mid-December transplanted crop during rabi season (Table. 1). Moreover, Relative humidity had shown negatively correlated with T. tabaci incidence. The minimum and maximum mean temperature was significantly positively correlated with immature (r = 0.637** min, 0.848** max) and adult T. tabaci incidence (r = 0.483** min, 0.759** max). The regression equations for immature T.tabaci at maximum temperature and minimum temperature being: [y = 1.80x-30.55 (R2 = 0.72) (Fig. 5)] and [y = 2.27x-12.04 (R2 = 0.40) (Fig. 6)].
Table 1
Simple correlation and regression between weather parameters and immature and adult thrips population during rabi season 2020-21
S. No. | Weather parameters | Adult | Nymph |
Intercept (a) | Slope (b) | ‘r’ value | Intercept (a) | Slope (b) | ‘r’ value |
1 | Maximum temperature | -10.173 | 0.609 | 0.759** | -30.557 | 1.802 | 0.848** |
2 | Minimum temperature | -2.649 | 0.650 | 0.483** | -12.046 | 2.271 | 0.637** |
3 | Morning relative humidity | 20.314 | -0.189 | -0.551** | 73.990 | -0.731 | -0.803** |
4 | Evening relative humidity | 11.443 | -0.150 | -0.616** | 33.295 | -0.442 | -0.686** |
5 | Wind velocity | 1.488 | 0.768 | 0.201 | 2.323 | 2.705 | 0.268 |
6 | Sunshine hours | -1.332 | 0.937 | 0.646** | -3.027 | -2.543 | 0.663** |
7 | Rainfall | - | - | - | - | - | - |
**(Significant) at 0.05% (r = correlation coefficient) |
From above the equations, it may be expressed that with every unit increase in maximum temperature and minimum temperature there was positively increase of 1.80 and 2.27 immature thrips per plant, respectively. Immature and adult thrips population and relative humidity were highly significantly negatively correlated during rabi season with (r= -0.686** evening, -0.803** morning), (r= -0.616** evening, -0.551** morning) respectively. Immature and adult thrips population and wind velocity was non-significantly postively correlated during rabi season (r = 0.268, r = 0.201) whereas immature and adult thrips population shows highly significant positive correlation with sunshine hours (r = 0.663**, r = 0.646**). The regression equation for sunshine hours being: [Y = 2.54x-3.027 (R2 = 0.44) (Fig. 7)]. From above equation, it may be expressed that with every unit increase in sunshine hours there was positive increase in 2.54 immature T.tabaci per plant.
Correlation studies for kharif season (Table 2) between immature, adult thrips population and weather parameter revealed that thrips population had a non-significant positive correlation with temperature [maximum (r = 0.050 for adults, r = 0.049 for nymphs), minimum (r = 0.022 for adults, r = 0.025 for nymphs)]. Non- significant negative correlation was found between thrips population and morning relative humidity (r= -0.173 for adults, r= -0.215 for nymphs) and evening relative humidity (r= -0.135 for adults, r= -0.165 for nymphs). This indicates that activity of thrips population decrease with the rise of morning and evening relative humidity. Immature and adult thrips population also showed non-significant negative correlation with wind velocity (r= -0.187 for adults, r= -0.081 for nymphs) and sunshine hours (r= -0.165 for adults, r= -0.034 for nymphs) during kharif season 2021-22. This suggests that population of thrips falls with increase in wind velocity. Occurrence of rainfall was negligible so no effect of rainfall was observed on pest population during period of study. Immature nymph and adult thrips showed non-significant negative correlation with the sunshine hours. This states that during kharif season of crop growth, the population of thrips decreases with increase in sunshine hours.
Table 2
Simple correlation and regression between weather parameters and immature and adult thrips population during kharif season 2020-21
S. No | Weather parameters | Adult | Nymph |
Intercept (a) | Slope (b) | ‘r’ value | Intercept (a) | Slope (b) | ‘r’ value |
1 | Maximum temperature | 3.035 | 0.038 | 0.050 | 11.109 | 0.145 | 0.049 |
2 | Minimum temperature | 4.022 | 0.009 | 0.022 | 14.794 | 0.039 | 0.025 |
3 | Morning relative humidity | 9.789 | -0.064 | -0.173 | 42.306 | -0.307 | -0.215 |
4 | Evening relative humidity | 5.458 | -0.026 | -0.135 | 21.522 | -0.124 | -0.165 |
5 | Wind velocity | 5.367 | -0.165 | -0.187 | 17.446 | -0.674 | -0.081 |
6 | Sunshine hours | 5.312 | -0.399 | -0.165 | 16.510 | -0.133 | -0.034 |
7 | Rainfall | - | - | - | | | - |
(r = correlation coefficient) |