Author
N. MANIKANDAN
School of Post Graduate Studies
Tamil Nadu Agricultural University
Coimbatore – 641 003.
2013
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Abstract
Experiments were undertaken to study the behaviour of the major rice pests under different temperature regimes and corresponding thermal accumulation for growth and development, and to develop a weather based forewarning model to forecast the pest population at Agro Climate Research Centre, Tamil Nadu Agricultural University, Coimbatore during 2011–13.
Experiments were carried out in Climate Control Chamber under five different constant temperatures (28.3°C, 30.6°C, 32.7°C, 34.3°C and 36°C) with major insects such as Yellow Stem Borer (YSB), Brown Planthopper (BPH) and Rice Leaffolder (RLF). Observations on demographic parameters of insects and thermal requirement were recorded at different temperature regimes.
Apart from this, data on weekly light trap catches of insects were collected from progress report of Directorate of Rice Research, Hyderabad for Cauvery Delta Zone (Aduthurai), and a weather based forewarning model had been developed.
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Results
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The number of eggs laid by YSB and RLF increased with increasing temperatures.
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In case of BPH, the number of eggs decreased with increasing temperatures.
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Hatching of eggs decreased with increasing temperatures irrespective of insect pests.
The total lifespan of:
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YSB extended to a maximum of 52 days at 28.3°C, whereas it was only 38 days at 36°C.
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BPH took 44 days to complete the generation at 28.3°C and as short as 32 days at 36°C.
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RLF took 36 days to complete the generation at 28.3°C, whereas it took only 27 days at 36°C.
The age-specific survivorship decreased with the progress of age at all the temperatures irrespective of the insect pests. The 50% mortality was observed to occur earlier at higher temperature regimes than at lower temperature regimes for all the insects.
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The survival fraction remained higher for all the stages at lower temperature regime of 28.3°C and drastically reduced at higher temperature regime of 34.3°C.
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The apparent mortality and mortality survivor ratio increased with increasing temperatures.
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Indispensable mortality decreased with increasing temperatures.
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YSB, BPH and RLF tend to be ‘K’ favoring at higher temperature from being ‘r’ favoring at lower temperature.
All the population growth parameters (finite rate of increase, intrinsic rate of natural increase and doubling time) increased with increasing temperatures. However, the increase had a turnaround after a threshold temperature (34.3°C), and hence, it was reduced at 36.0°C.
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Thermal Requirement
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YSB egg, larva and pupa required 136.3, 557.8 and 156.3 degree days, respectively.
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BPH eggs required 123.3 degree days to hatch into nymph, and the nymphal stage required 269 degree days to become adult.
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RLF eggs and larval stage required 98.1 and 344 degree days, respectively.
Weather Relationships
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Temperature (maximum and minimum of current, one and two weeks before) had a significant negative relationship with the number of light trap catches of all the insects.
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Relative humidity had a significant positive relationship.
Weather based pest forewarning models for YSB, BPH and RLF were developed by using Generalized Linear Model.
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Conclusion
From the above experiments, it is concluded that the temperature above the threshold of 32.7°C is detrimental to the growth and development of these insects.
The timing of control measures like chemical spray to control these insects could be planned based on the thermal accumulation (degree days) of the insects, which will reduce the cost of spray and improve the efficiency.
Weather based forewarning models developed for each insect could be used to forewarn the pest population at particular time, which will be useful in planning cost-effective control strategies.
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