Seasonal incidence of major insect pests, diversity of their natural enemies and persistence of imidacloprid in boro rice
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Date
2022
Authors
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Journal ISSN
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Publisher
DRPCAU, PUSA
Abstract
Out of the several pests observed in the boro rice ecosystem at Pusa, the yellow
stem borer, leaf folder, whorl maggot and gundhi bug were observed to cause
maximum damage to the crop. The maximum incidence of dead heart and white ears
was seen during the 14th standard week of April and 20th standard week of May
respectively. Dead heart incidence had a significant negative correlation with rainfall,
whereas white ear head incidence was seen to have a significant positive correlation
with rainfall, minimum temperature and relative humidity. The leaf folder incidence
peaked during the 17th standard week of April with 62.93 per cent incidence and
represented a positive and significant relation with respect to rainfall and bright
sunshine hours. In case of whorl maggot, maximum incidence of 3.43% was recorded
during the 16th standard week of April and there was a negative but significant
relationship with minimum temperature as well as relative humidity in the evening.
Gundhi bug incidence reached its peak point (4.05%) at the 21st standard week of
May and showed a significant positive correlation with minimum temperature.
The sweep net and handpicking methods used to capture natural enemies
revealed that lady bird beetles, mirid bugs, dragonfly, damselfly, spiders and rove
beetles were the dominant ones during Rabi 2021-22 in the boro rice production
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system. Among them, lady bird beetles had the highest relative abundance (30.07%),
followed by spiders (16.70%), rove beetles (14.59%), damselfly (14.05%), dragonfly
(13.64%) and mirid bugs (10.93%). The number of lady bird beetles, dragonfly and
damselfly were observed to be the maximum during the months of March to April
whereas spiders, mirid bugs and rove beetles reached maximum numbers during May.
Representative samples of water after the first application, field soil and paddy
leaves after the second application whereas that of straw, grain and husk at the
harvesting period were collected and analyzed using modified QuEChERS technique.
In water, at 0 day (2hrs after application) after the first application, the mean initial
residue was 0.03 mg/kg at double dose (90 g a.i/ha) whereas at 45 g a.i./ha and 60 g
a.i/ha, the residues were below the LOQ (0.01 mg/kg) and 1 day after applying the
insecticide, the insecticide was undetectable in case of all the treatments. In soil
samples, maximum residues were found at 0 day (2hrs after second application) for all
the doses. The residues dissipated below the LOQ (0.05 mg/kg) after 15 days of the
second application in case of the recommended dose. On the other hand, for 1.5 times
the recommended dose and double dose, the amount of residues reached below the
LOQ (0.05 mg/kg) after 30 days. In case of paddy leaves, the maximum level of
residue was found after 3 days of the second application. After fifteen days of second
application, residues in leaves were found to move below the LOQ (0.05 mg/kg) for
45 g a.i/ha as well as 60 g a.i/ha. However, at the double dose (90 g a.i/ha) the
residues were found to be undetectable (<0.05 mg/kg) after 30 days of second
application. Residue level was found to be below the limit of quantification
(0.05mg/kg) in the harvest samples of grains, straw and husk. Hence, imidacloprid
0.3G wasn‟t found to cause issues of residual toxicity in the paddy production
ecosystem.