Impact of thiamethoxam on Apis mellifera Linnaeus in mustard
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Date
2019
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Punjab Agricultural University, Ludhiana
Abstract
Bees, the important pollinators of oilseed brassica crops, are affected by the usage of thiamethoxam against sucking pests in mustard. Hence, studies were carried out to assess its impact on Apis melliefra Linnaeus. Laboratory studies carried out at Punjab Agricultural University, Ludhiana showed the median lethal values (LD50 dermal & oral) at 24 h to be 7.63 & 5.49 for A. mellifera and 5.45 & 1.98 ng bee-1 for Apis cerana cerana Fab., respectively. Hence, as per Environmental Protection Agency (EPA), thiamethoxam was highly toxic to hive bees. Hazard Quotient (HQ) categorization end point values were >50 which necessitated the conduct of higher tier studies. Chronically fed thiamethoxam to newly emerged bees at the highest consumption (1.09 ng bee-1 day-1) reduced the acini area by more than 50 per cent. Higher tier studies i.e. field trials conducted by applying thiamethoxam 25WG (Actara®) on Gobhi Sarson (GSC 7) revealed that the development of brood marked after 7 days of application of thiamethoxam @ 25 g a.i. ha-1 was unaffected. In contrary to this, brood marked at the day of application of thiamethoxam (@ 25 and 50 g a.i. ha-1) or on 7th day of application of thiamethoxam (@ 50 g a.i. ha-1) was adversely affected. Field collected bees on 3rd day of application from both the thiamethoxam treatments did not show PER response, however it improved with the passage of time and on 15th day of application, the response was the significantly higher than the values recorded at other days. At the recommended dose of thiamethoxam, mortality was very high (134.81-331.88 bees) for the first three days and decreased thereafter, to be statistically at par with untreated control on 8th day of application. This also affected the mean bee strength and unripe honey hoarding in a colony, those also started increasing on 7th day of application (6.95 bee-frames & 2787.41 g) and became significantly higher than pre-treatment on 14th day of application (8.19 bee-frames & 3780.96 g). The mean ripe honey hoarding did not decrease below pre-treatment. The mean unsealed worker brood also showed similar trend i.e. reduction upto 7th day of application (612.09 cm2) and statistically at par with pre-treatment on 14th day of application (688.20 cm2) but the pollen hoarding decreased upto 14 days (290.30 cm2) and on 21st day of application (372.79 cm2) became significantly more than pre-treatment. The initial residues (mg kg-1) recorded in dead bees (0.45), bee foraged nectar (0.02), bee foraged pollen (0.07) and bee bread (0.07) dissipated below LOQ (0.01 mg kg-1) at D5 , D3, D5 and D7 after application, respectively. Consequently bees' activity recorded at colony entrance and on crop blooms too decreased. The effect of double the recommended dose was more pronounced. Overall, it can be concluded that thiamethoxam had negative effect on A. mellifera colonies. Hence, farmers must not apply thiamethoxam on mustard crop during peak activity of bees.
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