The relationship between pellet size and the quantity of imidacloprid applied to sugar beet pellets and the consequences for seedling emergence

Abstract

When imidacloprid was applied to sugar beet pellets at 90 g a.i. per unit of 100,000 seeds, the quantity of imidacloprid applied to individual sugar beet pellets was directly proportional to the size of pellet, resulting in lower dosing of some pellets and high dosing of others. This variability in application resulted in a slowing of germination in a laboratory experiment but had no effect on the rate of emergence in a field trial of young seedlings produced from large pellets compared with that from smaller pellets. However, there was a significant reduction in total number of seedlings emerging from large imidacloprid-treated pellets in a spring-sown field trial suggesting that additional phytotoxicity had occurred. Uptake of imidacloprid in leaf tissue was also greater in plants produced from large pellets. In a field trial at Broom's Barn, sown in the autumn, imidacloprid at 30, 60, 90 and 120 g a.i. per unit significantly slowed the rate of emergence of three varieties of sugar beet but there was no effect on pellet size. However, in contrast to the earlier trial, small pellets of two of the three varieties produced significantly fewer seedlings than large pellets, irrespective of the rate of imidacloprid applied, and there were no interactions between pellet size and rate of imidacloprid applied. Similar results were observed in another field trial in Norfolk where small pellets of one of the varieties, untreated, or treated with 30, 60 and 90 g a.i. of imidacloprid, but not 120 g a.i., produced significantly fewer seedlings. Small pellets of another variety produced the highest number of seedlings. Thus the variability in application of imidacloprid to sugar beet pellets can sometimes cause increased phytotoxicity to plants produced from large pellets, but this effect was not consistent with different varieties and different batches of the same variety.