A long-term field study on the effects of dietary exposure of clothianidin to varroosis-weakened honey bee colonies

Abstract

Clothianidin is a commonly used systemic insecticide in seed treatments. Residues of clothianidin can occur in nectar and pollen as a result of within-plant-translocation. Foraging bees can collect contaminated nectar or pollen. Concerns have been brought forward that exposure to pesticide residues might affect colonies especially if they are weakened by varroosis. However, there are few scientific studies investigating such multiple-stressor scenarios in the context of the entire colony. To close this gapa field trial with 24 colonies was set up. The study design comprised four groups of six colonies each fed with uncontaminated sugar syrup ('C0'), or syrup spiked with 10 μg L−1 clothianidin ('C10'), 50 μg L−1 clothianidin ('C50') or 200 μg L−1 clothianidin ('C200'). C10 represented a residue concentration that may exceptionally occur and therefore a worst-case scenario, the higher dietary concentrations exceed and do not reflect fieldrealistic levels. A substantial load of 8 mites of Varroa destructor per ten gram bees in autumn was adjusted. The colonies were followed up for 328 days. The amount of brood and the strength of each colony were regularly assessed. Colony health, bee mortality, overwintering success, hive weights, and levels of in-hive residues were determined. Varroosis turned out to be the significant key factor for the endpoint colony strength. Clothianidin did not have a statistically significant impact on C0, C10 and C50 colonies. No statistical evidence was found for an interaction between varroosis andexposure to clothianidin.

Field-level clothianidin exposure affects bumblebees but generally not their pathogens

“…The surviving honeybee colonies were placed for an additional season by the maize fields of the same treatment (control/clothianidin or thiamethoxam) and Varroa abundance was again higher in colonies placed by neonicotinoid-treated fields with low levels of neonicotinoid exposure confirmed for apiaries at treated sites and for one of two control apiaries 59 . Experimentally induced exposure of honeybees to both Varroa and clothianidin spiked syrup showed no interactive effect between the two pressures 31 . In another study, long-lasting in-hive feeding of thiacloprid to honeybee colonies did not affect colony performance or the levels of parasites, pathogens and expressed immunity-related genes 30 .…”

Clothianidin seed-treatment has no detectable negative impact on honeybee colonies and their pathogens

“…Jones et al 30 demonstrated that although the gut microbial composition is influenced by the surrounding landscape, the relationship between environmental stressors, the gut symbiotic bacteria and its host is too complex and interactive for simple reductionist assessments. Two other field studies also found no effect of neonicotinoids on Varroa abundance 41,51 , while a third found a positive association 52 . In addition, in-hive feeding with imidacloprid-spiked pollen increased Varroa abundance but only at a very high imidacloprid concentration 49 .…”

Effect of Sub-lethal Doses of Imidacloprid on Learning and Memory Formation of Indigenous Arabian Bee (Apis mellifera jemenitica Ruttner) Adult Foragers

“…In certain field studies, realistic exposure levels to insecticides do not harm the bees at the colony level. But, when investigated at individual level, insecticide-induced effect can be observed (Blacquiere et al 2012, Fischer et al 2014, Lundin et al 2015, Retschnig et al 2015, Odemer et al 2018, Siede et al 2018, Rolke et al 2016. Social resilience or buffering obviously plays a crucial role at colony level which is not possible at individual level and these mechanisms are not yet fully understood (Manjon et al 2018, Straub et al 2015.…”