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.

Keywords: Clothianidin; Honey bee; Interaction; Varroosis.

Fig. 1

Varroa destructor infestation rate as a function of time and the exposure to clothianidin. error bars: + 1 SD; For group C200 there is only one data point at −2 DAI as these colonies dwindled rapidly. CLO = clothianidin

Fig. 2

Numbers of adult bees per colony (colony strength) assessed according to the Liebefeld method as a function of time and the exposure to clothianidin; error bars: + 1 standard deviation (SD). At DAI 0 the numbers of bees were converted from the bee weight of the shook swarms with the assumption that 1 kg bees corresponds to 10,000 bees. During the period of DAI 68 to DAI 320 there was no significant difference between the groups C0, C10, and C50 (p = 0.708). CLO = clothianidin

Fig. 3

Number of brood cells per colony as a function of time and the exposure to clothianidin; error bars: + 1 SD. Treatment groups C10 and C50 were not significantly different from group C0 (p = 0.593). CLO = clothianidin

Fig. 4

Net weight of the colonies as a function of time and the exposure to clothianidin; error bars: + 1 SD, Weights were corrected for addition or removal of the weights of hive material. CLO = clothianidin