Assessment of risks to honey bees posed by guttation

Abstract

Background: Besides their nectar and pollen collecting activities, honey bees also forage water. Guttation droplets may be used as a water source. Measurements of high residue levels of some intrinsically highly toxic, systemic insecticides in guttation droplets triggered research activities on the potential risk for honey bees. Since 2009, a large number of studies have been conducted on the environmental conditions and factors favoring guttation, foraging of guttation, the occurrence of guttation in different crops, the frequency of guttation events and residue measurements in guttation droplets in different crops, at different growth stages and with different active ingredients. Different approaches of laboratory, semi-field and field studies were set up to address the potential risk of guttation to bees and to gain clarification whether and how this concern would need to be specifically addressed in the risk assessment for bees. Results: Occasionally increased mortalities of worker bees were reported from single events in some trials, when colonies were placed directly next to the sown maize crop treated with a systemic insecticide. However, there were no long-term colony effects (e.g. on colony strength and brood development) reported from any of the realistic worst case exposure trials conducted by either public research institutes or industry. Conclusion: The potential risk for bees is in the first instance dependent on the distance of the colonies to treated crops. Maize is considered as the worst case crop in terms of frequency, duration and intensity of guttation and of residue level of compounds found in guttation liquid. Though increased worker bee mortality on individual days was seen in some of the field studies where hives were placed directly at guttating maize fields, adverse effects to colony vitality, colony and brood development were never observed. Keywords: Guttation, risk assessment, pesticides, honey bees.