Lethal effects of various tank mixtures including insecticides, fungicides and fertilizers on honey bees under laboratory, semi-field and field conditions

In agricultural crops, honey bees may be exposed to multiple pesticides. However, in field realistic conditions mixtures of products classified as harmless to bees can lead to a synergistic increase of toxicity on honey bees, as known for ergosterol biosynthesis-inhibiting (EBI) fungicides combined with pyrethroids or neonicotinoids. For substances like fertilizers, usually no information on potential side effects on bees is available. Initially, effects from tank mixes containing insecticides, fungicides and fertilizers were investigated under laboratory conditions by use of a spray chamber contact test. For selected combinations, higher tier studies were carried out. Mixtures containing thiacloprid, boscalid and dimoxystrobin with boron fertilizers showed no increase of mortality. In contrast, tank mixes of thiacloprid formulations and EBI-fungicides resulted within 24 h in synergistic mortality increase in laboratory, semi-field and field. Actually, a short time interval of 24 h between individually applied potentially synergistic products like pyrethroid insecticides and EBI fungicides led to an increase of mortality up to 100% under laboratory conditions, indicating that the detoxification was still ongoing, resulting in an increased susceptibility for other stressors during the metabolization process. In conclusion, tank mixtures do not always lead to an increase of honey bee toxicity. However, former published findings about synergistic impacts between neonicotinoides and EBI-fungicides could be confirmed. The findings of our and other higher tier studies finally prompted the competent regulatory authority (BVL) to regulate these mixtures by restriction NB6613 since September 2018. Nevertheless, the investigation of potential interactions between mixing partners and their additive or synergistic effects are the basis for a better understanding and a logical risk assessment to ensure protection of honey bees.