Assessment of the synergy and repellency of pyrethroid/fungicide mixtures

Abstract

There has been considerable concern over the last few years about the potential for synergism between pyrethroids and fungicides applied to flowering oilseed rape. Field data from honey bee (Apis mellifera L.) poisoning incidents in the UK have shown that mixtures of pyrethroids with fungicides that are not synergistic under laboratory conditions have been involved. This study aimed to determine if the mixing of fungicides with pyrethroids alters the repellent properties of the pyrethroid and thus increases the risk to honey bees when applied to flowering crops. Interpretation and regulation may be difficult if it is unclear whether synergism or a change in repellency is responsible for the incidents. Synergy and repellency of realistic combinations of two pyrethroids and eight fungicides were tested. Synergy was tested using a standard acute toxicity test method and repellency was tested using a novel in vitro test method. The results showed that in vitro, certain combinations of pyrethroids and fungicides did significantly increase the risk posed by pyrethroids alone, due to a reduction in the repellency of the pyrethroid. Further work is now in progress to see if this reduction in repellency seen in the laboratory occurs under semi-field conditions.