SYNERGISM BETWEEN EBI FUNGICIDES AND A PYRETHROID INSECTICIDE IN THE HONEYBEE (APIS-MELLIFERA)

The synergistic effect of a range of ergosterol-biosynthesis-inhibitin g (EBI) fungicides and a pyrethroid insecticide was studied in the hon eybee (Apis mellifera L.). Various EBI fungicides were combined separa tely with the pyrethroid lambda-cyhalothrin at ratios derived from the ir recommended application rates to represent tank-mixing in the field . The mixture was then applied topically to the thorax of honeybees, a nd mortality assessed 24 h post-treatment. All the fungicides tested i ncreased the toxicity of lambda-cyhalothrin to honeybees. The fungicid e propiconazole was found to have the strongest synergistic effect, de creasing the LD50 of lambda-cyhalothrin from 68.0 ng been to 4.2 ng, t hus having a synergistic ratio of 16.2. Hazard ratios were calculated for lambda-cyhalothrin and fungicide mixtures using a recommended appl ication rate of 7.5 g a.i. ha-1. The hazard ratio for lambda-cyhalothr in alone was 110, but when mixed with fungicide synergists, the hazard ratio ranged from 366 with flutriafol to 1786 with propiconazole. A b lank formulation of a fungicide (without the active ingredient prochlo raz) had little effect on the toxicity of lambda-cyhalothrin, indicati ng that it is primarily the fungicide active ingredient that is respon sible for the synergistic effect. The results are discussed in terms o f the potential hazard posed by pesticide synergism to honeybees in th e field.