Seasonal variations in susceptibility of apis mellifera to the synergistic action of prochloraz and deltamethrin

Abstract

The synergistic action of prochloraz and deltamethrin was investigated in summer and winter bees. Prochloraz and deltamethrin were used at sublethat doses that did not induce any significant mortality: Bees were treated with different doses of deltamethrin, either alone or in combination with prochloraz, at the constant field rate of 25 g/ha. In summer bees, the combination of prochloraz and deltamethrin at 125 mg/ha triggered a synergy that produced approx. 63 ± 5% mortality (corrected) after 24 h. At 62.5 mg/ha, deltamethrin still acted in synergy with prochloraz by inducing about 32.5 ± 3.5% mortality (corrected) after 24 h. The field rate of 31.25 mg/ha was the lowest dose at which deltamethrin acted in synergy with prochloraz in summer bees. In winter bees, no synergy occurred between prochloraz and deltamethrin at doses of 125 and 250 mg/ha. The deltamethrin dose had to be increased to 500 mg/ha to observe a synergy that produced only 47 ± 11.7% mortality (corrected) after 24 h. Considering the deltamethrin doses at which synergy occurred, summer bees appeared to be approximately eightfold more susceptible than winter bees to the synergistic action of prochloraz and deltamethrin.