Acute and chronic effects of spinosad on bumble bees, Bombus terrestris L. under laboratory conditions

Abstract

Under laboratory conditions, the acute toxicity of spinosad on adult workers of bumble bee Bombus terrestris L.was investigated through 96 hrs post-treatment by using three different exposure methods; orally, wet and dry contact. The results indicated that within 24 hrs, the 1/1 and 1/10 of maximum field recommended concentration (MFRC) of spinosad caused 100 % and 12-65 % mortality, respectively the three methods. While, the mortality in the control insects did not exceed 5 %. The highest LC50 value was detected in dry contact method at 6 hrs, the LC50=1046.15 ppm (2.62 MFRC). However, the lowest one in oral method was detected after 96 hrs, the LC50 values was 5.03 ppm (0.01 MFRC). At concentration 0.01 MFRC with dry contact,LT50 was 73 days but in the oral method had 14 days. The chronic effects of 1/100, 1/1000, 1/10000 of MFRC using the same three exposure methods were evaluated. The 1/100 of MFRC in oral method caused significant mortality in worker through 8 weeks. Moreover, reduction in survival of workers, drones produced, male delay emergency, and numbers of dead larvae. However, no significant differences were found between the control and the other treatment 1/1000 and 1/10000 MFRC using oral assay. In dry contact method, the concentrations from 1/1000 to 1/10 of MFRC spinosad had no negative effects. In wet contact method the concentrations 1/100 and 1/1000 had no negative effect, but the concentration 1/10 caused decreasing survival workers. Our results suggested that spinosad was highly toxic in wet contact method, although more safe in dry contact method to bumble bees under worse case laboratory condition with MFRC.