Acute toxicity of six flower/fruit thinners to Apis mellifera, Bombus terrestris (Hymenoptera: Apidae) and Osmia cornifrons (Hymenoptera: Megachilidae) in laboratory bioassays

Pollinators play a crucial role in the crop production. As the flower and fruit thinning is laborious and costly, dependency on the chemical flower/fruit thinning agents is increasing. At the same time, use of commercially produced buff-tailed bumblebee, Bombus terrestris in apple is increasing. This study was conduct to measure toxicity of chemical flower/fruit thinner agents (lime sulfur, ammonium thiosulphate, carbaryl, naphthalene acetamide, naphthalene acetic acid, and 6-benzyladenine + gibberellins) against bumblebee in laboratory bioassay. We assessed acute spray contact, topical contact, and acute oral toxicity. Spray Contact toxicity when applied in recommended concentration was not found. However, carbaryl applied in ten times of the recommended concentration showed 43% mortality. LC₅₀ values for carbaryl were 251 ug/g bee 48 hours after treatment. No mortality was observed in topical contact treatment as in the spray contact. However, carbaryl applied in hundreds times of the recommended concentration showed 46% mortality. In oral toxicity test with the recommended concentration carbaryl and lime sulfur showed 60% and 76% mortality, respectively. But no mortality was observed in ammonium thiosulphate, naphthalene acetic acid, and 6-benzyladenine + gibberellins, and naphthalene acetamide treatment. LC₅₀ values for carbaryl and lime sulfur were 543 and 1,631 ppm 48 hours after treatment. LD₅₀ values for carbaryl and lime sulfur were estimated 0.37 ug a.i./bee and 2.74 ug a.i./bee, respectively. Highest susceptibility was found in oral test. LC₅₀ values were 251 ug/g bee and 543 ppm for carbaryl spray and for oral 48 hours after treatment respectively. Similarly, LC₅₀ values for lime sulfur were oral 1,631 ppm. Least feeding was observed in lime sulfur. Ammonium thiosulphate, naphthalene acetamide, naphthalene acetic acid, and 6 benzyladenine + gibberellins were low susceptibility against B. terrestris, but Lime sulfur and carbaryl were high susceptibility against B. terrestris in laboratory test. Therefore, ammonium thisulphate, naphthalene acetamide, naphthalene acetic acid, 6 benzyladenine + gibberellins against flower/fruit thinner will use more safety for B. terrestris in apple orchard. Further intensive examination studiesare demanded.

Keywords: Apis mellifera, Bombus terristris, Osmia cornifrons, flower thinners, fruit thinners, acute toxicity