INFLUENCE OF TEMPERATURE ON HONEY BEE SUSCEPTIBILITY TO PESTICIDES

EPPO (2010 EPPO Bulletin 40(3): 313-319) and OECD (1998) are the European official guidelines that describe how to conduct trials for the evaluation of side-effects of plant protection products on honey bees. According to these guidelines, acute oral toxicity tests on adult honey bees should be carried out at 25±2°C. In nature, adult forager bees may be exposed to a wide range of temperatures: from about 15°C (when foragers fly in spring) to 35°C (brood nest temperature) or even more (outside in hot climates). Since forager bees may also be exposed to pesticides, the purpose of this work was to investigate the influence of temperature on the susceptibility of forager bees to these substances. Exiting forager bees from healthy and queen-right colonies were collected. Subsequently, acute oral LD50 tests were carried out at three different temperatures: 25±0.5, 30±0.5 and 35±0.5°C. Three active ingredients (fipronil, clothianidin and thiamethoxam) were separately tested. Following the European official guidelines (EPPO and OECD), five different doses of each a.i. and a control were provided via bulk administration in 10μL 50% w/w of sucrose solution per bee. Three or four replicates in different seasons were carried out. Mortality at 24 hours was assessed and LD50 with confidence intervals were calculated (Probit analysis, Polo LeOra software). The results show that the LD50 value depends on the test temperature. This relationship was confirmed statistically in all the replicates of fipronil and thiamethoxam and in 2 of 4 replicates of clothianidin. Furthermore, different substance groups have different LD50 trends in relation to the temperature. In fact, with the increase of the temperature A) the toxicity of fipronil (phenylpyrazole) increases, while B) the toxicities of clothianidin and thiamethoxam (neonicotinoids) decrease. To conclude, the toxicity of pesticides to forager bees is influenced by the temperature which the bees are exposed to. Interestingly, the strength and sign of this correlation depend on the characteristics of the a.i./substance group. The European official guidelines used in the pesticide registration process (EPPO, OECD) allow to perform toxicity tests at a single temperature within 23-27°C: this wide range gives the interested parts the opportunity to carry out the tests at the temperature that will cause the preferred effect. Thus, to carefully evaluate the effects of an a.i., toxicity tests should be carried out at least at two different temperatures distant by 10°C (e.g. 25°C and 35°C). Otherwise, the hazard which the bees are exposed to could be underestimated.