Influence of Pesticide Dose Preparation on Toxicity of Honeybee Species, Apis mellifera L. by Contact Method

Abstract

The aim of this study was to compare the effect of dose prepared from two different stocks in an acute contact toxicity test to honeybees, Apis mellifera L under laboratory condition. Dimethoate 30% EC contains an active ingredient of 90.18 % w/w which was used as the test substance. The test was conducted as per OECD Guideline 214 (1998). The test stock solution 69.92 ppm (Stock I) and 4.3ppm (Stock II) were prepared in deionized water. The toxicity was evaluated against honeybees at the doses viz., 0.045, 0.068, 0.101, 0.152 and 0.228 µg a.i./bee prepared from both stock solutions. Bees treated with Dimethoate 30% EC dissolved in deionized water were dosed by topical application on the dorsal side of thorax of each bee. The control (Deionized water) was maintained for comparison. Assessments of mortality and abnormal behavioural effects were carried out at 4, 24 and 48 hours after dosing. Dimethoate 30% EC (Stock I) at the highest tested nominal dose of 0.228 µg a.i./bee recorded the highest mortality of 70% and the lowest dose 0.045 µg a.i./bee recorded 0% mortality at 48 hours after dosing. Whereas dimethoate 30% EC (Stock II) at the highest tested nominal dose of 0.228 µg a.i./bee recorded the highest mortality of 23.33% and the lowest dose 0.045 µg a.i./bee recorded 0% mortality at 48 hours after dosing. The 48-hour Contact LD₅₀ (Stock I) was 0.160 µg (95% Confidence limits: 0.135 - 0.201) dimethoate/bee at 48 hours after dosing. The 48-hour Contact LD₅₀ (Stock II) was 0.319 µg (95% Confidence limits: 0.135 - 0.201) dimethoate/bee at 48 hours after dosing. All the control groups (Deionized water) recorded 0.0% mortality during the assessment made up to 48 hours after dosing. Observations of sublethal (abnormal behavioural) effects included moribund, knockdown and apathy at 24 hours after dosing the test substance. No sub lethal effects were observed at 4 and 48 hours after dosing. The test met all the validity criteria as described in OECD 214.