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(19-07) Toxic effects of dimethoate on egg-laying and hatching in the springtail Mesaphorura macrochaeta. Martin, J.1, Camargo, J.*,1, 1 Universidad de Alcala, Alcala de Henares (madrid), Spain ABSTRACT- In this study we have carried out laboratory experiments to examine the toxic effects of dimethoate (organophosphorus insecticide) on egg-laying and hatching in sexual and parthenogenetic populations of the springtail Mesaphorura macrochaeta (Collembola, Onychiuridae). Springtails were obtained from wild populations inhabiting a Denmark uncontaminated beach area. Terrestrial toxicity bioassays were conducted using four dimethoate dosages (representing 140, 280, 1400 and 2800 g of active ingredient per ha; the maximum dosage recommended for agricultural purpose in Denmark is 280 g of active ingredient per ha) and five controls. Each dosage consisted of five replicates, with then females per replicate for the parthenogenetic population and ten individuals (females and males) per replicate for the sexual population. Toxicity bioassays were performed at 20 ± 1°C in a dark incubator. Animals were not fed during laboratory experiments. Adult mortality and egg-laying were recorded every day during 23 days. Egg-hatching was recorded every day for a maximum period of 33 days. Statistical analyses were performed using the SPSS 10.0 (1999) software in order to compare the effects on egg-laying and hatching between sexual and parthenogenetic populations. There were significant differences (p<0.05; M-W u-test) between both populations at 140 and 280 dosages. However, because of the high toxicity of dimethoate, there were no significant difference between both populations at 1400 and 2800 dosages. ED50 days-23 days (egg-laying; probit analysis) values (as g of active ingredient per ha) and their 95% confidence limits were 473.8 (422.7-530.3) for the parthenogenetic population and 121.1 (74.6-170.9) for the sexual population. Overall, we can conclude that the reproduction process is more adversely affected by dimethoate toxicity in sexual populations than in parthenogenetic populations. Key words: dimethoate toxicity, Colembola, egg-laying |
