Earthworm extraction with onion solution

► Onion extract solution is a low cost and non-toxic method to collect earthworms. ► Sampling efficiency is related to the concentration of onion extract in the solution. ► Highest numbers were obtained with 175gL−1 onion extract solution. ► Onion extract at 175gL−1 had similar efficiency to formaldehyde 0.5%. Earthworm populations can be assessed using various extraction solutions of which the most widely used is formaldehyde (0.5%). Although formaldehyde extraction has been proposed as a standard method by ISO, its use has been questioned due to its toxic and carcinogenic properties, and the associated risks to human health and the environment. Alternative, safer methods are needed, so the present study was undertaken to evaluate the efficiency of a low cost and non-toxic onion (Allium cepa L.) extract solution in collecting earthworms in two soil types in Southern Brazil. The study was divided in three parts. In the first, we examined the efficiency of solutions with 60, 80, 100, 170 and 200g onion extract L−1 H2O in extracting earthworms from a sandy Ultisol underlying a citrus orchard. The second experiment evaluated the effectiveness of solutions of 25, 75, 125 and 175g onion extract L−1 H2O compared with the standard formaldehyde solution (0.5%) in extracting earthworms from a clayey Oxisol cultivated with corn. The third experiment compared the onion extract solution at concentration of 175gL−1 with formaldehyde (0.5%) in extracting earthworms from the sandy Ultisol. Increasing concentrations of onion extract solution led to higher extraction of earthworms in both soil types, but the highest numbers were obtained with 175gL−1 H2O, a concentration that had similar efficiency to the standard solution of formaldehyde (0.5%). The onion extract solution is a low cost and non-toxic alternative to the use formaldehyde in extracting worms from soils, although further research is warranted to assess its use in other soils, climates and vegetation conditions.