Whalen, J. K., Benslim, H. and Vanasse, A. 2012. Insecticides (dimethoate and lambda-cyhalothrin) for soybean aphid control - are they toxic to earthworms? Evidence from laboratory and field bioassays. Can. J. Soil Sci. 92: 751-758. Outbreaks of the exotic soybean aphid (Aphis glycines Matsumura) cause significant yield loss of soybean [Glycine max (L.) Merr.] grown in North America. This study assessed the toxicity of dimethoate and lambda cyhalothrin, recommended to control soybean aphid, to earthworms, a non-target organism present in soybean fields. Laboratory bioassays showed the lethal and sub-lethal concentrations of dimethoate and lambda cyhalothrin to adult Eisenia fetida (Savigny) and their offspring. The most sensitive toxicity endpoint was adult E. fetida growth rates, which had a half maximal effective concentration (EC₅₀) of 6.39 mg active ingredient (a.i.) kg^-1 soil for dimethoate and 5.41 mg a.i. kg^-1 soil for lambda cyhalothrin after 28 d exposure. The field bioassay was done on two commercial farms with naturally occurring earthworms (including Aporrectodea turgida,Allolobophora chlorotica, Aporrectodea rosea, Lumbricus terrestris and Eiseniella tetraedra). Soybean fields received insecticidal sprays during 2007 and 2008. Timely application of dimethoate (both farms) and lambda cyhalothrin (one farm) controlled the aphid outbreak in 2007 and increased soybean yield by 9 to 22% over the untreated control. These insecticide treatments had no effect on field earthworm populations, probably because application rates recommended to control soybean aphid were up to three orders of magnitude lower than the EC₅₀ determined in laboratory tests. In conclusion, recommended application levels of dimethoate and lambda cyhalothrin for soybean aphid control are not toxic to earthworms in soybean fields.