The role of predators and parasites was evaluated by artificially excluding them for the estimation of the DBM mortality. The results showed the importance of ground-dwelling predators such as the lycosid spider which was a major biotic mortality agent of DBM at immature stages based on an immunological test; the incidence of this spider decreased by the application of methomyl. The LC₅₀ of the lycosid Pardosa astrigera was around 10 ppm against about 7,500 ppm for the fourth instar DBM larvae using the dipping method and about 20,000 ppm for third instar larvae by feeding methomylcontaminated cabbage leaves. Applications of sublethal concentrations of methomyl to fourth instar larvae and pupae of DBM resulted in the increase of the fecundity of the adults which emerged. The adult females derived from the treated pupae laid a larger number of eggs with a higher rate of fertilization than the untreated check. The application of methomyl thus may actually cause a resurgence of the insect population through the following mechanisms: (1) insecticide resistance, (2) differential mortality levels between DBM and its predator which is a major factor in the control , and (3) stimulation of the reproductive potential.