A 13-d continuous dietary exposure bioassay using nymphs of the insidious flower bug, Orius insidiosus (Say) (Heteroptera: Anthocoridae), was developed to assess the potential dietary effects of insecticidal substances that have little or no contact toxicity. The nymphs were fed a bee pollen diet treated with different concentrations of an inorganic stomach poison, potassium arsenate, and a cysteine protease inhibitor, E-64. The results showed that the test system was capable of detecting the dietary effects of both substances on the survival and development of O. insidiosus from the nymph to the adult stage in a dose-dependent manner. For the potassium arsenate treatments, ≈25% of the nymphs survived and developed to the adult stage by 13 d of dietary exposure at 3.8 μg/g of diet, whereas no test nymphs survived to adulthood at or above 15 μg/g of diet. The assay time required for a 75% mortality response ranged from ≈7 d at 30 μg/g of diet to 13 d at 3.8 μg/g of diet. For the E-64 treatments, no test insects survived to adulthood at any of the concentration tested (75–600 μg/g of diet) by 13 d of dietary exposure, and the assay time required for a 75% mortality response ranged from 5 to 11 d at dietary rates of 600 and 75 μg/g, respectively. The research presented here describes a robust test system that is useful for evaluating potential adverse effects (or toxicity) of plant-incorporated protectants on nontarget heteropteran predators such as O. insidiosus.