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Podisus nigrispinus (Dallas) (Hemiptera: Pentatomidae) and Palmistichus elaeisis (Delvare & LaSalle) (Hymenoptera: Eulophidae) are commercially used in the biological control of defoliating lepidopterans in Brazil. The combination of these natural enemies in integrated pest management systems and the risks of intraguild interactions in simultaneous releases were evaluated in the laboratory. The objective was to verify the possible biological effects of intraguild interaction between P. nigrispinus and P. elaeisis. Pupae of natural host Thyrinteina arnobia (Stoll) (Lepidoptera: Geometridae) and of the alternative host Tenebrio molitor L. (Coleoptera: Tenebrionidae), aged 24 hours, were individualized and exposed to parasitism by females of P. elaeisis for 24 hours. The selectivity of P. nigrispinus to parasitized prey was tested with T. arnobia, T. molitor and with volatile compounds extracted from pupae parasitized or healthy. The biomass of T. molitor pupae, healthy or parasitized by P. elaeisis (at one, nine or 18 days), consumed by P. nigrispinus was calculated. The selectivity of each P. nigrispinus female for healthy or parasitized pupae was observed and the effect on the biology of the predator was analysed. Podisus nigrispinus preferred healthy prey to those with more than one day of parasitism and was attracted to pupae washed with hexane extract from healthy hosts. The body mass of this predator was higher when feeding on healthy pupae. The intraguild interaction between P. nigrispinus and P. elaeisis is symmetrical, indicating that these natural enemies can be used, simultaneously, in the integrated management of defoliating caterpillars without reducing their efficiency. [ABSTRACT FROM AUTHOR]