Peristenus stygicus Loan develops in Mirinae species Lygus hesperus , Knight, L. lineolaris , (Palisot de Beauvois) and Polymerus basalis (Reuter), Orthotylinae species Labopidicola geminata (Johnston), Phylinae species Pseudatomoscelis seriatus (Reuter), and partially develops in the mirine Dichrooscytus sp. The parasitoids attack Microphylellus maculipennis (Knight) (Phylinae) and another orthotyline species, but do not develop in these. Taedia johnstoni (Knight) (Mirinae), 2 species of Brycorinae and 3 species of Lygaeidae are not acceptable as hosts. Olfactometer studies show the parasites are significantly more attracted to lygus nymphs when Phaseolus vulgaris L. is present than either the nymphs or bean alone. Host color, mobility, size, previously infested plant material, and host plant species are rejected as host selection criteria.