none Helicoverpa zea; FOOD-PLANTS; Gossypium hirsutum (cotton) & Vigna unguiculata (cowpea); Plant volatiles, role in host location by hymenopteran parasitoid; HYMENOPTERAN-PARASITES; Microplitis croceipes; Host location, role of food plant volatiles Microplitis croceipes; CHEMORECEPTION-; Discriminatory ability in relation to lepidopteran host location; LEARNING-; Lepidopteran host location, relationship; LEPIDOPTERAN-HOSTS; Helicoverpa zea; Host location, role of plant volatiles & effect of learning. A study was conducted to determine the primary source of volatile cues within the plant-host complex used by host-seeking free-flying female Microplitis croceipes Cresson in flight tunnel bioassays. In single-source and two-choice tests, using wasps given an oviposition experience on either cotton (Gossypium hirsutum) or cowpea (Vigna unguiculata) seedlings damaged by corn earworm (CEW: Helicoverpa zea Boddie), the damaged seedlings were significantly more attractive than the CEW frass, which was in turn more attractive than the larvae themselves. In series of two-choice wind-tunnel tests, the discriminatory ability of the wasps was examined, following various oviposition experiences. Significantly more wasps flew to plants with "old" damage than to plants with "fresh" damage, regardless of whether they had experience on fresh or old damage. In a comparison of plant species, wasps with only one experience on either host-damaged cotton or host-damaged cowpea were unable to distinguish between them, and showed no preference for either plant, whereas wasps with multiple experiences on a particular plant preferentially flew to that plant in the choice test. In comparing hosts with nonhosts, wasps successfully learned to distinguish CEW from beet armyworm (BAW; Spodoptera exigua) on cotton but were unable to distinguish CEW from either BAW or cabbage looper (Trichoplusia ni) on cowpea. The results show the important role played by plant volatiles in the location of hosts by M. croceipes and indicate the wasps' limitations in discriminating among the various odors. The ecological advantages and disadvantages of this behavior are discussed.