none Helicoverpa zea; Heliothis virescens; HYMENOPTERAN-PARASITES; Microplitis croceipes; Host location, role of olfactory response to plant volatiles Microplitis croceipes; ANTENNAE-; CHEMORECEPTION-; TAXIS-; Olfactory response to plant volatiles, role in lepidopteran host location; LEPIDOPTERAN-HOSTS; Helicoverpa zea & Heliothis virescens; Host location, role of olfactory response to plant volatiles. Antennal olfactory responses of the parasitoid, Microplitis croceipes (Cresson) (Hymenoptera: Braconidae), to 29 cotton volatile compounds were measured by electroantennogram (EAG) techniques. No significant sexual differences were found in EAGs of males and females to volatiles emanating from 100-mu-g stimulus loads of the 29 cotton compounds. Green leaf volatiles (saturated and monounsaturated six-carbon alcohols, aldehydes, and their acetate derivatives), heptanal, and the benzene derivatives, benzaldehyde and acetophenone, elicited the largest EAGs. Monoterpenes elicited moderate EAGs with beta-ocimene being the most effective monoterpene tested. Among the sesquiterpenes tested, beta-bisabolol was the most effective stimulus at the 100-mu-g dose. Dose-response curves constructed from EAGs of females revealed a low threshold for (Z)-3-hexenyl acetate, a compound previously shown to be an effective attractant in wind-tunnel bioassays. Comparison of relative volatilities of the various odorants indicated differential selectivity and sensitivity of M. croceipes antennal receptors to them. The roles of cotton plant volatiles in host habitat location of M. croceipes are discussed.