AbstractThe lethal doses of the pyrethroid deltamethrin were estimated for the egg parasitoid Trissolcus basalis (Wollaston) (Hymenoptera: Scelionidae), and the sub‐lethal effects of an LD25 on female walking behaviour were evaluated. Linear speed of treated parasitoids was reduced compared with that of untreated ones for a period of up to 24 h. The sub‐lethal effects of deltamethrin on parasitoid response to patches contaminated by a contact kairomone from its host, Nezara viridula L (Heteroptera: Pentatomidae), was also evaluated. Both treated and untreated parasitoids responded to host‐contaminated patches by increasing residence time and decreasing linear speed. However, treated females showed a significantly lower residence time compared with untreated females. Both treated and untreated females showed a similar decrease over time in the proportion of time spent in the host‐contaminated patch. Hence, we hypothesise that the sub‐lethal effect of deltamethrin on host foraging may decrease T basalis efficacy in controlling N viridula.© 2002 Society of Chemical Industry