none Diaeretus leucopterus; CONTROL-OF; Eulachnus (Hemiptera); Potential, prevalence & searching behaviour implications; HEMIPTERAN-HOSTS; Eulachnus agilis; Prevalence, searching behaviour & biocontrol potential; GERMANY-; Bayreuth area; Hemipteran host, prevalence & biocontrol potential Eulachnus; CONTROL-BY; Diaeretus leucopterus (Hymenoptera); Potential, parasite prevalence & searching behaviour implications, Germany Eulachnus agilis; HYMENOPTERAN-PARASITES; Diaeretus leucopterus; Prevalence, searching behaviour & biocontrol potential; PLANT-HOSTS; Pinus sylvestris; Hymenopteran parasite prevalence, foraging & biocontrol potential; GERMANY-; Bayreuth area; Hymenopteran parasite prevalence & biocontrol potential, plant host The Palaearctic pine aphids, Eulachnus agilis (Kaltenbach) and Eulachnus rileyi (Williams) have both been introduced into other continents where they have been reported causing damage to economically important pines. In Europe, they are attacked by the specialist parasitoid Diaeretus leucopterus (Haliday) which has been suggested as a possible biological control agent. Here we report on several aspects of the ecology of the parasitoid, conducted on E. agilis in Germany in 1993-94, to provide a more scientific basis for judging its potential for use in biological control. Parasitism of all instars in the field rarely exceeded 10% and was independent of host density. A high percentage of parasitized aphids were hyperparasitized. Measurements of the impact over 16 weeks in a greenhouse release experiment showed that parasitism rates increased from 2 to 19% but were insufficient to suppress the aphid population below a damaging level. There was evidence of a density-dependent response. Studies on foraging behaviour showed that female parasitoids searched pines by quite extensive walks. There was no relationship between the number of aphids per tree and number of ovipositions, and the mean number of ovipositions per female per tree was 2.5 +- 0.4 eggs. The majority (55.1%) of encountered aphids did not respond to the parasitoids. However, female parasitoids attacked a much higher proportion of aphids that did respond but oviposition success on this group was poor. The number of aphid-infested needles on seedlings increased significantly owing to the parasitoid's foraging activity. On the basis of these results, it is suggested that D. leucopterus is only likely to be of benefit in biological control if used in conjunction with other complementary natural enemies.