The effects of flowering plants on the longevity and fecundity of the proovigenic egg parasitoid Copidosoma koehleri Blanchard, an important parasite of potato moth, Phthorimaea operculella (Zeller), were studied in the laboratory and in the field. Laboratory experimentation showed that fecundity was increased in the presence of honey. When C. koehleri females were deprived of hosts for the first 5 days of their adult lives, neither the total number of eggs laid nor longevity were significantly affected. Longevity was significantly increased when adults were caged on flowering plants of dill, borage, or coriander. Field cage experiments supported these observations with rates of parasitism in P. operculella being significantly higher in fed treatments. A second field trial found that rates of parasitism were greater among P. operculella larvae recovered from potato plants growing close to a strip of flowers than in larvae 20 m distant. Pest populations and levels of crop damage were, however, increased by proximity of flowering plants. Laboratory studies subsequently quantified the effect of some flowers on P. operculella, finding that buckwheat and coriander were at least equivalent to honey in increasing fecundity and adult longevity. Findings are discussed in relation to the use of flowering plants as a tool in habitat management for pest control. The use of 'selective food plants,' which allow only beneficial insects access to nectaries, is proposed as a new strategy.