Insects have become increasingly resistant to chemical control methods, while at the same time public awareness of the harmful effects of synthetic pesticides has increased. The search for more environmentally 'friendly' means of pest suppression is gaining momentum and biological control (the use of natural enemies to reduce populations of noxious organisms) has become an increasingly sought-after option. Despite an increase in the establishment rate of insect natural enemies, classical biological control of arthropods is currently no more successful than it was one hundred years ago. Ecological theory relevant to population biology, food webs and diversity provides insight into how biological control agents can be made more effective, yet this theory has often been absent from the biological control literature. Examples of the use of ecological concepts (including intraguild predation, life-history omnivory and resource subsidies) in practical biological control are reviewed, and aspects of theory not yet considered in this context are discussed. Cereals are important as primary food crops, globally and within New Zealand. Possibly the greatest amount of damage sustained by cereal crops in New Zealand is caused by aphids and chemical control of these pests is very expensive relative to biological control. This thesis examines how biological control of the rose-grain aphid Metopolophium dirhodum (Walker) (Hemiptera: Aphidiidae) by the koinobiont, synovigenic endoparasitoid Aphidius rhopalosiphi De Stefani-Perez (Hymenoptera: Aphidiidae) can be enhanced by floral resource subsidies. The mechanisms underlying this enhancement were determined in a series of laboratory experiments and then tested in laboratory microcosms and in the field. Sugar resources significantly increased longevity and egg load in A. rhopalosiphi and another species of aphidiid, Diaeretiella rapae McIntosh. Pollen had no significant effect on longevity or egg load in these species. These results are discussed in terms of the effects of resource subsidies on egg- versus time-limitation. Laboratory microcosm experiments tested whether the mechanisms of increased potential fecundity via enhanced egg load and longevity translate into increased rates of parasitism (i.e., realised fecundity). Only treatments receiving sugar showed increased reproductive success. The presence of flowering buckwheat Fagopyrum esculentum Moench (c.v. Kitawase) (Polygonaceae) plants caused a slight, non-significant increase in rates of parasitism. Field surveys of natural aphid populations in a wheat Triticum aestivum (L.) (c.v. Otane) (Gramineae) field showed that proximity to floral buckwheat patches, distance to the nearest edge or the leeward end of the field were not significantly correlated with rates of parasitism. These variables were significantly correlated with aphid density in some surveys. Rates of parasitism were not correlated with aphid density. When aphid population density was controlled by experimental placement of aphids, proximity to floral resource patches significantly affected rates of parasitism. Parasitism rates were highest at the edges of buckwheat patches and declined exponentially with distance, eventually reaching zero beyond 14 m. Lower levels of parasitism were observed within the floral patches than at their edges. This phenomenon is compatible with the concept of spatio-temporal partitioning between parasitoid feeding and host-searching behaviours. Potential costs (increased predation risk, opportunity costs) and benefits (increased fecundity and longevity) associated with floral feeding by parasitoids, and temporal variation in these factors, are discussed. It is concluded that resource subsidies are not only effective in the practical enhancement of the efficacy of a specific biological control agent, but their use is based on a sound foundation in ecological theory that allows extension of these principles across taxa.