Flowers in tri-trophic systems: mechanisms allowing selective exploitationby insect natural enemies for conservation biological control

Many insects have coevolved with certain angiosperm taxa to act as pollinat ors. However, the nectar and pollen from such flowers is also widely fed up on by other insects, including entomophagous species. Conservation biologic al control seeks to maximise the impact of these natural enemies on crop pe sts by enhancing availability of nectar and pollen-rich plants in agroecosy stems. A risk with this approach is that pests may also benefit from the fo od resource. We show that the flowers of some plants (viz., buckwheat, Fago pyron esculentum Moench and dill, Anethum graveolens L.), and the extraflor al nectaries of faba bean (Vicia faba L.) benefit both Copidosoma koehleri Blanchard (Hymenoptera: Encyrtidae) and its host, the potato pest, Phthorim aea operculella Zeller (Lepidoptera: Gelechiidae). In contrast, phacelia (P hacelia tanacetifolia Benth) and nasturtium (Tropaeoleum majus L.) benefite d only the parasitoid. When adult moths of P. operculella were caged with f lowers of phacelia or nasturtium, longevity of males and females, egg layin g life, fecundity, average oviposition rate, and number of eggs in ovaries at death were no greater than in the control treatment with access to shoot s without flowers plus water. All the foregoing measures were increased com pared to the control when the moths were allowed access to dill, buckwheat or faba bean extrafloral nectaries. Such `selectivity' has the potential to make the use of floral resources in conservation biological control more s trategic. We present morphometric and observational evidence to illustrate how such mechanisms may operate.