1. Dicyphus hesperus Knight (Heteroptera: Miridae) nymphs were fed from egg hatch to the adult stage on Ephestia kuehniella eggs provided either alone or in combination with tomato leaves or with a supplementary water source.
2. Only 6% of individuals completed nymphal development on a diet of eggs alone. In contrast, a high proportion of nymphs completed development on a diet of eggs when either tomato leaves (97%) or a supplementary water source (88%) were provided.
3. The development times of nymphs given access to leaves were significantly shorter than those of nymphs given access only to supplementary water.
4. Adult female D. hesperus that were given access to tomato leaves prior to feeding trials consumed significantly more eggs in a 4-h period than females that were dehydrated before trials. Dehydrated females that were allowed access to water for 3 h before trials consumed an intermediate number of eggs.
5. Plant feeding or access to some other water source is required for prey feeding, growth, and development in D. hesperus, and acquisition of water is proposed as a primary function of plant feeding. In addition, D. hesperus derives nutrients from plant feeding that increase the rate of nymphal development, although nymphs cannot complete development when provided only with tomato leaves.
6. Three simple models are presented of feeding behaviour in predatory Heteroptera where the amount of plant feeding either decreases, increases, or is constant as a function of the amount of prey feeding. The models are discussed with reference to the results and the probable multifunctional nature of plant feeding in predatory Heteroptera.