In their natural habitat in central Mexico, adult Diabrotica porracea Harold (Chrysomelidae: Galerucinae) feed exclusively within the trenches cut by Epilachna tredecimnotata (Latreille) (Coccinellidae) on green, turgent leaves of their common host Cucurbita okeechobeensis ssp. martinezii L. Bailey. In laboratory experiments, we observed that adult E. tredecimnotata cut their characteristic trenches into turgent leaves of this plant. Furthermore, D. porracea beetles only fed inside trenched circles or on leaves without turgor pressure. No sap flow was detected in wilted leaves or leaves without turgor pressure. The phloem sap emanating from cut leaf veins is sticky and chrystalizes quickly. Due to its hardening on the insectś mouthparts it impedes further feeding. Our results support the hypothesis that herbivorous insects on Cucurbitaceae avoid feeding on turgent leaves due to the sticky phloem sap. The opportunistic feeding behavior of D. porracea is discussed in the context of the coexistance of both beetle species on the same plant.