none no species In northwest Florida, Hydrellia valida Loew (Diptera: Ephydridae) frequently mines the leaves of salt marsh cord grass, Spartina alterniflora . Larvae and pupae are more commonly found in shoreline plants within 3 m of the sea than in inland plants. The authors experimentally increased foliar nitrogen content on inland Spartina patches to levels above those of shoreline plants. Leafminer densities on experimental centres remained far below those on control edges. Larval parasitism by Opius sp. (Hymenoptera: Braconidae) and Pteromalus sp. (Hymenoptera: Pteromalidae) reaches over 90% with parasitism increasing from shoreline to inland plants. The authors suggest that gradients in leafminer densities are caused by differences in parasitoid abundance. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships; Ecology; Population dynamics; Habitat; Brackish habitat; Land and freshwater zones; Nearctic region; North America; USA Hydrellia valida (Ephydridae); Hymenopteran parasites; Opius & Pteromalus, distribution within habitat relationship; Plant hosts; Spartina alterniflora; Nitrogen content, hymenopteran parasite distribution relationship; Prevalence; Opius & Pteromalus (Hymenoptera) distribution relationship; Population density; Larvae, plant host nitrogen content relationship; Distribution within habitat; Plant host nitrogen content, & hymenopteran parasites, relationships; Salt marsh; Distribution patterns on host plant, factors; Florida; North west; Distribution on host plants, leaf nitrogen content & parasite relationships Opius (Braconidae ); Pteromalus (Pteromalidae ); Dipteran hosts; Hydrellia valida, host larval distribution relationship; Prevalence; Hydrellia valida (Diptera), host distribution relationship; Florida; North west; Prevalence effect on dipteran host distribution patterns