Foliar nitrogen and larval parasitism as determinants of leafminer distribution patterns on Spartina alterniflora

ABSTRACT, 1. In northwest Florida, Hydrellia valida hew (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. 2. Shoreline plants contain over twice as much total foliar nitrogen as inland plants. In transects from inland to shoreline plants, leafminer density is positively correlated with total leaf nitrogen content. 3. We 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. 4. Larval parasitism by Opius sp. (Hymenoptera: Braconidae) and Pteromalus sp. (Hymenoptera: Pteromalidae) reaches over 90%, with parasitism increasing from shoreline to inland plants. We suggest that gradients in leafminer densities are caused by differences in parasitoid abundance.