Plant water stress and gall formation (Cecidomyiidae: Asphondylia spp.) on creosote bush

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Abstract
1. Populations of creosote bush (Larrea tridentata (DC) Coville), were studied in Arizona to determine whether associated gallformers (Cecidomyiidae: Asphondylia spp.) were more abundant on water‐stressed or nonstressed plants. Gall densities were measured along a steep elevational gradient that extended from mesic, higher elevations to lower elevations in the desert; and in the Grand Canyon where severely water‐stressed and relatively unstressed plants occurred adjacently. At the Grand Canyon site, the responses of creosote bush to water stress were also studied.2. The number and densities of Asphondylia species increased both at lower elevations and locally on water‐stressed plants in the Grand Canyon, indicating that climatic and local conditions influence gallformer abundance in the same way.3. Five of the eight Asphondylia species studied at the Grand Canyon site were more abundant on stressed plants, two species were more abundant on nonstressed plants and one species showed no preference for either plant type.4. Densities of most species on stressed plants were positively correlated with the number of meristematic terminals per branch, which were more numerous on stressed plants, due to a bushier architecture. Flower gallformers were more abundant on nonstressed plants, which produced more flowers. Gall densities did not correlate with chemistry measurements, although these parameters also varied with level of stress.5. These results suggest that gallforming species respond variably to plant stress, even within a closely‐related lineage, and that there are effects of stress on plants, including architectural changes, that may be more important to herbivores than biochemical effects emphasized by White (1984) and others.