Insect-induced galls are observed on plants throughout the world, but patterns of gall-inducing insect species richness are not random. In the USA and Brazil, species richness increases with decreasing altitude, which is associated with increasing temperature and aridity. At a given elevation the number of gall-inducing insect species is also higher in drier habitats than mesic habitats. However, variations in the number of potential host plant species, related to soil fertility, may be the cause of these patterns, not differences in temperature and aridity.

We examined patterns of species richness of gall-inducing insects by counting the number of gall-inducing insect species and plant species, and measuring soil phosphorus, in replicate dry and mesic plots at five locations along an altitudinal aridity gradient in Big Bend National Park, Texas. Almost all galls were on woody plant species (trees or shrubs). We found the greatest number of gall-inducing insect species at intermediate elevations, and in more mesic habitats, rather than at low altitudes or in drier habitats. The number of woody plant species was also highest at intermediate elevations and in mesic habitats. Soil phosphorus was high at both extremes of the altitudinal gradient, where few gall-inducing insect species occurred.

Our results show that patterns of species richness of gall-inducing insects may largely be a function of the number of woody plant species present. The chance of a gall-inducing insect finding its specific host plant species increases as the number of woody plant species increases. The effect of soil fertility requires further study but the findings suggest that high soil fertility does not favor gall-inducing insects.