Diversity of gall-inducing insects in a Mexican tropical dry forest: the importance of plant species richness, life-forms, host plant age and plant density

1 We tested four hypotheses concerning the variation in species richness of gall-inducing insects (GII) in plant communities. We sampled deciduous and riparian habitats in a tropical dry forest at Chamela-Cuixmala Biosphere Reserve in western Mexico, that differ in phenology and moisture availability. 2 GII species richness might be expected to increase with the richness of host plant species, with life-form complexity of host plants (trees to shrubs, herbs and climbers), with host age or with host plant density. 3 We found 172 plant species, of which more were present in deciduous than in riparian habitats, but 34 (19.8%) occurred in both. A different GII species colonized each of 39 host species. Most GII species (69.2%) were gall midges (Cecidomyiidae). 4 We found a significant positive correlation between GII species richness and plant species richness in both deciduous and riparian habitats, suggesting that radiation of GII species may be associated with plant species richness. 5 Most of the GII species occurred on trees or shrubs rather than herbs or climbers, consistent with structural complexity providing more colonization sites. 6 The frequency of GII was greater on saplings, which may have more undifferentiated meristems susceptible to gall induction. 7 Both the frequency and intensity of damage by galls were greater in deciduous than riparian habitats, indicating a preference of GII species for plants in the more xeric habitat. 8 The frequency of GII increased with host plant density in only 18 (46.2%) of GII species. 9 Richness of GII depends on all proposed factors, although density is often the least important. The effects of host richness and host age are similar to those seen in many specialist folivorous insects, although the unique association of a single insect with a single host is only seen for GII.