Insect galls in xeric and mesic habitats in a Cerrado-Caatinga transition in northern Minas Gerais, Brazil

Galls are morphological changes caused by endoparasites on plants, mainly related to insects. Several hypotheses have been proposed to explain the patterns of gall richness. In this study, we tested the harsh environment hypothesis, which suggests higher galling richness in xeric habitats when compared to mesic habitats. In addition, we describe the richness of gall induced-insects on their host plants in a transition Cerrado-Caatinga, Brazil. Sampling was performed along of five trajectory of an “imaginary line,” traveled at a natural walking pace over the course of one hour in each of the three adjacent habitats (cerrado, dry forest and riparian forest). We found 98 different morphospecies of galling insects on 70 plant species belonging to 20 families. Gall richness was higher in the xeric habitats (cerrado and dry forest) when compared to mesic habitat (riparian forest). Gall richness differed significantly among the gall-inducing taxa; most galls morphospecies were induced by the Diptera (Cecidomyiidae). In general, richness of all gall-inducing taxa was higher in the xeric habitats than in the mesic habitat. The highest gall richness was found on leaves of the host plants of the riparian forest and cerrado, while the stem represented the organ most attacked in the dry forest. Although there is lower availability of sites for oviposition by female galling in the dry forest during the dry season, we found higher gall richness in this environment when compared to riparian forest. In this way, confirming the harsh environment hypothesis despite the absence of leaves. Likely, there was high gall irradiation on organ exposed continuously to gall attack during ecological time in dry forest. However, further studies are needed to clarify the forces related to differential gall performance among habitats, particularly in threatened Cerrado-Caatinga transition. Key words: Cerrado, seasonally dry tropical forests, harsh environment hypothesis, galling insects, host plants.