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1. Our understanding of the structure and spatial organisation of biological assemblages in human‐modified tropical landscapes has critical importance to improve conservation actions. Investigations on this topic have focused on local (α) diversity patterns, overlooking the changes in species turnover (β diversity) between sites, and its consequences on total (γ) diversity. 2. This study assessed the differences in α, β and γ diversities of galling insects and their host plants (saplings) in a fragmented Atlantic forest landscape in northeast Brazil. Both assemblages were recorded in 30 plots (total of 0.1 ha for each forest type) located in the interior and on the edges of a large fragment and small forest fragments (10 plots per forest type). 3. α diversity of host plants and galling insect assemblages was significantly higher in interior (reference) plots than in edge and fragment plots. Yet, both assemblages showed higher β diversity in fragment and edge plots than in reference plots – a finding potentially associated with the hyperdynamism of fragmented forests and consistent with the landscape divergence hypothesis. 4. However, biotic differentiation of host plant and galling insects was not great enough to compensate the loss of α diversity, and thus γ diversity, because most host plant and galling insect species in forest fragments were also registered in reference plots. Our findings indicate that, despite each small forest fragment being very dissimilar from each other, they have low importance for the conservation of plant assemblages and their specialized herbivores at landscape scale. Connectivity among fragments and plot‐edge distance seem not to have any effect on host plant and galling insect diversity.Host plants and galling insects of edge‐affected habitats are subsets of the species pool recorded in interior forest.Differentiation of host plant and galling insects in edge‐affected habitats was not large enough to compensate for the loss of α and γ diversity. [ABSTRACT FROM AUTHOR]