Triangle-free graphs with large independent domination number

Let G be a simple graph of order n and minimum degree δ. The independent domination number i (G) is defined as the minimum cardinality of an independent dominating set of G. We prove the following conjecture due to Haviland [J. Haviland, Independent domination in triangle-free graphs, Discrete Mathematics 308 (2008), 3545-3550]: If G is a triangle-free graph of order n and minimum degree δ, then i (G) ≤ n - 2 δ for n / 4 ≤ δ ≤ n / 3, while i (G) ≤ δ for n / 3 < δ ≤ 2 n / 5. Moreover, the extremal graphs achieving these upper bounds are also characterized.